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_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 2.3.1 Function Linkage Constant Values)

• 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_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 2.3.2 Variable Linkage Constant Values.

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

• 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 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_type encoding requirement:

• name_off: offset to a non-empty string

• info.kind_flag: 0

• 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.

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

2.3.1 Function Linkage Constant Values

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

2.3.2 Variable Linkage Constant Values

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 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.

4.3 .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.