BPF Type Format (BTF)

1. Introduction

BTF (BPF Type Format) is the metadata format which encodes the debug inforelated to BPF program/map. The name BTF was used initially to describe datatypes. The BTF was later extended to include function info for definedsubroutines, and line info for source/line information.

The debug info is used for map pretty print, function signature, etc. Thefunction signature enables better bpf program/function kernel symbol. The lineinfo helps generate source annotated translated byte code, jited code andverifier 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 kernelverifies the BTF info before using it. The ELF file format is a user spacecontract between ELF file and libbpf loader.

The type and string sections are part of the BTF kernel API, describing thedebug info (mostly types related) referenced by the bpf program. These twosections are discussed in details in2. BTF Type and String Encoding.

2. BTF Type and String Encoding

The fileinclude/uapi/linux/btf.h provides high-level definition of howtypes/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 is0xeB9F, which has different encoding for big and littleendian systems, and can be used to test whether BTF is generated for big- orlittle-endian target. Thebtf_header is designed to be extensible withhdr_len equal tosizeof(structbtf_header) when a data blob isgenerated.

2.1 String Encoding

The first string in the string section must be a null string. The rest ofstring table is a concatenation of other null-terminated strings.

2.2 Type Encoding

The type id0 is reserved forvoid type. The type section is parsedsequentially and type id is assigned to each recognized type starting from id1. 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       */#define BTF_KIND_VAR            14      /* Variable     */#define BTF_KIND_DATASEC        15      /* Section      */

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. Thename_off instructbtf_type specifies the offset in the string table.The following sections detail encoding of each kind.

2.2.1 BTF_KIND_INT

structbtf_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 au32 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)

TheBTF_INT_ENCODING has the following attributes:

#define BTF_INT_SIGNED  (1 << 0)#define BTF_INT_CHAR    (1 << 1)#define BTF_INT_BOOL    (1 << 2)

TheBTF_INT_ENCODING() provides extra information: signedness, char, orbool, for the int type. The char and bool encoding are mostly useful forpretty print. At most one encoding can be specified for the int type.

TheBTF_INT_BITS() specifies the number of actual bits held by this inttype. For example, a 4-bit bitfield encodesBTF_INT_BITS() equals to 4.Thebtf_type.size*8 must be equal to or greater thanBTF_INT_BITS()for the type. The maximum value ofBTF_INT_BITS() is 128.

TheBTF_INT_OFFSET() specifies the starting bit offset to calculate valuesfor 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 hasBTF_INT_OFFSET()=2 andBTF_INT_BITS()=4

Then in the struct memory layout, this member will occupy4 bits startingfrom bits100+2=102.

Alternatively, the bitfield struct member can be the following to access thesame bits as the above:

  • btf member bit offset 102,
  • btf member pointing to an int type,
  • the int type hasBTF_INT_OFFSET()=0 andBTF_INT_BITS()=4

The original intention ofBTF_INT_OFFSET() is to provide flexibility ofbitfield encoding. Currently, both llvm and pahole generateBTF_INT_OFFSET()=0 for all int types.

2.2.2 BTF_KIND_PTR

structbtf_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 followbtf_type.

2.2.3 BTF_KIND_ARRAY

structbtf_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 onestructbtf_array:

struct btf_array {    __u32   type;    __u32   index_type;    __u32   nelems;};
Thestructbtf_array encoding:
  • type: the element type
  • index_type: the index type
  • nelems: the number of elements for this array (0 is also allowed).

Theindex_type can be any regular int type (u8,u16,u32,u64,unsigned__int128). The original design of includingindex_type follows DWARF, which has anindex_type for its array type.Currently in BTF, beyond type verification, theindex_type is not used.

Thestructbtf_array allows chaining through element type to representmultidimensional arrays. For example, forinta[5][6], the following typeinformation 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 intoone-dimensional array, e.g., fora[5][6], thebtf_array.nelems isequal to30. This is because the original use case is map pretty printwhere the whole array is dumped out so one-dimensional array is enough. Asmore BTF usage is explored, pahole and llvm can be changed to generate properchained representation for multidimensional arrays.

2.2.4 BTF_KIND_STRUCT

2.2.5 BTF_KIND_UNION

structbtf_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 byinfo.vlen number ofstructbtf_member.:

struct btf_member {    __u32   name_off;    __u32   type;    __u32   offset;};
structbtf_member encoding:
  • name_off: offset to a valid C identifier
  • type: the member type
  • offset: <see below>

If the type infokind_flag is not set, the offset contains only bit offsetof the member. Note that the base type of the bitfield can only be int or enumtype. If the bitfield size is 32, the base type can be either int or enumtype. If the bitfield size is not 32, the base type must be int, and int typeBTF_INT_BITS() encodes the bitfield size.

If thekind_flag is set, thebtf_member.offset contains both memberbitfield size and bit offset. The bitfield size and bit offset are calculatedas 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 introducedkind_flag and explained why bothmodes exist:

2.2.6 BTF_KIND_ENUM

structbtf_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 byinfo.vlen number ofstructbtf_enum.:

struct btf_enum {    __u32   name_off;    __s32   val;};
Thebtf_enum encoding:
  • name_off: offset to a valid C identifier
  • val: any value

2.2.7 BTF_KIND_FWD

structbtf_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 followbtf_type.

2.2.8 BTF_KIND_TYPEDEF

structbtf_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 atname_off

No additional type data followbtf_type.

2.2.9 BTF_KIND_VOLATILE

structbtf_type encoding requirement:
  • name_off: 0
  • info.kind_flag: 0
  • info.kind: BTF_KIND_VOLATILE
  • info.vlen: 0
  • type: the type withvolatile qualifier

No additional type data followbtf_type.

2.2.10 BTF_KIND_CONST

structbtf_type encoding requirement:
  • name_off: 0
  • info.kind_flag: 0
  • info.kind: BTF_KIND_CONST
  • info.vlen: 0
  • type: the type withconst qualifier

No additional type data followbtf_type.

2.2.11 BTF_KIND_RESTRICT

structbtf_type encoding requirement:
  • name_off: 0
  • info.kind_flag: 0
  • info.kind: BTF_KIND_RESTRICT
  • info.vlen: 0
  • type: the type withrestrict qualifier

No additional type data followbtf_type.

2.2.12 BTF_KIND_FUNC

structbtf_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 followbtf_type.

A BTF_KIND_FUNC defines not a type, but a subprogram (function) whosesignature is defined bytype. The subprogram is thus an instance of thattype. The BTF_KIND_FUNC may in turn be referenced by a func_info in the4.2 .BTF.ext section (ELF) or in the arguments to3.3 BPF_PROG_LOAD(ABI).

2.2.13 BTF_KIND_FUNC_PROTO

structbtf_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 byinfo.vlen number ofstructbtf_param.:

struct btf_param {    __u32   name_off;    __u32   type;};

If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, thenbtf_param.name_off must point to a valid C identifier except for thepossible last argument representing the variable argument. The btf_param.typerefers to parameter type.

If the function has variable arguments, the last parameter is encoded withname_off=0 andtype=0.

2.2.14 BTF_KIND_VAR

structbtf_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 singlestructbtf_variable with thefollowing data:

struct btf_var {    __u32   linkage;};
structbtf_var encoding:
  • linkage: currently only static variable 0, or globally allocated
    variable 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 amap definition.

2.2.15 BTF_KIND_DATASEC

structbtf_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 byinfo.vlen number ofstructbtf_var_secinfo.:

struct btf_var_secinfo {    __u32   type;    __u32   offset;    __u32   size;};
structbtf_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

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_infoand 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 in2. BTF Type and String Encoding, can be directly loaded into the kernel. Abtf_fdis returned to a userspace.

3.2 BPF_MAP_CREATE

A map can be created withbtf_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 andvalue types for the map. During ELF parsing, libbpf is able to extractkey/value type_id’s and assign them to BPF_MAP_CREATE attributesautomatically.

3.3 BPF_PROG_LOAD

During prog_load, func_info and line_info can be passed to kernel with propervalues 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, andline_info_rec_size is the size of each line_info record. Passing the recordsize 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 matchesbpf 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’tchange during the lifetime of a program, map, or btf.

The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id’s, one foreach command, to user space, for bpf program or maps, respectively, so aninspection 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 detailedinformation from kernel about this fd, some of which are BTF-related. Forexample,bpf_map_info returnsbtf_id and key/value type ids.bpf_prog_info returnsbtf_id, func_info, and line info for translatedbpf byte codes, and jited_line_info.

3.7 BPF_BTF_GET_FD_BY_ID

Withbtf_id obtained inbpf_map_info andbpf_prog_info, bpfsyscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, withcommand BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into thekernel with BPF_BTF_LOAD, can be retrieved.

With the btf blob,bpf_map_info, andbpf_prog_info, an introspectiontool has full btf knowledge and is able to pretty print map key/values, dumpfunc 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 issame as the one describe in2. BTF Type and String Encoding.

4.2 .BTF.ext section

The .BTF.ext section encodes func_info and line_info which needs loadermanipulation before loading into the kernel.

The specification for .BTF.ext section is defined attools/lib/bpf/btf.handtools/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;};

It is very similar to .BTF section. Instead of type/string section, itcontains func_info and line_info section. See3.3 BPF_PROG_LOAD for detailsabout func_info and line_info record format.

The func_info is organized as below.:

func_info_rec_sizebtf_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 ofbpf_func_info structure when.BTF.ext is generated.btf_ext_info_sec, defined below, is a collection offunc_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_sizebtf_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 ofbpf_line_info structure when.BTF.ext is generated.

The interpretation ofbpf_func_info->insn_off andbpf_line_info->insn_off is different between kernel API and ELF API. Forkernel API, theinsn_off is the instruction offset in the unit ofstructbpf_insn. For ELF API, theinsn_off is the byte offset from thebeginning of section (btf_ext_info_sec->sec_name_off).

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 ofmacros defined ininclude/linux/btf_ids.h header file. Kernel code canuse them to create lists and sets (sorted lists) of BTF ID values.

TheBTF_ID_LIST andBTF_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

Theu32list[]; variable is defined to access the list.

TheBTF_ID_UNUSED macro defines 4 zero bytes. It’s used when wewant to define unused entry in BTF_ID_LIST, like:

BTF_ID_LIST(bpf_skb_output_btf_ids)BTF_ID(struct, sk_buff)BTF_ID_UNUSEDBTF_ID(struct, task_struct)

All the BTF ID lists and sets are compiled in the .BTF_ids section andresolved during the linking phase of kernel build byresolve_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 simplyraw bytes. This is especially valuable for large structure or if your datastructure 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 bpf_map_def SEC("maps") tmpmap = {     .type = BPF_MAP_TYPE_ARRAY,     .key_size = sizeof(__u32),     .value_size = sizeof(struct tmp_t),     .max_entries = 1,};BPF_ANNOTATE_KV_PAIR(tmpmap, int, struct tmp_t);

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 progdump with better kernel symbol names, function prototypes and lineinformation.:

$ 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 verificationfailure.:

   /* 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’tsupport .BTF.ext and btf BTF_KIND_FUNC type yet. For example,:

-bash-4.4$ cat t.cstruct 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.oFile 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 targetonly. The assembly code (-S) is able to show the BTF encoding in assemblyformat.:

-bash-4.4$ cat t2.ctypedef 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

Kernel bpf selftesttest_btf.c provides extensive set of BTF-related tests.