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 2. BTF Type and String Encoding.
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, fwd, enum and enum64.
*/
__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_typeencoding requirement:name_off: any valid offsetinfo.kind_flag: 0info.kind: BTF_KIND_INTinfo.vlen: 0size: 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() = 2andBTF_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() = 0andBTF_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_typeencoding requirement:name_off: 0info.kind_flag: 0info.kind: BTF_KIND_PTRinfo.vlen: 0type: the pointee type of the pointer
No additional type data follow btf_type.
2.2.3 BTF_KIND_ARRAY¶
struct btf_typeencoding requirement:name_off: 0info.kind_flag: 0info.kind: BTF_KIND_ARRAYinfo.vlen: 0size/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_arrayencoding: type: the element typeindex_type: the index typenelems: the number of elements for this array (0is 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_typeencoding requirement:name_off: 0 or offset to a valid C identifierinfo.kind_flag: 0 or 1info.kind: BTF_KIND_STRUCT or BTF_KIND_UNIONinfo.vlen: the number of struct/union membersinfo.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_memberencoding:name_off: offset to a valid C identifiertype: the member typeoffset: <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_typeencoding requirement:name_off: 0 or offset to a valid C identifierinfo.kind_flag: 0 for unsigned, 1 for signedinfo.kind: BTF_KIND_ENUMinfo.vlen: number of enum valuessize: 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_enumencoding: name_off: offset to a valid C identifierval: 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_typeencoding requirement:name_off: offset to a valid C identifierinfo.kind_flag: 0 for struct, 1 for unioninfo.kind: BTF_KIND_FWDinfo.vlen: 0type: 0
No additional type data follow btf_type.
2.2.8 BTF_KIND_TYPEDEF¶
struct btf_typeencoding requirement:name_off: offset to a valid C identifierinfo.kind_flag: 0info.kind: BTF_KIND_TYPEDEFinfo.vlen: 0type: the type which can be referred by name atname_off
No additional type data follow btf_type.
2.2.9 BTF_KIND_VOLATILE¶
struct btf_typeencoding requirement:name_off: 0info.kind_flag: 0info.kind: BTF_KIND_VOLATILEinfo.vlen: 0type: the type withvolatilequalifier
No additional type data follow btf_type.
2.2.10 BTF_KIND_CONST¶
struct btf_typeencoding requirement:name_off: 0info.kind_flag: 0info.kind: BTF_KIND_CONSTinfo.vlen: 0type: the type withconstqualifier
No additional type data follow btf_type.
2.2.11 BTF_KIND_RESTRICT¶
struct btf_typeencoding requirement:name_off: 0info.kind_flag: 0info.kind: BTF_KIND_RESTRICTinfo.vlen: 0type: the type withrestrictqualifier
No additional type data follow btf_type.
2.2.12 BTF_KIND_FUNC¶
struct btf_typeencoding requirement:name_off: offset to a valid C identifierinfo.kind_flag: 0info.kind: BTF_KIND_FUNCinfo.vlen: linkage information (BTF_FUNC_STATIC, BTF_FUNC_GLOBALor BTF_FUNC_EXTERN)
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
4.2 .BTF.ext section (ELF) or in the arguments to 3.3 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_typeencoding requirement:name_off: 0info.kind_flag: 0info.kind: BTF_KIND_FUNC_PROTOinfo.vlen: # of parameterstype: 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_typeencoding requirement:name_off: offset to a valid C identifierinfo.kind_flag: 0info.kind: BTF_KIND_VARinfo.vlen: 0type: the type of the variable
btf_type is followed by a single struct btf_variable with the
following data:
struct btf_var {
__u32 linkage;
};
struct btf_varencoding:linkage: currently only static variable 0, or globally allocatedvariable in ELF sections 1
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_typeencoding requirement:name_off: offset to a valid name associated with a variable orone of .data/.bss/.rodata
info.kind_flag: 0info.kind: BTF_KIND_DATASECinfo.vlen: # of variablessize: total section size in bytes (0 at compilation time, patchedto 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_secinfoencoding:type: the type of the BTF_KIND_VAR variableoffset: the in-section offset of the variablesize: the size of the variable in bytes
2.2.16 BTF_KIND_FLOAT¶
struct btf_typeencoding requirement:name_off: any valid offsetinfo.kind_flag: 0info.kind: BTF_KIND_FLOATinfo.vlen: 0size: 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_typeencoding requirement:name_off: offset to a non-empty stringinfo.kind_flag: 0info.kind: BTF_KIND_DECL_TAGinfo.vlen: 0type:struct,union,func,varortypedef
btf_type is followed by struct btf_decl_tag.:
struct btf_decl_tag {
__u32 component_idx;
};
The name_off encodes btf_decl_tag attribute string.
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.
2.2.18 BTF_KIND_TYPE_TAG¶
struct btf_typeencoding requirement:name_off: offset to a non-empty stringinfo.kind_flag: 0info.kind: BTF_KIND_TYPE_TAGinfo.vlen: 0type: the type withbtf_type_tagattribute
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.
2.2.19 BTF_KIND_ENUM64¶
struct btf_typeencoding requirement:name_off: 0 or offset to a valid C identifierinfo.kind_flag: 0 for unsigned, 1 for signedinfo.kind: BTF_KIND_ENUM64info.vlen: number of enum valuessize: 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_enum64encoding: name_off: offset to a valid C identifierval_lo32: lower 32-bit value for a 64-bit valueval_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.
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
2. BTF Type and String Encoding, 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.
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 2. BTF Type and String Encoding.
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 3.3 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 Documentation/bpf/llvm_reloc.rst for more information on CO-RE relocations.
4.2 .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.
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
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.