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|
=====================
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 */
#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 */
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 24-27: kind (e.g. int, ptr, array...etc)
* bits 28-30: unused
* bit 31: kind_flag, currently used by
* struct, union and fwd
*/
__u32 info;
/* "size" is used by INT, ENUM, STRUCT and UNION.
* "size" tells the size of the type it is describing.
*
* "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
* FUNC and FUNC_PROTO.
* "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``.
The following kernel patch introduced ``kind_flag`` and
explained why both modes exist:
https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3#diff-fa650a64fdd3968396883d2fe8215ff3
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
* ``info.kind``: BTF_KIND_ENUM
* ``info.vlen``: number of enum values
* ``size``: 4
``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
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``: 0
* ``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).
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``.
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 bpf_map_def SEC("maps") btf_map = {
.type = BPF_MAP_TYPE_ARRAY,
.key_size = sizeof(int),
.value_size = sizeof(struct ipv_counts),
.max_entries = 4,
};
BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts);
Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name,
key and value types for the map.
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 si
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