요약·해설과 원문, 전문 번역을 서로 분리했습니다. API 이름, symbol, source path는 원문 표기를 사용합니다.
1. 요약·해설
원문의 핵심 논리와 kernel programming 관점의 보충 설명입니다. 아래의 전문 번역과는 별도로 작성했습니다.
2. 영어 원문 전체
번역 기준이 된 Linux v6.18.37 원문입니다. 줄 번호는 이 버전의 파일 좌표입니다.
원문 전체 펼치기
=====================
BPF Type Format (BTF)
=====================
1. Introduction
===============
BTF (BPF Type Format) is the metadata format which encodes the debug info
related to BPF program/map. The name BTF was used initially to describe data
types. The BTF was later extended to include function info for defined
subroutines, and line info for source/line information.
The debug info is used for map pretty print, function signature, etc. The
function signature enables better bpf program/function kernel symbol. The line
info helps generate source annotated translated byte code, jited code and
verifier log.
The BTF specification contains two parts,
* BTF kernel API
* BTF ELF file format
The kernel API is the contract between user space and kernel. The kernel
verifies the BTF info before using it. The ELF file format is a user space
contract between ELF file and libbpf loader.
The type and string sections are part of the BTF kernel API, describing the
debug info (mostly types related) referenced by the bpf program. These two
sections are discussed in details in :ref:`BTF_Type_String`.
.. _BTF_Type_String:
2. BTF Type and String Encoding
===============================
The file ``include/uapi/linux/btf.h`` provides high-level definition of how
types/strings are encoded.
The beginning of data blob must be::
struct btf_header {
__u16 magic;
__u8 version;
__u8 flags;
__u32 hdr_len;
/* All offsets are in bytes relative to the end of this header */
__u32 type_off; /* offset of type section */
__u32 type_len; /* length of type section */
__u32 str_off; /* offset of string section */
__u32 str_len; /* length of string section */
};
The magic is ``0xeB9F``, which has different encoding for big and little
endian systems, and can be used to test whether BTF is generated for big- or
little-endian target. The ``btf_header`` is designed to be extensible with
``hdr_len`` equal to ``sizeof(struct btf_header)`` when a data blob is
generated.
2.1 String Encoding
-------------------
The first string in the string section must be a null string. The rest of
string table is a concatenation of other null-terminated strings.
2.2 Type Encoding
-----------------
The type id ``0`` is reserved for ``void`` type. The type section is parsed
sequentially and type id is assigned to each recognized type starting from id
``1``. Currently, the following types are supported::
#define BTF_KIND_INT 1 /* Integer */
#define BTF_KIND_PTR 2 /* Pointer */
#define BTF_KIND_ARRAY 3 /* Array */
#define BTF_KIND_STRUCT 4 /* Struct */
#define BTF_KIND_UNION 5 /* Union */
#define BTF_KIND_ENUM 6 /* Enumeration up to 32-bit values */
#define BTF_KIND_FWD 7 /* Forward */
#define BTF_KIND_TYPEDEF 8 /* Typedef */
#define BTF_KIND_VOLATILE 9 /* Volatile */
#define BTF_KIND_CONST 10 /* Const */
#define BTF_KIND_RESTRICT 11 /* Restrict */
#define BTF_KIND_FUNC 12 /* Function */
#define BTF_KIND_FUNC_PROTO 13 /* Function Proto */
#define BTF_KIND_VAR 14 /* Variable */
#define BTF_KIND_DATASEC 15 /* Section */
#define BTF_KIND_FLOAT 16 /* Floating point */
#define BTF_KIND_DECL_TAG 17 /* Decl Tag */
#define BTF_KIND_TYPE_TAG 18 /* Type Tag */
#define BTF_KIND_ENUM64 19 /* Enumeration up to 64-bit values */
Note that the type section encodes debug info, not just pure types.
``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
Each type contains the following common data::
struct btf_type {
__u32 name_off;
/* "info" bits arrangement
* bits 0-15: vlen (e.g. # of struct's members)
* bits 16-23: unused
* bits 24-28: kind (e.g. int, ptr, array...etc)
* bits 29-30: unused
* bit 31: kind_flag, currently used by
* struct, union, enum, fwd, enum64,
* decl_tag and type_tag
*/
__u32 info;
/* "size" is used by INT, ENUM, STRUCT, UNION and ENUM64.
* "size" tells the size of the type it is describing.
*
* "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
* FUNC, FUNC_PROTO, DECL_TAG and TYPE_TAG.
* "type" is a type_id referring to another type.
*/
union {
__u32 size;
__u32 type;
};
};
For certain kinds, the common data are followed by kind-specific data. The
``name_off`` in ``struct btf_type`` specifies the offset in the string table.
The following sections detail encoding of each kind.
2.2.1 BTF_KIND_INT
~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: any valid offset
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_INT
* ``info.vlen``: 0
* ``size``: the size of the int type in bytes.
``btf_type`` is followed by a ``u32`` with the following bits arrangement::
#define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
#define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
#define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)
The ``BTF_INT_ENCODING`` has the following attributes::
#define BTF_INT_SIGNED (1 << 0)
#define BTF_INT_CHAR (1 << 1)
#define BTF_INT_BOOL (1 << 2)
The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
bool, for the int type. The char and bool encoding are mostly useful for
pretty print. At most one encoding can be specified for the int type.
The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
for the type. The maximum value of ``BTF_INT_BITS()`` is 128.
The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
for this int. For example, a bitfield struct member has:
* btf member bit offset 100 from the start of the structure,
* btf member pointing to an int type,
* the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4``
Then in the struct memory layout, this member will occupy ``4`` bits starting
from bits ``100 + 2 = 102``.
Alternatively, the bitfield struct member can be the following to access the
same bits as the above:
* btf member bit offset 102,
* btf member pointing to an int type,
* the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4``
The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of
bitfield encoding. Currently, both llvm and pahole generate
``BTF_INT_OFFSET() = 0`` for all int types.
2.2.2 BTF_KIND_PTR
~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_PTR
* ``info.vlen``: 0
* ``type``: the pointee type of the pointer
No additional type data follow ``btf_type``.
2.2.3 BTF_KIND_ARRAY
~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_ARRAY
* ``info.vlen``: 0
* ``size/type``: 0, not used
``btf_type`` is followed by one ``struct btf_array``::
struct btf_array {
__u32 type;
__u32 index_type;
__u32 nelems;
};
The ``struct btf_array`` encoding:
* ``type``: the element type
* ``index_type``: the index type
* ``nelems``: the number of elements for this array (``0`` is also allowed).
The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``,
``u64``, ``unsigned __int128``). The original design of including
``index_type`` follows DWARF, which has an ``index_type`` for its array type.
Currently in BTF, beyond type verification, the ``index_type`` is not used.
The ``struct btf_array`` allows chaining through element type to represent
multidimensional arrays. For example, for ``int a[5][6]``, the following type
information illustrates the chaining:
* [1]: int
* [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6``
* [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5``
Currently, both pahole and llvm collapse multidimensional array into
one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is
equal to ``30``. This is because the original use case is map pretty print
where the whole array is dumped out so one-dimensional array is enough. As
more BTF usage is explored, pahole and llvm can be changed to generate proper
chained representation for multidimensional arrays.
2.2.4 BTF_KIND_STRUCT
~~~~~~~~~~~~~~~~~~~~~
2.2.5 BTF_KIND_UNION
~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0 or offset to a valid C identifier
* ``info.kind_flag``: 0 or 1
* ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION
* ``info.vlen``: the number of struct/union members
* ``info.size``: the size of the struct/union in bytes
``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.::
struct btf_member {
__u32 name_off;
__u32 type;
__u32 offset;
};
``struct btf_member`` encoding:
* ``name_off``: offset to a valid C identifier
* ``type``: the member type
* ``offset``: <see below>
If the type info ``kind_flag`` is not set, the offset contains only bit offset
of the member. Note that the base type of the bitfield can only be int or enum
type. If the bitfield size is 32, the base type can be either int or enum
type. If the bitfield size is not 32, the base type must be int, and int type
``BTF_INT_BITS()`` encodes the bitfield size.
If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member
bitfield size and bit offset. The bitfield size and bit offset are calculated
as below.::
#define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24)
#define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff)
In this case, if the base type is an int type, it must be a regular int type:
* ``BTF_INT_OFFSET()`` must be 0.
* ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``.
Commit 9d5f9f701b18 introduced ``kind_flag`` and explains why both modes
exist.
2.2.6 BTF_KIND_ENUM
~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0 or offset to a valid C identifier
* ``info.kind_flag``: 0 for unsigned, 1 for signed
* ``info.kind``: BTF_KIND_ENUM
* ``info.vlen``: number of enum values
* ``size``: 1/2/4/8
``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.::
struct btf_enum {
__u32 name_off;
__s32 val;
};
The ``btf_enum`` encoding:
* ``name_off``: offset to a valid C identifier
* ``val``: any value
If the original enum value is signed and the size is less than 4,
that value will be sign extended into 4 bytes. If the size is 8,
the value will be truncated into 4 bytes.
2.2.7 BTF_KIND_FWD
~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: offset to a valid C identifier
* ``info.kind_flag``: 0 for struct, 1 for union
* ``info.kind``: BTF_KIND_FWD
* ``info.vlen``: 0
* ``type``: 0
No additional type data follow ``btf_type``.
2.2.8 BTF_KIND_TYPEDEF
~~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: offset to a valid C identifier
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_TYPEDEF
* ``info.vlen``: 0
* ``type``: the type which can be referred by name at ``name_off``
No additional type data follow ``btf_type``.
2.2.9 BTF_KIND_VOLATILE
~~~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_VOLATILE
* ``info.vlen``: 0
* ``type``: the type with ``volatile`` qualifier
No additional type data follow ``btf_type``.
2.2.10 BTF_KIND_CONST
~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_CONST
* ``info.vlen``: 0
* ``type``: the type with ``const`` qualifier
No additional type data follow ``btf_type``.
2.2.11 BTF_KIND_RESTRICT
~~~~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_RESTRICT
* ``info.vlen``: 0
* ``type``: the type with ``restrict`` qualifier
No additional type data follow ``btf_type``.
2.2.12 BTF_KIND_FUNC
~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: offset to a valid C identifier
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_FUNC
* ``info.vlen``: linkage information (BTF_FUNC_STATIC, BTF_FUNC_GLOBAL
or BTF_FUNC_EXTERN - see :ref:`BTF_Function_Linkage_Constants`)
* ``type``: a BTF_KIND_FUNC_PROTO type
No additional type data follow ``btf_type``.
A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose
signature is defined by ``type``. The subprogram is thus an instance of that
type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the
:ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load`
(ABI).
Currently, only linkage values of BTF_FUNC_STATIC and BTF_FUNC_GLOBAL are
supported in the kernel.
2.2.13 BTF_KIND_FUNC_PROTO
~~~~~~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_FUNC_PROTO
* ``info.vlen``: # of parameters
* ``type``: the return type
``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.::
struct btf_param {
__u32 name_off;
__u32 type;
};
If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then
``btf_param.name_off`` must point to a valid C identifier except for the
possible last argument representing the variable argument. The btf_param.type
refers to parameter type.
If the function has variable arguments, the last parameter is encoded with
``name_off = 0`` and ``type = 0``.
2.2.14 BTF_KIND_VAR
~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: offset to a valid C identifier
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_VAR
* ``info.vlen``: 0
* ``type``: the type of the variable
``btf_type`` is followed by a single ``struct btf_variable`` with the
following data::
struct btf_var {
__u32 linkage;
};
``btf_var.linkage`` may take the values: BTF_VAR_STATIC, BTF_VAR_GLOBAL_ALLOCATED or BTF_VAR_GLOBAL_EXTERN -
see :ref:`BTF_Var_Linkage_Constants`.
Not all type of global variables are supported by LLVM at this point.
The following is currently available:
* static variables with or without section attributes
* global variables with section attributes
The latter is for future extraction of map key/value type id's from a
map definition.
2.2.15 BTF_KIND_DATASEC
~~~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: offset to a valid name associated with a variable or
one of .data/.bss/.rodata
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_DATASEC
* ``info.vlen``: # of variables
* ``size``: total section size in bytes (0 at compilation time, patched
to actual size by BPF loaders such as libbpf)
``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.::
struct btf_var_secinfo {
__u32 type;
__u32 offset;
__u32 size;
};
``struct btf_var_secinfo`` encoding:
* ``type``: the type of the BTF_KIND_VAR variable
* ``offset``: the in-section offset of the variable
* ``size``: the size of the variable in bytes
2.2.16 BTF_KIND_FLOAT
~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: any valid offset
* ``info.kind_flag``: 0
* ``info.kind``: BTF_KIND_FLOAT
* ``info.vlen``: 0
* ``size``: the size of the float type in bytes: 2, 4, 8, 12 or 16.
No additional type data follow ``btf_type``.
2.2.17 BTF_KIND_DECL_TAG
~~~~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: offset to a non-empty string
* ``info.kind_flag``: 0 or 1
* ``info.kind``: BTF_KIND_DECL_TAG
* ``info.vlen``: 0
* ``type``: ``struct``, ``union``, ``func``, ``var`` or ``typedef``
``btf_type`` is followed by ``struct btf_decl_tag``.::
struct btf_decl_tag {
__u32 component_idx;
};
The ``type`` should be ``struct``, ``union``, ``func``, ``var`` or ``typedef``.
For ``var`` or ``typedef`` type, ``btf_decl_tag.component_idx`` must be ``-1``.
For the other three types, if the btf_decl_tag attribute is
applied to the ``struct``, ``union`` or ``func`` itself,
``btf_decl_tag.component_idx`` must be ``-1``. Otherwise,
the attribute is applied to a ``struct``/``union`` member or
a ``func`` argument, and ``btf_decl_tag.component_idx`` should be a
valid index (starting from 0) pointing to a member or an argument.
If ``info.kind_flag`` is 0, then this is a normal decl tag, and the
``name_off`` encodes btf_decl_tag attribute string.
If ``info.kind_flag`` is 1, then the decl tag represents an arbitrary
__attribute__. In this case, ``name_off`` encodes a string
representing the attribute-list of the attribute specifier. For
example, for an ``__attribute__((aligned(4)))`` the string's contents
is ``aligned(4)``.
2.2.18 BTF_KIND_TYPE_TAG
~~~~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: offset to a non-empty string
* ``info.kind_flag``: 0 or 1
* ``info.kind``: BTF_KIND_TYPE_TAG
* ``info.vlen``: 0
* ``type``: the type with ``btf_type_tag`` attribute
Currently, ``BTF_KIND_TYPE_TAG`` is only emitted for pointer types.
It has the following btf type chain:
::
ptr -> [type_tag]*
-> [const | volatile | restrict | typedef]*
-> base_type
Basically, a pointer type points to zero or more
type_tag, then zero or more const/volatile/restrict/typedef
and finally the base type. The base type is one of
int, ptr, array, struct, union, enum, func_proto and float types.
Similarly to decl tags, if the ``info.kind_flag`` is 0, then this is a
normal type tag, and the ``name_off`` encodes btf_type_tag attribute
string.
If ``info.kind_flag`` is 1, then the type tag represents an arbitrary
__attribute__, and the ``name_off`` encodes a string representing the
attribute-list of the attribute specifier.
2.2.19 BTF_KIND_ENUM64
~~~~~~~~~~~~~~~~~~~~~~
``struct btf_type`` encoding requirement:
* ``name_off``: 0 or offset to a valid C identifier
* ``info.kind_flag``: 0 for unsigned, 1 for signed
* ``info.kind``: BTF_KIND_ENUM64
* ``info.vlen``: number of enum values
* ``size``: 1/2/4/8
``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum64``.::
struct btf_enum64 {
__u32 name_off;
__u32 val_lo32;
__u32 val_hi32;
};
The ``btf_enum64`` encoding:
* ``name_off``: offset to a valid C identifier
* ``val_lo32``: lower 32-bit value for a 64-bit value
* ``val_hi32``: high 32-bit value for a 64-bit value
If the original enum value is signed and the size is less than 8,
that value will be sign extended into 8 bytes.
2.3 Constant Values
-------------------
.. _BTF_Function_Linkage_Constants:
2.3.1 Function Linkage Constant Values
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. table:: Function Linkage Values and Meanings
=================== ===== ===========
kind value description
=================== ===== ===========
``BTF_FUNC_STATIC`` 0x0 definition of subprogram not visible outside containing compilation unit
``BTF_FUNC_GLOBAL`` 0x1 definition of subprogram visible outside containing compilation unit
``BTF_FUNC_EXTERN`` 0x2 declaration of a subprogram whose definition is outside the containing compilation unit
=================== ===== ===========
.. _BTF_Var_Linkage_Constants:
2.3.2 Variable Linkage Constant Values
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. table:: Variable Linkage Values and Meanings
============================ ===== ===========
kind value description
============================ ===== ===========
``BTF_VAR_STATIC`` 0x0 definition of global variable not visible outside containing compilation unit
``BTF_VAR_GLOBAL_ALLOCATED`` 0x1 definition of global variable visible outside containing compilation unit
``BTF_VAR_GLOBAL_EXTERN`` 0x2 declaration of global variable whose definition is outside the containing compilation unit
============================ ===== ===========
3. BTF Kernel API
=================
The following bpf syscall command involves BTF:
* BPF_BTF_LOAD: load a blob of BTF data into kernel
* BPF_MAP_CREATE: map creation with btf key and value type info.
* BPF_PROG_LOAD: prog load with btf function and line info.
* BPF_BTF_GET_FD_BY_ID: get a btf fd
* BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info
and other btf related info are returned.
The workflow typically looks like:
::
Application:
BPF_BTF_LOAD
|
v
BPF_MAP_CREATE and BPF_PROG_LOAD
|
V
......
Introspection tool:
......
BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's)
|
V
BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd)
|
V
BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id)
| |
V |
BPF_BTF_GET_FD_BY_ID (get btf_fd) |
| |
V |
BPF_OBJ_GET_INFO_BY_FD (get btf) |
| |
V V
pretty print types, dump func signatures and line info, etc.
3.1 BPF_BTF_LOAD
----------------
Load a blob of BTF data into kernel. A blob of data, described in
:ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd``
is returned to a userspace.
3.2 BPF_MAP_CREATE
------------------
A map can be created with ``btf_fd`` and specified key/value type id.::
__u32 btf_fd; /* fd pointing to a BTF type data */
__u32 btf_key_type_id; /* BTF type_id of the key */
__u32 btf_value_type_id; /* BTF type_id of the value */
In libbpf, the map can be defined with extra annotation like below:
::
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__type(key, int);
__type(value, struct ipv_counts);
__uint(max_entries, 4);
} btf_map SEC(".maps");
During ELF parsing, libbpf is able to extract key/value type_id's and assign
them to BPF_MAP_CREATE attributes automatically.
.. _BPF_Prog_Load:
3.3 BPF_PROG_LOAD
-----------------
During prog_load, func_info and line_info can be passed to kernel with proper
values for the following attributes:
::
__u32 insn_cnt;
__aligned_u64 insns;
......
__u32 prog_btf_fd; /* fd pointing to BTF type data */
__u32 func_info_rec_size; /* userspace bpf_func_info size */
__aligned_u64 func_info; /* func info */
__u32 func_info_cnt; /* number of bpf_func_info records */
__u32 line_info_rec_size; /* userspace bpf_line_info size */
__aligned_u64 line_info; /* line info */
__u32 line_info_cnt; /* number of bpf_line_info records */
The func_info and line_info are an array of below, respectively.::
struct bpf_func_info {
__u32 insn_off; /* [0, insn_cnt - 1] */
__u32 type_id; /* pointing to a BTF_KIND_FUNC type */
};
struct bpf_line_info {
__u32 insn_off; /* [0, insn_cnt - 1] */
__u32 file_name_off; /* offset to string table for the filename */
__u32 line_off; /* offset to string table for the source line */
__u32 line_col; /* line number and column number */
};
func_info_rec_size is the size of each func_info record, and
line_info_rec_size is the size of each line_info record. Passing the record
size to kernel make it possible to extend the record itself in the future.
Below are requirements for func_info:
* func_info[0].insn_off must be 0.
* the func_info insn_off is in strictly increasing order and matches
bpf func boundaries.
Below are requirements for line_info:
* the first insn in each func must have a line_info record pointing to it.
* the line_info insn_off is in strictly increasing order.
For line_info, the line number and column number are defined as below:
::
#define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10)
#define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff)
3.4 BPF_{PROG,MAP}_GET_NEXT_ID
------------------------------
In kernel, every loaded program, map or btf has a unique id. The id won't
change during the lifetime of a program, map, or btf.
The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for
each command, to user space, for bpf program or maps, respectively, so an
inspection tool can inspect all programs and maps.
3.5 BPF_{PROG,MAP}_GET_FD_BY_ID
-------------------------------
An introspection tool cannot use id to get details about program or maps.
A file descriptor needs to be obtained first for reference-counting purpose.
3.6 BPF_OBJ_GET_INFO_BY_FD
--------------------------
Once a program/map fd is acquired, an introspection tool can get the detailed
information from kernel about this fd, some of which are BTF-related. For
example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids.
``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated
bpf byte codes, and jited_line_info.
3.7 BPF_BTF_GET_FD_BY_ID
------------------------
With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf
syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with
command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the
kernel with BPF_BTF_LOAD, can be retrieved.
With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection
tool has full btf knowledge and is able to pretty print map key/values, dump
func signatures and line info, along with byte/jit codes.
4. ELF File Format Interface
============================
4.1 .BTF section
----------------
The .BTF section contains type and string data. The format of this section is
same as the one describe in :ref:`BTF_Type_String`.
.. _BTF_Ext_Section:
4.2 .BTF.ext section
--------------------
The .BTF.ext section encodes func_info, line_info and CO-RE relocations
which needs loader manipulation before loading into the kernel.
The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h``
and ``tools/lib/bpf/btf.c``.
The current header of .BTF.ext section::
struct btf_ext_header {
__u16 magic;
__u8 version;
__u8 flags;
__u32 hdr_len;
/* All offsets are in bytes relative to the end of this header */
__u32 func_info_off;
__u32 func_info_len;
__u32 line_info_off;
__u32 line_info_len;
/* optional part of .BTF.ext header */
__u32 core_relo_off;
__u32 core_relo_len;
};
It is very similar to .BTF section. Instead of type/string section, it
contains func_info, line_info and core_relo sub-sections.
See :ref:`BPF_Prog_Load` for details about func_info and line_info
record format.
The func_info is organized as below.::
func_info_rec_size /* __u32 value */
btf_ext_info_sec for section #1 /* func_info for section #1 */
btf_ext_info_sec for section #2 /* func_info for section #2 */
...
``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when
.BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of
func_info for each specific ELF section.::
struct btf_ext_info_sec {
__u32 sec_name_off; /* offset to section name */
__u32 num_info;
/* Followed by num_info * record_size number of bytes */
__u8 data[0];
};
Here, num_info must be greater than 0.
The line_info is organized as below.::
line_info_rec_size /* __u32 value */
btf_ext_info_sec for section #1 /* line_info for section #1 */
btf_ext_info_sec for section #2 /* line_info for section #2 */
...
``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when
.BTF.ext is generated.
The interpretation of ``bpf_func_info->insn_off`` and
``bpf_line_info->insn_off`` is different between kernel API and ELF API. For
kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
beginning of section (``btf_ext_info_sec->sec_name_off``).
The core_relo is organized as below.::
core_relo_rec_size /* __u32 value */
btf_ext_info_sec for section #1 /* core_relo for section #1 */
btf_ext_info_sec for section #2 /* core_relo for section #2 */
``core_relo_rec_size`` specifies the size of ``bpf_core_relo``
structure when .BTF.ext is generated. All ``bpf_core_relo`` structures
within a single ``btf_ext_info_sec`` describe relocations applied to
section named by ``btf_ext_info_sec->sec_name_off``.
See :ref:`Documentation/bpf/llvm_reloc.rst <btf-co-re-relocations>`
for more information on CO-RE relocations.
4.3 .BTF_ids section
--------------------
The .BTF_ids section encodes BTF ID values that are used within the kernel.
This section is created during the kernel compilation with the help of
macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can
use them to create lists and sets (sorted lists) of BTF ID values.
The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values,
with following syntax::
BTF_ID_LIST(list)
BTF_ID(type1, name1)
BTF_ID(type2, name2)
resulting in following layout in .BTF_ids section::
__BTF_ID__type1__name1__1:
.zero 4
__BTF_ID__type2__name2__2:
.zero 4
The ``u32 list[];`` variable is defined to access the list.
The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we
want to define unused entry in BTF_ID_LIST, like::
BTF_ID_LIST(bpf_skb_output_btf_ids)
BTF_ID(struct, sk_buff)
BTF_ID_UNUSED
BTF_ID(struct, task_struct)
The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values
and their count, with following syntax::
BTF_SET_START(set)
BTF_ID(type1, name1)
BTF_ID(type2, name2)
BTF_SET_END(set)
resulting in following layout in .BTF_ids section::
__BTF_ID__set__set:
.zero 4
__BTF_ID__type1__name1__3:
.zero 4
__BTF_ID__type2__name2__4:
.zero 4
The ``struct btf_id_set set;`` variable is defined to access the list.
The ``typeX`` name can be one of following::
struct, union, typedef, func
and is used as a filter when resolving the BTF ID value.
All the BTF ID lists and sets are compiled in the .BTF_ids section and
resolved during the linking phase of kernel build by ``resolve_btfids`` tool.
4.4 .BTF.base section
---------------------
Split BTF - where the .BTF section only contains types not in the associated
base .BTF section - is an extremely efficient way to encode type information
for kernel modules, since they generally consist of a few module-specific
types along with a large set of shared kernel types. The former are encoded
in split BTF, while the latter are encoded in base BTF, resulting in more
compact representations. A type in split BTF that refers to a type in
base BTF refers to it using its base BTF ID, and split BTF IDs start
at last_base_BTF_ID + 1.
The downside of this approach however is that this makes the split BTF
somewhat brittle - when the base BTF changes, base BTF ID references are
no longer valid and the split BTF itself becomes useless. The role of the
.BTF.base section is to make split BTF more resilient for cases where
the base BTF may change, as is the case for kernel modules not built every
time the kernel is for example. .BTF.base contains named base types; INTs,
FLOATs, STRUCTs, UNIONs, ENUM[64]s and FWDs. INTs and FLOATs are fully
described in .BTF.base sections, while composite types like structs
and unions are not fully defined - the .BTF.base type simply serves as
a description of the type the split BTF referred to, so structs/unions
have 0 members in the .BTF.base section. ENUM[64]s are similarly recorded
with 0 members. Any other types are added to the split BTF. This
distillation process then leaves us with a .BTF.base section with
such minimal descriptions of base types and .BTF split section which refers
to those base types. Later, we can relocate the split BTF using both the
information stored in the .BTF.base section and the new .BTF base; the type
information in the .BTF.base section allows us to update the split BTF
references to point at the corresponding new base BTF IDs.
BTF relocation happens on kernel module load when a kernel module has a
.BTF.base section, and libbpf also provides a btf__relocate() API to
accomplish this.
As an example consider the following base BTF::
[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[2] STRUCT 'foo' size=8 vlen=2
'f1' type_id=1 bits_offset=0
'f2' type_id=1 bits_offset=32
...and associated split BTF::
[3] PTR '(anon)' type_id=2
i.e. split BTF describes a pointer to struct foo { int f1; int f2 };
.BTF.base will consist of::
[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[2] STRUCT 'foo' size=8 vlen=0
If we relocate the split BTF later using the following new base BTF::
[1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
[2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[3] STRUCT 'foo' size=8 vlen=2
'f1' type_id=2 bits_offset=0
'f2' type_id=2 bits_offset=32
...we can use our .BTF.base description to know that the split BTF reference
is to struct foo, and relocation results in new split BTF::
[4] PTR '(anon)' type_id=3
Note that we had to update BTF ID and start BTF ID for the split BTF.
So we see how .BTF.base plays the role of facilitating later relocation,
leading to more resilient split BTF.
.BTF.base sections will be generated automatically for out-of-tree kernel module
builds - i.e. where KBUILD_EXTMOD is set (as it would be for "make M=path/2/mod"
cases). .BTF.base generation requires pahole support for the "distilled_base"
BTF feature; this is available in pahole v1.28 and later.
5. Using BTF
============
5.1 bpftool map pretty print
----------------------------
With BTF, the map key/value can be printed based on fields rather than simply
raw bytes. This is especially valuable for large structure or if your data
structure has bitfields. For example, for the following map,::
enum A { A1, A2, A3, A4, A5 };
typedef enum A ___A;
struct tmp_t {
char a1:4;
int a2:4;
int :4;
__u32 a3:4;
int b;
___A b1:4;
enum A b2:4;
};
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__type(key, int);
__type(value, struct tmp_t);
__uint(max_entries, 1);
} tmpmap SEC(".maps");
bpftool is able to pretty print like below:
::
[{
"key": 0,
"value": {
"a1": 0x2,
"a2": 0x4,
"a3": 0x6,
"b": 7,
"b1": 0x8,
"b2": 0xa
}
}
]
5.2 bpftool prog dump
---------------------
The following is an example showing how func_info and line_info can help prog
dump with better kernel symbol names, function prototypes and line
information.::
$ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
[...]
int test_long_fname_2(struct dummy_tracepoint_args * arg):
bpf_prog_44a040bf25481309_test_long_fname_2:
; static int test_long_fname_2(struct dummy_tracepoint_args *arg)
0: push %rbp
1: mov %rsp,%rbp
4: sub $0x30,%rsp
b: sub $0x28,%rbp
f: mov %rbx,0x0(%rbp)
13: mov %r13,0x8(%rbp)
17: mov %r14,0x10(%rbp)
1b: mov %r15,0x18(%rbp)
1f: xor %eax,%eax
21: mov %rax,0x20(%rbp)
25: xor %esi,%esi
; int key = 0;
27: mov %esi,-0x4(%rbp)
; if (!arg->sock)
2a: mov 0x8(%rdi),%rdi
; if (!arg->sock)
2e: cmp $0x0,%rdi
32: je 0x0000000000000070
34: mov %rbp,%rsi
; counts = bpf_map_lookup_elem(&btf_map, &key);
[...]
5.3 Verifier Log
----------------
The following is an example of how line_info can help debugging verification
failure.::
/* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
* is modified as below.
*/
data = (void *)(long)xdp->data;
data_end = (void *)(long)xdp->data_end;
/*
if (data + 4 > data_end)
return XDP_DROP;
*/
*(u32 *)data = dst->dst;
$ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp
; data = (void *)(long)xdp->data;
224: (79) r2 = *(u64 *)(r10 -112)
225: (61) r2 = *(u32 *)(r2 +0)
; *(u32 *)data = dst->dst;
226: (63) *(u32 *)(r2 +0) = r1
invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0)
R2 offset is outside of the packet
6. BTF Generation
=================
You need latest pahole
https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't
support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,::
-bash-4.4$ cat t.c
struct t {
int a:2;
int b:3;
int c:2;
} g;
-bash-4.4$ gcc -c -O2 -g t.c
-bash-4.4$ pahole -JV t.o
File t.o:
[1] STRUCT t kind_flag=1 size=4 vlen=3
a type_id=2 bitfield_size=2 bits_offset=0
b type_id=2 bitfield_size=3 bits_offset=2
c type_id=2 bitfield_size=2 bits_offset=5
[2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target
only. The assembly code (-S) is able to show the BTF encoding in assembly
format.::
-bash-4.4$ cat t2.c
typedef int __int32;
struct t2 {
int a2;
int (*f2)(char q1, __int32 q2, ...);
int (*f3)();
} g2;
int main() { return 0; }
int test() { return 0; }
-bash-4.4$ clang -c -g -O2 --target=bpf t2.c
-bash-4.4$ readelf -S t2.o
......
[ 8] .BTF PROGBITS 0000000000000000 00000247
000000000000016e 0000000000000000 0 0 1
[ 9] .BTF.ext PROGBITS 0000000000000000 000003b5
0000000000000060 0000000000000000 0 0 1
[10] .rel.BTF.ext REL 0000000000000000 000007e0
0000000000000040 0000000000000010 16 9 8
......
-bash-4.4$ clang -S -g -O2 --target=bpf t2.c
-bash-4.4$ cat t2.s
......
.section .BTF,"",@progbits
.short 60319 # 0xeb9f
.byte 1
.byte 0
.long 24
.long 0
.long 220
.long 220
.long 122
.long 0 # BTF_KIND_FUNC_PROTO(id = 1)
.long 218103808 # 0xd000000
.long 2
.long 83 # BTF_KIND_INT(id = 2)
.long 16777216 # 0x1000000
.long 4
.long 16777248 # 0x1000020
......
.byte 0 # string offset=0
.ascii ".text" # string offset=1
.byte 0
.ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7
.byte 0
.ascii "int main() { return 0; }" # string offset=33
.byte 0
.ascii "int test() { return 0; }" # string offset=58
.byte 0
.ascii "int" # string offset=83
......
.section .BTF.ext,"",@progbits
.short 60319 # 0xeb9f
.byte 1
.byte 0
.long 24
.long 0
.long 28
.long 28
.long 44
.long 8 # FuncInfo
.long 1 # FuncInfo section string offset=1
.long 2
.long .Lfunc_begin0
.long 3
.long .Lfunc_begin1
.long 5
.long 16 # LineInfo
.long 1 # LineInfo section string offset=1
.long 2
.long .Ltmp0
.long 7
.long 33
.long 7182 # Line 7 Col 14
.long .Ltmp3
.long 7
.long 58
.long 8206 # Line 8 Col 14
7. Testing
==========
The kernel BPF selftest `tools/testing/selftests/bpf/prog_tests/btf.c`_
provides an extensive set of BTF-related tests.
.. Links
.. _tools/testing/selftests/bpf/prog_tests/btf.c:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/btf.c
3. 한국어 전문 번역
영어 원문의 문단 순서와 의미를 유지한 전체 번역입니다. 코드, 함수명, symbol과 URL은 원문 표기를 유지합니다.
BTF 소개와 specification 범위
1-31BTF(BPF Type Format)는 BPF program과 map 관련 debug info를 encode하는 metadata format입니다. 처음에는 data type을 설명하는 이름으로 사용됐지만, 이후 정의된 subroutine의 function info와 source·line 정보를 위한 line info까지 포함하도록 확장됐습니다.
debug info는 map pretty print와 function signature 등에 사용됩니다. function signature는 BPF program/function의 kernel symbol을 더 잘 표현하게 하고, line info는 source annotation이 붙은 translated bytecode, JIT code, verifier log를 생성하는 데 도움을 줍니다.
BTF specification은 두 부분으로 구성됩니다.
- BTF kernel API
- BTF ELF file format
kernel API는 userspace와 kernel 사이의 contract이며 kernel은 BTF info를 사용하기 전에 verify합니다. ELF file format은 ELF file과 libbpf loader 사이의 userspace contract입니다.
type section과 string section은 BTF kernel API의 일부로서 BPF program이 참조하는, 주로 type 관련 debug info를 설명합니다. 이 두 section은 아래 BTF Type and String Encoding에서 자세히 다룹니다.
BTF header와 string encoding
32-64`include/uapi/linux/btf.h`는 type과 string을 encode하는 방식을 높은 수준에서 정의합니다. data blob은 다음 header로 시작해야 합니다.
struct btf_header {
__u16 magic;
__u8 version;
__u8 flags;
__u32 hdr_len;
/* All offsets are in bytes relative to the end of this header */
__u32 type_off; /* offset of type section */
__u32 type_len; /* length of type section */
__u32 str_off; /* offset of string section */
__u32 str_len; /* length of string section */
};magic은 `0xeB9F`입니다. big-endian과 little-endian system에서 encoding이 달라 BTF가 어느 endian target용으로 생성됐는지 검사하는 데 사용할 수 있습니다.
`btf_header`는 확장 가능하도록 설계됐으며 data blob을 생성할 때 `hdr_len`은 `sizeof(struct btf_header)`와 같습니다.
string section의 첫 string은 반드시 null string이어야 합니다. 나머지 string table은 다른 null-terminated string을 이어 붙인 형태입니다.
Type ID와 공통 btf_type encoding
65-125type id `0`은 `void` type용으로 예약됩니다. type section을 순서대로 parse하면서 인식한 각 type에 id `1`부터 type id를 부여합니다. 현재 지원하는 kind는 다음과 같습니다.
#define BTF_KIND_INT 1 /* Integer */
#define BTF_KIND_PTR 2 /* Pointer */
#define BTF_KIND_ARRAY 3 /* Array */
#define BTF_KIND_STRUCT 4 /* Struct */
#define BTF_KIND_UNION 5 /* Union */
#define BTF_KIND_ENUM 6 /* Enumeration up to 32-bit values */
#define BTF_KIND_FWD 7 /* Forward */
#define BTF_KIND_TYPEDEF 8 /* Typedef */
#define BTF_KIND_VOLATILE 9 /* Volatile */
#define BTF_KIND_CONST 10 /* Const */
#define BTF_KIND_RESTRICT 11 /* Restrict */
#define BTF_KIND_FUNC 12 /* Function */
#define BTF_KIND_FUNC_PROTO 13 /* Function Proto */
#define BTF_KIND_VAR 14 /* Variable */
#define BTF_KIND_DATASEC 15 /* Section */
#define BTF_KIND_FLOAT 16 /* Floating point */
#define BTF_KIND_DECL_TAG 17 /* Decl Tag */
#define BTF_KIND_TYPE_TAG 18 /* Type Tag */
#define BTF_KIND_ENUM64 19 /* Enumeration up to 64-bit values */type section은 순수 type만이 아니라 debug info를 encode합니다. `BTF_KIND_FUNC`는 type이 아니며 정의된 subprogram을 나타냅니다.
각 type에는 다음 공통 data가 있습니다.
struct btf_type {
__u32 name_off;
/* "info" bits arrangement
* bits 0-15: vlen (e.g. # of struct's members)
* bits 16-23: unused
* bits 24-28: kind (e.g. int, ptr, array...etc)
* bits 29-30: unused
* bit 31: kind_flag, currently used by
* struct, union, enum, fwd, enum64,
* decl_tag and type_tag
*/
__u32 info;
/* "size" is used by INT, ENUM, STRUCT, UNION and ENUM64.
* "size" tells the size of the type it is describing.
*
* "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
* FUNC, FUNC_PROTO, DECL_TAG and TYPE_TAG.
* "type" is a type_id referring to another type.
*/
union {
__u32 size;
__u32 type;
};
};특정 kind에서는 공통 data 뒤에 kind-specific data가 이어집니다. `struct btf_type`의 `name_off`는 string table offset을 지정합니다. 다음 절에서 각 kind의 encoding을 설명합니다.
BTF_KIND_INT와 bitfield
126-177`BTF_KIND_INT`의 `struct btf_type` encoding requirement는 다음과 같습니다.
- `name_off`: 유효한 offset
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_INT`
- `info.vlen`: 0
- `size`: int type의 byte 단위 size
`btf_type` 뒤에는 다음 bit arrangement를 가진 `u32`가 옵니다.
#define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
#define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
#define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)`BTF_INT_ENCODING`에는 다음 attribute가 있습니다.
#define BTF_INT_SIGNED (1 << 0)
#define BTF_INT_CHAR (1 << 1)
#define BTF_INT_BOOL (1 << 2)`BTF_INT_ENCODING()`은 int type의 signedness, char, bool 추가 정보를 제공합니다. char와 bool encoding은 주로 pretty print에 유용하며 int type에는 최대 하나의 encoding만 지정할 수 있습니다.
`BTF_INT_BITS()`는 이 int type이 실제로 보유한 bit 수를 지정합니다. 예를 들어 4-bit bitfield는 `BTF_INT_BITS() = 4`로 encode합니다. type의 `btf_type.size * 8`은 `BTF_INT_BITS()` 이상이어야 하며 최대값은 128입니다.
`BTF_INT_OFFSET()`은 이 int에서 value를 계산할 시작 bit offset을 지정합니다. 예를 들어 다음 bitfield struct member를 생각할 수 있습니다.
- structure 시작에서 BTF member bit offset은 100입니다.
- BTF member는 int type을 가리킵니다.
- int type은 `BTF_INT_OFFSET() = 2`, `BTF_INT_BITS() = 4`입니다.
이 member는 struct memory layout에서 `100 + 2 = 102` bit부터 4 bits를 차지합니다. 같은 bit에 access하려면 다음처럼 encode할 수도 있습니다.
- BTF member bit offset은 102입니다.
- BTF member는 int type을 가리킵니다.
- int type은 `BTF_INT_OFFSET() = 0`, `BTF_INT_BITS() = 4`입니다.
`BTF_INT_OFFSET()`의 원래 목적은 bitfield encoding에 flexibility를 주는 것입니다. 현재 LLVM과 pahole은 모든 int type에 `BTF_INT_OFFSET() = 0`을 생성합니다.
BTF_KIND_PTR와 BTF_KIND_ARRAY
178-232`BTF_KIND_PTR`의 encoding requirement는 다음과 같습니다.
- `name_off`: 0
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_PTR`
- `info.vlen`: 0
- `type`: pointer가 가리키는 pointee type
`btf_type` 뒤에 추가 type data는 없습니다.
`BTF_KIND_ARRAY`의 encoding requirement는 다음과 같습니다.
- `name_off`: 0
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_ARRAY`
- `info.vlen`: 0
- `size/type`: 사용하지 않으므로 0
`btf_type` 뒤에는 `struct btf_array` 하나가 옵니다.
struct btf_array {
__u32 type;
__u32 index_type;
__u32 nelems;
};`struct btf_array`의 field는 다음과 같습니다.
- `type`: element type
- `index_type`: index type
- `nelems`: array의 element 수이며 `0`도 허용
`index_type`은 `u8`, `u16`, `u32`, `u64`, `unsigned __int128` 같은 regular int type일 수 있습니다. `index_type`을 둔 초기 설계는 array type에 `index_type`이 있는 DWARF를 따랐습니다. 현재 BTF에서는 type verification 외에 `index_type`을 사용하지 않습니다.
`struct btf_array`는 element type을 따라 chain을 만들어 multidimensional array를 표현할 수 있습니다. `int a[5][6]`의 type 정보는 다음과 같습니다.
- [1]: int
- [2]: array, `btf_array.type = [1]`, `btf_array.nelems = 6`
- [3]: array, `btf_array.type = [2]`, `btf_array.nelems = 5`
현재 pahole과 LLVM은 multidimensional array를 one-dimensional array로 collapse합니다. 예를 들어 `a[5][6]`의 `btf_array.nelems`는 30입니다. 초기 use case인 map pretty print에서는 array 전체를 dump하므로 one-dimensional representation이면 충분했기 때문입니다. BTF 사용이 확대되면 proper chained representation을 생성하도록 pahole과 LLVM을 바꿀 수 있습니다.
BTF_KIND_STRUCT와 BTF_KIND_UNION
233-278struct와 union의 `struct btf_type` encoding requirement는 다음과 같습니다.
- `name_off`: 0 또는 valid C identifier의 offset
- `info.kind_flag`: 0 또는 1
- `info.kind`: `BTF_KIND_STRUCT` 또는 `BTF_KIND_UNION`
- `info.vlen`: struct/union member 수
- `info.size`: struct/union의 byte 단위 size
`btf_type` 뒤에는 `info.vlen`개의 `struct btf_member`가 옵니다.
struct btf_member {
__u32 name_off;
__u32 type;
__u32 offset;
};`struct btf_member` encoding은 다음과 같습니다.
- `name_off`: valid C identifier의 offset
- `type`: member type
- `offset`: 아래 규칙에 따른 값
type info의 `kind_flag`가 설정되지 않으면 offset에는 member의 bit offset만 들어갑니다. bitfield base type은 int 또는 enum만 가능합니다. bitfield size가 32이면 base type이 int 또는 enum일 수 있습니다. size가 32가 아니면 base type은 int여야 하며 int type의 `BTF_INT_BITS()`가 bitfield size를 encode합니다.
`kind_flag`가 설정되면 `btf_member.offset`에 member bitfield size와 bit offset이 모두 들어가며 다음처럼 계산합니다.
#define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24)
#define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff)이 경우 base type이 int라면 regular int type이어야 하며 다음을 만족해야 합니다.
- `BTF_INT_OFFSET()`은 0이어야 합니다.
- `BTF_INT_BITS()`는 `{1,2,4,8,16} * 8` 중 하나여야 합니다.
commit `9d5f9f701b18`이 `kind_flag`를 도입했으며 두 mode가 모두 존재하는 이유를 설명합니다.
BTF_KIND_ENUM과 BTF_KIND_FWD
279-316`BTF_KIND_ENUM`의 encoding requirement는 다음과 같습니다.
- `name_off`: 0 또는 valid C identifier의 offset
- `info.kind_flag`: unsigned이면 0, signed이면 1
- `info.kind`: `BTF_KIND_ENUM`
- `info.vlen`: enum value 수
- `size`: 1, 2, 4, 8 중 하나
`btf_type` 뒤에는 `info.vlen`개의 `struct btf_enum`이 옵니다.
struct btf_enum {
__u32 name_off;
__s32 val;
};`btf_enum` encoding은 다음과 같습니다.
- `name_off`: valid C identifier의 offset
- `val`: 임의의 value
원래 enum value가 signed이고 size가 4보다 작으면 value를 4 bytes로 sign-extend합니다. size가 8이면 value를 4 bytes로 truncate합니다.
`BTF_KIND_FWD`의 encoding requirement는 다음과 같습니다.
- `name_off`: valid C identifier의 offset
- `info.kind_flag`: struct이면 0, union이면 1
- `info.kind`: `BTF_KIND_FWD`
- `info.vlen`: 0
- `type`: 0
`btf_type` 뒤에 추가 type data는 없습니다.
Alias·qualifier와 BTF_KIND_FUNC
317-386`BTF_KIND_TYPEDEF`의 encoding requirement는 다음과 같습니다.
- `name_off`: valid C identifier의 offset
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_TYPEDEF`
- `info.vlen`: 0
- `type`: `name_off`의 이름으로 참조할 type
`BTF_KIND_VOLATILE`의 encoding requirement는 다음과 같습니다.
- `name_off`: 0
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_VOLATILE`
- `info.vlen`: 0
- `type`: `volatile` qualifier가 붙은 type
`BTF_KIND_CONST`의 encoding requirement는 다음과 같습니다.
- `name_off`: 0
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_CONST`
- `info.vlen`: 0
- `type`: `const` qualifier가 붙은 type
`BTF_KIND_RESTRICT`의 encoding requirement는 다음과 같습니다.
- `name_off`: 0
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_RESTRICT`
- `info.vlen`: 0
- `type`: `restrict` qualifier가 붙은 type
위 네 kind 모두 `btf_type` 뒤에 추가 type data가 없습니다. `BTF_KIND_FUNC`의 encoding requirement는 다음과 같습니다.
- `name_off`: valid C identifier의 offset
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_FUNC`
- `info.vlen`: `BTF_FUNC_STATIC`, `BTF_FUNC_GLOBAL`, `BTF_FUNC_EXTERN` 중 linkage 정보
- `type`: `BTF_KIND_FUNC_PROTO` type
`BTF_KIND_FUNC`는 type이 아니라 `type`이 signature를 정의하는 subprogram(function)을 나타냅니다. 즉 subprogram은 해당 type의 instance입니다. `BTF_KIND_FUNC`는 `.BTF.ext`의 `func_info` 또는 `BPF_PROG_LOAD` ABI argument에서 참조될 수 있습니다.
현재 kernel은 `BTF_FUNC_STATIC`과 `BTF_FUNC_GLOBAL` linkage value만 지원합니다.
BTF_KIND_FUNC_PROTO
387-410`BTF_KIND_FUNC_PROTO`의 encoding requirement는 다음과 같습니다.
- `name_off`: 0
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_FUNC_PROTO`
- `info.vlen`: parameter 수
- `type`: return type
`btf_type` 뒤에는 `info.vlen`개의 `struct btf_param`이 옵니다.
struct btf_param {
__u32 name_off;
__u32 type;
};`BTF_KIND_FUNC_PROTO` type을 `BTF_KIND_FUNC`가 참조하면 variable argument를 나타낼 수 있는 마지막 argument를 제외하고 `btf_param.name_off`는 valid C identifier를 가리켜야 합니다. `btf_param.type`은 parameter type을 참조합니다.
function에 variable argument가 있으면 마지막 parameter를 `name_off = 0`, `type = 0`으로 encode합니다.
BTF_KIND_VAR와 BTF_KIND_DATASEC
411-464`BTF_KIND_VAR`의 encoding requirement는 다음과 같습니다.
- `name_off`: valid C identifier의 offset
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_VAR`
- `info.vlen`: 0
- `type`: variable type
`btf_type` 뒤에는 다음 `struct btf_var` 하나가 옵니다.
struct btf_var {
__u32 linkage;
};`btf_var.linkage`는 `BTF_VAR_STATIC`, `BTF_VAR_GLOBAL_ALLOCATED`, `BTF_VAR_GLOBAL_EXTERN` 중 하나입니다.
현재 LLVM이 지원하는 global variable type은 다음과 같습니다.
- section attribute가 있거나 없는 static variable
- section attribute가 있는 global variable
두 번째 항목은 앞으로 map definition에서 map key/value type id를 추출하기 위한 것입니다.
`BTF_KIND_DATASEC`의 encoding requirement는 다음과 같습니다.
- `name_off`: variable과 연결된 valid name 또는 `.data`, `.bss`, `.rodata` 중 하나의 offset
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_DATASEC`
- `info.vlen`: variable 수
- `size`: 전체 section의 byte 단위 size. compile 시에는 0이고 libbpf 같은 BPF loader가 실제 size로 patch합니다.
`btf_type` 뒤에는 `info.vlen`개의 `struct btf_var_secinfo`가 옵니다.
struct btf_var_secinfo {
__u32 type;
__u32 offset;
__u32 size;
};`struct btf_var_secinfo` encoding은 다음과 같습니다.
- `type`: `BTF_KIND_VAR` variable의 type
- `offset`: section 안에서 variable의 offset
- `size`: variable의 byte 단위 size
BTF_KIND_FLOAT와 BTF_KIND_DECL_TAG
465-510`BTF_KIND_FLOAT`의 encoding requirement는 다음과 같습니다.
- `name_off`: 유효한 offset
- `info.kind_flag`: 0
- `info.kind`: `BTF_KIND_FLOAT`
- `info.vlen`: 0
- `size`: float type의 byte 단위 size로 2, 4, 8, 12, 16 중 하나
`btf_type` 뒤에 추가 type data는 없습니다.
`BTF_KIND_DECL_TAG`의 encoding requirement는 다음과 같습니다.
- `name_off`: 비어 있지 않은 string의 offset
- `info.kind_flag`: 0 또는 1
- `info.kind`: `BTF_KIND_DECL_TAG`
- `info.vlen`: 0
- `type`: `struct`, `union`, `func`, `var`, `typedef` 중 하나
`btf_type` 뒤에는 다음 `struct btf_decl_tag`가 옵니다.
struct btf_decl_tag {
__u32 component_idx;
};`type`은 `struct`, `union`, `func`, `var`, `typedef` 중 하나여야 합니다. `var` 또는 `typedef`에서는 `btf_decl_tag.component_idx`가 `-1`이어야 합니다.
나머지 세 type에서 attribute를 `struct`, `union`, `func` 자체에 적용하면 `component_idx`는 `-1`입니다. member 또는 function argument에 적용하면 0부터 시작하는 valid member/argument index여야 합니다.
`info.kind_flag`가 0이면 normal decl tag이고 `name_off`는 `btf_decl_tag` attribute string을 encode합니다. 1이면 decl tag가 임의의 `__attribute__`를 나타내며 `name_off`는 attribute specifier의 attribute-list string을 encode합니다. 예를 들어 `__attribute__((aligned(4)))`의 string 내용은 `aligned(4)`입니다.
BTF_KIND_TYPE_TAG와 BTF_KIND_ENUM64
511-567`BTF_KIND_TYPE_TAG`의 encoding requirement는 다음과 같습니다.
- `name_off`: 비어 있지 않은 string의 offset
- `info.kind_flag`: 0 또는 1
- `info.kind`: `BTF_KIND_TYPE_TAG`
- `info.vlen`: 0
- `type`: `btf_type_tag` attribute가 붙은 type
현재 `BTF_KIND_TYPE_TAG`는 pointer type에만 emit되며 다음 BTF type chain을 가집니다.
ptr -> [type_tag]*
-> [const | volatile | restrict | typedef]*
-> base_typepointer type은 0개 이상의 type tag, 이어서 0개 이상의 const·volatile·restrict·typedef, 마지막으로 base type을 가리킵니다. base type은 int, ptr, array, struct, union, enum, func_proto, float 중 하나입니다.
decl tag와 마찬가지로 `info.kind_flag`가 0이면 normal type tag이고 `name_off`가 `btf_type_tag` attribute string을 encode합니다. 1이면 임의의 `__attribute__`를 나타내며 `name_off`는 attribute specifier의 attribute-list string을 encode합니다.
`BTF_KIND_ENUM64`의 encoding requirement는 다음과 같습니다.
- `name_off`: 0 또는 valid C identifier의 offset
- `info.kind_flag`: unsigned이면 0, signed이면 1
- `info.kind`: `BTF_KIND_ENUM64`
- `info.vlen`: enum value 수
- `size`: 1, 2, 4, 8 중 하나
`btf_type` 뒤에는 `info.vlen`개의 `struct btf_enum64`가 옵니다.
struct btf_enum64 {
__u32 name_off;
__u32 val_lo32;
__u32 val_hi32;
};`btf_enum64` encoding은 다음과 같습니다.
- `name_off`: valid C identifier의 offset
- `val_lo32`: 64-bit value의 lower 32-bit
- `val_hi32`: 64-bit value의 high 32-bit
원래 enum value가 signed이고 size가 8보다 작으면 value를 8 bytes로 sign-extend합니다.
Function·variable linkage constant
568-599function linkage constant의 value와 의미는 다음과 같습니다.
| kind | value | description |
|---|---|---|
| `BTF_FUNC_STATIC` | 0x0 | containing compilation unit 밖에서 보이지 않는 subprogram definition |
| `BTF_FUNC_GLOBAL` | 0x1 | containing compilation unit 밖에서도 보이는 subprogram definition |
| `BTF_FUNC_EXTERN` | 0x2 | definition이 containing compilation unit 밖에 있는 subprogram declaration |
variable linkage constant의 value와 의미는 다음과 같습니다.
| kind | value | description |
|---|---|---|
| `BTF_VAR_STATIC` | 0x0 | containing compilation unit 밖에서 보이지 않는 global variable definition |
| `BTF_VAR_GLOBAL_ALLOCATED` | 0x1 | containing compilation unit 밖에서도 보이는 global variable definition |
| `BTF_VAR_GLOBAL_EXTERN` | 0x2 | definition이 containing compilation unit 밖에 있는 global variable declaration |
BTF kernel API workflow
600-642BTF와 관련된 `bpf` syscall command는 다음과 같습니다.
- `BPF_BTF_LOAD`: BTF data blob을 kernel에 load합니다.
- `BPF_MAP_CREATE`: BTF key/value type info와 함께 map을 생성합니다.
- `BPF_PROG_LOAD`: BTF function info와 line info를 포함해 program을 load합니다.
- `BPF_BTF_GET_FD_BY_ID`: BTF fd를 얻습니다.
- `BPF_OBJ_GET_INFO_BY_FD`: BTF, `func_info`, `line_info`와 그 밖의 BTF 관련 정보를 반환합니다.
일반적인 workflow는 application load 경로와 introspection 경로로 나뉩니다.
application은 BTF를 먼저 load한 뒤 map과 program을 만들고, introspection tool은 ID에서 fd와 BTF blob을 얻어 type·signature·line info를 해석합니다.
BPF_BTF_LOAD와 BPF_MAP_CREATE
643-673`BPF_BTF_LOAD`는 BTF Type and String Encoding에서 설명한 BTF data blob을 kernel에 직접 load하고 userspace에 `btf_fd`를 반환합니다.
map은 `btf_fd`와 지정한 key/value type id로 생성할 수 있습니다.
__u32 btf_fd; /* fd pointing to a BTF type data */
__u32 btf_key_type_id; /* BTF type_id of the key */
__u32 btf_value_type_id; /* BTF type_id of the value */libbpf에서는 다음처럼 추가 annotation으로 map을 정의할 수 있습니다.
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__type(key, int);
__type(value, struct ipv_counts);
__uint(max_entries, 4);
} btf_map SEC(".maps");ELF를 parse하는 동안 libbpf는 key/value `type_id`를 추출해 `BPF_MAP_CREATE` attribute에 자동으로 할당할 수 있습니다.
BPF_PROG_LOAD의 func_info와 line_info
674-723`prog_load` 중에는 다음 attribute에 적절한 값을 넣어 `func_info`와 `line_info`를 kernel에 전달할 수 있습니다.
__u32 insn_cnt;
__aligned_u64 insns;
......
__u32 prog_btf_fd; /* fd pointing to BTF type data */
__u32 func_info_rec_size; /* userspace bpf_func_info size */
__aligned_u64 func_info; /* func info */
__u32 func_info_cnt; /* number of bpf_func_info records */
__u32 line_info_rec_size; /* userspace bpf_line_info size */
__aligned_u64 line_info; /* line info */
__u32 line_info_cnt; /* number of bpf_line_info records */`func_info`와 `line_info`는 각각 다음 structure의 array입니다.
struct bpf_func_info {
__u32 insn_off; /* [0, insn_cnt - 1] */
__u32 type_id; /* pointing to a BTF_KIND_FUNC type */
};
struct bpf_line_info {
__u32 insn_off; /* [0, insn_cnt - 1] */
__u32 file_name_off; /* offset to string table for the filename */
__u32 line_off; /* offset to string table for the source line */
__u32 line_col; /* line number and column number */
};`func_info_rec_size`와 `line_info_rec_size`는 각 record의 size입니다. record size를 kernel에 전달하면 미래에 record 자체를 확장할 수 있습니다.
`func_info` requirement는 다음과 같습니다.
- `func_info[0].insn_off`는 0이어야 합니다.
- `func_info`의 `insn_off`는 strictly increasing order이며 BPF function boundary와 일치해야 합니다.
`line_info` requirement는 다음과 같습니다.
- 각 function의 첫 instruction을 가리키는 `line_info` record가 있어야 합니다.
- `line_info`의 `insn_off`는 strictly increasing order여야 합니다.
`line_info`의 line number와 column number는 다음처럼 정의합니다.
#define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10)
#define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff)ID·fd 기반 introspection API
724-760kernel에서 load된 모든 program, map, BTF는 unique id를 가집니다. 이 id는 program, map, BTF의 lifetime 동안 바뀌지 않습니다.
`BPF_{PROG,MAP}_GET_NEXT_ID` syscall command는 BPF program과 map의 모든 id를 userspace에 각각 반환하므로 inspection tool이 모든 program과 map을 조사할 수 있습니다.
introspection tool은 id만으로 program이나 map detail을 얻을 수 없습니다. reference counting을 위해 먼저 `BPF_{PROG,MAP}_GET_FD_BY_ID`로 file descriptor를 얻어야 합니다.
program/map fd를 얻으면 `BPF_OBJ_GET_INFO_BY_FD`로 해당 fd의 detail을 kernel에서 가져올 수 있으며 일부는 BTF 관련 정보입니다. `bpf_map_info`는 `btf_id`와 key/value type id를, `bpf_prog_info`는 translated BPF bytecode의 `btf_id`, `func_info`, line info와 `jited_line_info`를 반환합니다.
`bpf_map_info`와 `bpf_prog_info`에서 얻은 `btf_id`를 사용해 `BPF_BTF_GET_FD_BY_ID`로 BTF fd를 가져옵니다. 이어 `BPF_OBJ_GET_INFO_BY_FD`로 원래 `BPF_BTF_LOAD`가 kernel에 load한 BTF blob을 가져올 수 있습니다.
BTF blob, `bpf_map_info`, `bpf_prog_info`를 갖춘 introspection tool은 전체 BTF 정보를 이용해 map key/value를 pretty print하고 byte/JIT code와 함께 function signature와 line info를 dump할 수 있습니다.
.BTF와 .BTF.ext header
761-804ELF `.BTF` section은 type data와 string data를 포함하며 format은 BTF Type and String Encoding에서 설명한 것과 같습니다.
`.BTF.ext` section은 kernel에 load하기 전에 loader가 조작해야 하는 `func_info`, `line_info`, CO-RE relocation을 encode합니다. specification은 `tools/lib/bpf/btf.h`와 `tools/lib/bpf/btf.c`에 정의돼 있습니다.
현재 `.BTF.ext` section header는 다음과 같습니다.
struct btf_ext_header {
__u16 magic;
__u8 version;
__u8 flags;
__u32 hdr_len;
/* All offsets are in bytes relative to the end of this header */
__u32 func_info_off;
__u32 func_info_len;
__u32 line_info_off;
__u32 line_info_len;
/* optional part of .BTF.ext header */
__u32 core_relo_off;
__u32 core_relo_len;
};`.BTF` section과 매우 비슷하지만 type/string section 대신 `func_info`, `line_info`, `core_relo` sub-section을 포함합니다. `func_info`와 `line_info` record format은 `BPF_PROG_LOAD` 절을 참조하십시오.
.BTF.ext sub-section layout
805-854`func_info`는 다음처럼 구성됩니다.
func_info_rec_size /* __u32 value */
btf_ext_info_sec for section #1 /* func_info for section #1 */
btf_ext_info_sec for section #2 /* func_info for section #2 */
...`func_info_rec_size`는 `.BTF.ext`를 생성할 때의 `bpf_func_info` structure size입니다. `btf_ext_info_sec`은 각 ELF section별 `func_info` collection입니다.
struct btf_ext_info_sec {
__u32 sec_name_off; /* offset to section name */
__u32 num_info;
/* Followed by num_info * record_size number of bytes */
__u8 data[0];
};여기서 `num_info`는 0보다 커야 합니다. `line_info`는 다음처럼 구성됩니다.
line_info_rec_size /* __u32 value */
btf_ext_info_sec for section #1 /* line_info for section #1 */
btf_ext_info_sec for section #2 /* line_info for section #2 */
...`line_info_rec_size`는 `.BTF.ext`를 생성할 때의 `bpf_line_info` structure size입니다.
`bpf_func_info->insn_off`와 `bpf_line_info->insn_off`의 해석은 kernel API와 ELF API에서 다릅니다. kernel API에서는 `struct bpf_insn` 단위의 instruction offset이고, ELF API에서는 section 시작인 `btf_ext_info_sec->sec_name_off`부터의 byte offset입니다.
`core_relo`는 다음처럼 구성됩니다.
core_relo_rec_size /* __u32 value */
btf_ext_info_sec for section #1 /* core_relo for section #1 */
btf_ext_info_sec for section #2 /* core_relo for section #2 */`core_relo_rec_size`는 `.BTF.ext` 생성 시 `bpf_core_relo` structure size입니다. 하나의 `btf_ext_info_sec` 안에 있는 모든 `bpf_core_relo` structure는 `btf_ext_info_sec->sec_name_off`가 이름을 지정한 section에 적용할 relocation을 설명합니다.
CO-RE relocation에 관한 자세한 내용은 `Documentation/bpf/llvm_reloc.rst`의 `btf-co-re-relocations`를 참조하십시오.
.BTF_ids list와 set
855-915`.BTF_ids` section은 kernel 안에서 사용하는 BTF ID value를 encode합니다. kernel compile 중 `include/linux/btf_ids.h`의 macro로 생성하며 kernel code는 이 macro로 BTF ID value의 list와 sorted list인 set을 만들 수 있습니다.
`BTF_ID_LIST`와 `BTF_ID` macro는 다음 syntax로 unsorted BTF ID list를 정의합니다.
BTF_ID_LIST(list)
BTF_ID(type1, name1)
BTF_ID(type2, name2)그 결과 `.BTF_ids` section에는 다음 layout이 생깁니다.
__BTF_ID__type1__name1__1:
.zero 4
__BTF_ID__type2__name2__2:
.zero 4list에 access하도록 `u32 list[];` variable이 정의됩니다.
`BTF_ID_UNUSED` macro는 zero byte 4개를 정의하며 `BTF_ID_LIST` 안의 unused entry를 만들 때 사용합니다.
BTF_ID_LIST(bpf_skb_output_btf_ids)
BTF_ID(struct, sk_buff)
BTF_ID_UNUSED
BTF_ID(struct, task_struct)`BTF_SET_START/END` macro pair는 다음 syntax로 BTF ID value의 sorted list와 count를 정의합니다.
BTF_SET_START(set)
BTF_ID(type1, name1)
BTF_ID(type2, name2)
BTF_SET_END(set)그 결과 `.BTF_ids` section에는 다음 layout이 생깁니다.
__BTF_ID__set__set:
.zero 4
__BTF_ID__type1__name1__3:
.zero 4
__BTF_ID__type2__name2__4:
.zero 4list에 access하도록 `struct btf_id_set set;` variable이 정의됩니다.
`typeX` name은 다음 중 하나이며 BTF ID value를 resolve할 때 filter로 사용합니다.
struct, union, typedef, func모든 BTF ID list와 set은 `.BTF_ids` section에 compile되며 kernel build의 linking phase에서 `resolve_btfids` tool이 resolve합니다.
Split BTF와 .BTF.base relocation
916-990split BTF는 `.BTF` section에 연결된 base `.BTF` section에 없는 type만 담는 방식입니다. module-specific type은 적고 공유 kernel type은 많은 kernel module의 type 정보를 매우 효율적으로 encode합니다. 전자는 split BTF에, 후자는 base BTF에 encode돼 더 compact한 representation을 만듭니다.
split BTF의 type이 base BTF type을 참조할 때 base BTF ID를 사용하며 split BTF ID는 `last_base_BTF_ID + 1`부터 시작합니다.
단점은 base BTF가 바뀌면 base BTF ID reference가 더 이상 valid하지 않아 split BTF가 쓸모없어지는 brittle한 구조라는 점입니다. `.BTF.base` section은 kernel이 매번 build될 때 함께 build되지 않는 out-of-tree module처럼 base BTF가 바뀔 수 있는 경우 split BTF를 더 resilient하게 만듭니다.
`.BTF.base`에는 이름 있는 base type인 INT, FLOAT, STRUCT, UNION, ENUM/ENUM64, FWD를 담습니다. INT와 FLOAT는 완전히 설명하지만 struct·union 같은 composite type은 완전히 정의하지 않습니다. split BTF가 참조한 type을 식별하는 설명만 제공하므로 struct/union은 member 0개로 기록하며 ENUM/ENUM64도 value 0개로 기록합니다. 그 밖의 type은 split BTF에 들어갑니다.
이 distillation 결과 base type의 최소 설명을 담은 `.BTF.base`와 이를 참조하는 split `.BTF`가 남습니다. 나중에 `.BTF.base`의 정보와 새 base BTF를 함께 사용해 split BTF를 relocate할 수 있습니다. `.BTF.base` type 정보로 split BTF reference가 대응하는 새 base BTF ID를 가리키도록 update합니다.
kernel module에 `.BTF.base` section이 있으면 module load 시 BTF relocation을 수행합니다. libbpf도 같은 작업을 하는 `btf__relocate()` API를 제공합니다.
예를 들어 다음 base BTF를 생각합니다.
[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[2] STRUCT 'foo' size=8 vlen=2
'f1' type_id=1 bits_offset=0
'f2' type_id=1 bits_offset=32연결된 split BTF는 다음과 같습니다.
[3] PTR '(anon)' type_id=2즉 split BTF는 `struct foo { int f1; int f2; }`를 가리키는 pointer를 설명합니다. `.BTF.base`는 다음과 같이 구성됩니다.
[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[2] STRUCT 'foo' size=8 vlen=0나중에 다음 새 base BTF로 split BTF를 relocate한다고 가정합니다.
[1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
[2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[3] STRUCT 'foo' size=8 vlen=2
'f1' type_id=2 bits_offset=0
'f2' type_id=2 bits_offset=32`.BTF.base` description을 사용하면 split BTF reference가 `struct foo`를 대상으로 함을 알 수 있고 relocation 결과는 다음 새 split BTF가 됩니다.
[4] PTR '(anon)' type_id=3split BTF의 BTF ID와 시작 BTF ID를 update해야 했습니다. 이처럼 `.BTF.base`는 이후 relocation을 가능하게 해 split BTF를 더 resilient하게 만듭니다.
`.BTF.base` section은 `KBUILD_EXTMOD`가 설정되는 out-of-tree kernel module build, 예를 들어 `make M=path/2/mod`에서 자동 생성됩니다. 생성에는 pahole의 `distilled_base` BTF feature가 필요하며 pahole v1.28 이상에서 지원합니다.
bpftool map pretty print
991-1034BTF를 사용하면 map key/value를 raw byte가 아니라 field를 기준으로 출력할 수 있습니다. 큰 structure나 bitfield가 있는 data structure에서 특히 유용합니다. 다음 map을 예로 듭니다.
enum A { A1, A2, A3, A4, A5 };
typedef enum A ___A;
struct tmp_t {
char a1:4;
int a2:4;
int :4;
__u32 a3:4;
int b;
___A b1:4;
enum A b2:4;
};
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__type(key, int);
__type(value, struct tmp_t);
__uint(max_entries, 1);
} tmpmap SEC(".maps");bpftool은 다음처럼 pretty print할 수 있습니다.
[{
"key": 0,
"value": {
"a1": 0x2,
"a2": 0x4,
"a3": 0x6,
"b": 7,
"b1": 0x8,
"b2": 0xa
}
}
]bpftool prog dump
1035-1068다음 예제는 `func_info`와 `line_info`를 이용해 더 나은 kernel symbol name, function prototype, line 정보를 포함한 program dump를 만드는 방법을 보여 줍니다.
$ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
[...]
int test_long_fname_2(struct dummy_tracepoint_args * arg):
bpf_prog_44a040bf25481309_test_long_fname_2:
; static int test_long_fname_2(struct dummy_tracepoint_args *arg)
0: push %rbp
1: mov %rsp,%rbp
4: sub $0x30,%rsp
b: sub $0x28,%rbp
f: mov %rbx,0x0(%rbp)
13: mov %r13,0x8(%rbp)
17: mov %r14,0x10(%rbp)
1b: mov %r15,0x18(%rbp)
1f: xor %eax,%eax
21: mov %rax,0x20(%rbp)
25: xor %esi,%esi
; int key = 0;
27: mov %esi,-0x4(%rbp)
; if (!arg->sock)
2a: mov 0x8(%rdi),%rdi
; if (!arg->sock)
2e: cmp $0x0,%rdi
32: je 0x0000000000000070
34: mov %rbp,%rsi
; counts = bpf_map_lookup_elem(&btf_map, &key);
[...]Verifier log의 source line 정보
1069-1094다음 예제는 `line_info`가 verification failure debug에 어떻게 도움을 주는지 보여 줍니다. `tools/testing/selftests/bpf/test_xdp_noinline.c`에서 packet boundary check를 주석 처리하면 verifier log가 문제가 난 source line과 invalid packet access를 함께 표시합니다.
/* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
* is modified as below.
*/
data = (void *)(long)xdp->data;
data_end = (void *)(long)xdp->data_end;
/*
if (data + 4 > data_end)
return XDP_DROP;
*/
*(u32 *)data = dst->dst;
$ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp
; data = (void *)(long)xdp->data;
224: (79) r2 = *(u64 *)(r10 -112)
225: (61) r2 = *(u32 *)(r2 +0)
; *(u32 *)data = dst->dst;
226: (63) *(u32 *)(r2 +0) = r1
invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0)
R2 offset is outside of the packetpahole과 LLVM의 BTF 생성
1095-1201BTF를 생성하려면 최신 pahole `https://git.kernel.org/pub/scm/devel/pahole/pahole.git/` 또는 LLVM 8.0 이상이 필요합니다.
pahole은 DWARF-to-BTF converter로 동작합니다. 이 문서의 설명 시점에는 `.BTF.ext`와 BTF `BTF_KIND_FUNC` type을 지원하지 않습니다. 다음은 bitfield structure를 compile하고 pahole로 BTF를 표시하는 예입니다.
-bash-4.4$ cat t.c
struct t {
int a:2;
int b:3;
int c:2;
} g;
-bash-4.4$ gcc -c -O2 -g t.c
-bash-4.4$ pahole -JV t.o
File t.o:
[1] STRUCT t kind_flag=1 size=4 vlen=3
a type_id=2 bitfield_size=2 bits_offset=0
b type_id=2 bitfield_size=3 bits_offset=2
c type_id=2 bitfield_size=2 bits_offset=5
[2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNEDLLVM은 BPF target에 `-g`를 사용할 때만 `.BTF`와 `.BTF.ext`를 직접 생성할 수 있습니다. assembly code를 생성하는 `-S`를 사용하면 BTF encoding을 assembly format으로 확인할 수 있습니다.
-bash-4.4$ cat t2.c
typedef int __int32;
struct t2 {
int a2;
int (*f2)(char q1, __int32 q2, ...);
int (*f3)();
} g2;
int main() { return 0; }
int test() { return 0; }
-bash-4.4$ clang -c -g -O2 --target=bpf t2.c
-bash-4.4$ readelf -S t2.o
......
[ 8] .BTF PROGBITS 0000000000000000 00000247
000000000000016e 0000000000000000 0 0 1
[ 9] .BTF.ext PROGBITS 0000000000000000 000003b5
0000000000000060 0000000000000000 0 0 1
[10] .rel.BTF.ext REL 0000000000000000 000007e0
0000000000000040 0000000000000010 16 9 8
......
-bash-4.4$ clang -S -g -O2 --target=bpf t2.c
-bash-4.4$ cat t2.s
......
.section .BTF,"",@progbits
.short 60319 # 0xeb9f
.byte 1
.byte 0
.long 24
.long 0
.long 220
.long 220
.long 122
.long 0 # BTF_KIND_FUNC_PROTO(id = 1)
.long 218103808 # 0xd000000
.long 2
.long 83 # BTF_KIND_INT(id = 2)
.long 16777216 # 0x1000000
.long 4
.long 16777248 # 0x1000020
......
.byte 0 # string offset=0
.ascii ".text" # string offset=1
.byte 0
.ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7
.byte 0
.ascii "int main() { return 0; }" # string offset=33
.byte 0
.ascii "int test() { return 0; }" # string offset=58
.byte 0
.ascii "int" # string offset=83
......
.section .BTF.ext,"",@progbits
.short 60319 # 0xeb9f
.byte 1
.byte 0
.long 24
.long 0
.long 28
.long 28
.long 44
.long 8 # FuncInfo
.long 1 # FuncInfo section string offset=1
.long 2
.long .Lfunc_begin0
.long 3
.long .Lfunc_begin1
.long 5
.long 16 # LineInfo
.long 1 # LineInfo section string offset=1
.long 2
.long .Ltmp0
.long 7
.long 33
.long 7182 # Line 7 Col 14
.long .Ltmp3
.long 7
.long 58
.long 8206 # Line 8 Col 14BTF selftest
1202-1210kernel BPF selftest `tools/testing/selftests/bpf/prog_tests/btf.c`는 광범위한 BTF 관련 test를 제공합니다.
source는 `https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/btf.c`에서 확인할 수 있습니다.
요약과 해설
btf.rst:1-1210BTF는 BPF program과 map의 type·function·source line debug metadata를 compact하게 encode합니다. type id와 string table, 19개 `BTF_KIND_*`의 field 규칙을 통해 kernel과 userspace loader가 동일한 type contract를 공유합니다.
kernel API는 `BPF_BTF_LOAD`로 BTF blob을 load한 뒤 map/program 생성과 introspection에 연결합니다. ELF의 `.BTF.ext`는 function·line info와 CO-RE relocation을, `.BTF_ids`는 kernel 내부 ID list/set을, `.BTF.base`는 base BTF가 바뀌어도 split BTF를 relocate할 최소 type 설명을 담습니다.
BTF를 활용하면 bpftool이 map field를 pretty print하고 JIT dump와 verifier failure에 source line을 표시할 수 있습니다. pahole은 DWARF에서 BTF를 변환하며 LLVM BPF target은 `.BTF`와 `.BTF.ext`를 직접 생성할 수 있습니다.