8 BTF (BPF Type Format) is the metadata format which encodes the debug info
9 related to BPF program/map. The name BTF was used initially to describe data
10 types. The BTF was later extended to include function info for defined
11 subroutines, and line info for source/line information.
13 The debug info is used for map pretty print, function signature, etc. The
14 function signature enables better bpf program/function kernel symbol. The line
15 info helps generate source annotated translated byte code, jited code and
18 The BTF specification contains two parts,
22 The kernel API is the contract between user space and kernel. The kernel
23 verifies the BTF info before using it. The ELF file format is a user space
24 contract between ELF file and libbpf loader.
26 The type and string sections are part of the BTF kernel API, describing the
27 debug info (mostly types related) referenced by the bpf program. These two
28 sections are discussed in details in :ref:`BTF_Type_String`.
32 2. BTF Type and String Encoding
33 *******************************
35 The file ``include/uapi/linux/btf.h`` provides high-level definition of how
36 types/strings are encoded.
38 The beginning of data blob must be::
46 /* All offsets are in bytes relative to the end of this header */
47 __u32 type_off; /* offset of type section */
48 __u32 type_len; /* length of type section */
49 __u32 str_off; /* offset of string section */
50 __u32 str_len; /* length of string section */
53 The magic is ``0xeB9F``, which has different encoding for big and little
54 endian systems, and can be used to test whether BTF is generated for big- or
55 little-endian target. The ``btf_header`` is designed to be extensible with
56 ``hdr_len`` equal to ``sizeof(struct btf_header)`` when a data blob is
62 The first string in the string section must be a null string. The rest of
63 string table is a concatenation of other null-terminated strings.
68 The type id ``0`` is reserved for ``void`` type. The type section is parsed
69 sequentially and type id is assigned to each recognized type starting from id
70 ``1``. Currently, the following types are supported::
72 #define BTF_KIND_INT 1 /* Integer */
73 #define BTF_KIND_PTR 2 /* Pointer */
74 #define BTF_KIND_ARRAY 3 /* Array */
75 #define BTF_KIND_STRUCT 4 /* Struct */
76 #define BTF_KIND_UNION 5 /* Union */
77 #define BTF_KIND_ENUM 6 /* Enumeration */
78 #define BTF_KIND_FWD 7 /* Forward */
79 #define BTF_KIND_TYPEDEF 8 /* Typedef */
80 #define BTF_KIND_VOLATILE 9 /* Volatile */
81 #define BTF_KIND_CONST 10 /* Const */
82 #define BTF_KIND_RESTRICT 11 /* Restrict */
83 #define BTF_KIND_FUNC 12 /* Function */
84 #define BTF_KIND_FUNC_PROTO 13 /* Function Proto */
85 #define BTF_KIND_VAR 14 /* Variable */
86 #define BTF_KIND_DATASEC 15 /* Section */
88 Note that the type section encodes debug info, not just pure types.
89 ``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
91 Each type contains the following common data::
95 /* "info" bits arrangement
96 * bits 0-15: vlen (e.g. # of struct's members)
98 * bits 24-27: kind (e.g. int, ptr, array...etc)
100 * bit 31: kind_flag, currently used by
101 * struct, union and fwd
104 /* "size" is used by INT, ENUM, STRUCT and UNION.
105 * "size" tells the size of the type it is describing.
107 * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
108 * FUNC and FUNC_PROTO.
109 * "type" is a type_id referring to another type.
117 For certain kinds, the common data are followed by kind-specific data. The
118 ``name_off`` in ``struct btf_type`` specifies the offset in the string table.
119 The following sections detail encoding of each kind.
124 ``struct btf_type`` encoding requirement:
125 * ``name_off``: any valid offset
126 * ``info.kind_flag``: 0
127 * ``info.kind``: BTF_KIND_INT
129 * ``size``: the size of the int type in bytes.
131 ``btf_type`` is followed by a ``u32`` with the following bits arrangement::
133 #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
134 #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
135 #define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)
137 The ``BTF_INT_ENCODING`` has the following attributes::
139 #define BTF_INT_SIGNED (1 << 0)
140 #define BTF_INT_CHAR (1 << 1)
141 #define BTF_INT_BOOL (1 << 2)
143 The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
144 bool, for the int type. The char and bool encoding are mostly useful for
145 pretty print. At most one encoding can be specified for the int type.
147 The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
148 type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
149 The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
150 for the type. The maximum value of ``BTF_INT_BITS()`` is 128.
152 The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
153 for this int. For example, a bitfield struct member has:
154 * btf member bit offset 100 from the start of the structure,
155 * btf member pointing to an int type,
156 * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4``
158 Then in the struct memory layout, this member will occupy ``4`` bits starting
159 from bits ``100 + 2 = 102``.
161 Alternatively, the bitfield struct member can be the following to access the
162 same bits as the above:
163 * btf member bit offset 102,
164 * btf member pointing to an int type,
165 * the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4``
167 The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of
168 bitfield encoding. Currently, both llvm and pahole generate
169 ``BTF_INT_OFFSET() = 0`` for all int types.
174 ``struct btf_type`` encoding requirement:
176 * ``info.kind_flag``: 0
177 * ``info.kind``: BTF_KIND_PTR
179 * ``type``: the pointee type of the pointer
181 No additional type data follow ``btf_type``.
186 ``struct btf_type`` encoding requirement:
188 * ``info.kind_flag``: 0
189 * ``info.kind``: BTF_KIND_ARRAY
191 * ``size/type``: 0, not used
193 ``btf_type`` is followed by one ``struct btf_array``::
201 The ``struct btf_array`` encoding:
202 * ``type``: the element type
203 * ``index_type``: the index type
204 * ``nelems``: the number of elements for this array (``0`` is also allowed).
206 The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``,
207 ``u64``, ``unsigned __int128``). The original design of including
208 ``index_type`` follows DWARF, which has an ``index_type`` for its array type.
209 Currently in BTF, beyond type verification, the ``index_type`` is not used.
211 The ``struct btf_array`` allows chaining through element type to represent
212 multidimensional arrays. For example, for ``int a[5][6]``, the following type
213 information illustrates the chaining:
216 * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6``
217 * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5``
219 Currently, both pahole and llvm collapse multidimensional array into
220 one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is
221 equal to ``30``. This is because the original use case is map pretty print
222 where the whole array is dumped out so one-dimensional array is enough. As
223 more BTF usage is explored, pahole and llvm can be changed to generate proper
224 chained representation for multidimensional arrays.
226 2.2.4 BTF_KIND_STRUCT
227 ~~~~~~~~~~~~~~~~~~~~~
231 ``struct btf_type`` encoding requirement:
232 * ``name_off``: 0 or offset to a valid C identifier
233 * ``info.kind_flag``: 0 or 1
234 * ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION
235 * ``info.vlen``: the number of struct/union members
236 * ``info.size``: the size of the struct/union in bytes
238 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.::
246 ``struct btf_member`` encoding:
247 * ``name_off``: offset to a valid C identifier
248 * ``type``: the member type
249 * ``offset``: <see below>
251 If the type info ``kind_flag`` is not set, the offset contains only bit offset
252 of the member. Note that the base type of the bitfield can only be int or enum
253 type. If the bitfield size is 32, the base type can be either int or enum
254 type. If the bitfield size is not 32, the base type must be int, and int type
255 ``BTF_INT_BITS()`` encodes the bitfield size.
257 If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member
258 bitfield size and bit offset. The bitfield size and bit offset are calculated
261 #define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24)
262 #define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff)
264 In this case, if the base type is an int type, it must be a regular int type:
266 * ``BTF_INT_OFFSET()`` must be 0.
267 * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``.
269 The following kernel patch introduced ``kind_flag`` and explained why both
272 https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3#diff-fa650a64fdd3968396883d2fe8215ff3
277 ``struct btf_type`` encoding requirement:
278 * ``name_off``: 0 or offset to a valid C identifier
279 * ``info.kind_flag``: 0
280 * ``info.kind``: BTF_KIND_ENUM
281 * ``info.vlen``: number of enum values
284 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.::
291 The ``btf_enum`` encoding:
292 * ``name_off``: offset to a valid C identifier
298 ``struct btf_type`` encoding requirement:
299 * ``name_off``: offset to a valid C identifier
300 * ``info.kind_flag``: 0 for struct, 1 for union
301 * ``info.kind``: BTF_KIND_FWD
305 No additional type data follow ``btf_type``.
307 2.2.8 BTF_KIND_TYPEDEF
308 ~~~~~~~~~~~~~~~~~~~~~~
310 ``struct btf_type`` encoding requirement:
311 * ``name_off``: offset to a valid C identifier
312 * ``info.kind_flag``: 0
313 * ``info.kind``: BTF_KIND_TYPEDEF
315 * ``type``: the type which can be referred by name at ``name_off``
317 No additional type data follow ``btf_type``.
319 2.2.9 BTF_KIND_VOLATILE
320 ~~~~~~~~~~~~~~~~~~~~~~~
322 ``struct btf_type`` encoding requirement:
324 * ``info.kind_flag``: 0
325 * ``info.kind``: BTF_KIND_VOLATILE
327 * ``type``: the type with ``volatile`` qualifier
329 No additional type data follow ``btf_type``.
331 2.2.10 BTF_KIND_CONST
332 ~~~~~~~~~~~~~~~~~~~~~
334 ``struct btf_type`` encoding requirement:
336 * ``info.kind_flag``: 0
337 * ``info.kind``: BTF_KIND_CONST
339 * ``type``: the type with ``const`` qualifier
341 No additional type data follow ``btf_type``.
343 2.2.11 BTF_KIND_RESTRICT
344 ~~~~~~~~~~~~~~~~~~~~~~~~
346 ``struct btf_type`` encoding requirement:
348 * ``info.kind_flag``: 0
349 * ``info.kind``: BTF_KIND_RESTRICT
351 * ``type``: the type with ``restrict`` qualifier
353 No additional type data follow ``btf_type``.
358 ``struct btf_type`` encoding requirement:
359 * ``name_off``: offset to a valid C identifier
360 * ``info.kind_flag``: 0
361 * ``info.kind``: BTF_KIND_FUNC
363 * ``type``: a BTF_KIND_FUNC_PROTO type
365 No additional type data follow ``btf_type``.
367 A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose
368 signature is defined by ``type``. The subprogram is thus an instance of that
369 type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the
370 :ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load`
373 2.2.13 BTF_KIND_FUNC_PROTO
374 ~~~~~~~~~~~~~~~~~~~~~~~~~~
376 ``struct btf_type`` encoding requirement:
378 * ``info.kind_flag``: 0
379 * ``info.kind``: BTF_KIND_FUNC_PROTO
380 * ``info.vlen``: # of parameters
381 * ``type``: the return type
383 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.::
390 If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then
391 ``btf_param.name_off`` must point to a valid C identifier except for the
392 possible last argument representing the variable argument. The btf_param.type
393 refers to parameter type.
395 If the function has variable arguments, the last parameter is encoded with
396 ``name_off = 0`` and ``type = 0``.
401 ``struct btf_type`` encoding requirement:
402 * ``name_off``: offset to a valid C identifier
403 * ``info.kind_flag``: 0
404 * ``info.kind``: BTF_KIND_VAR
406 * ``type``: the type of the variable
408 ``btf_type`` is followed by a single ``struct btf_variable`` with the
415 ``struct btf_var`` encoding:
416 * ``linkage``: currently only static variable 0, or globally allocated
417 variable in ELF sections 1
419 Not all type of global variables are supported by LLVM at this point.
420 The following is currently available:
422 * static variables with or without section attributes
423 * global variables with section attributes
425 The latter is for future extraction of map key/value type id's from a
428 2.2.15 BTF_KIND_DATASEC
429 ~~~~~~~~~~~~~~~~~~~~~~~
431 ``struct btf_type`` encoding requirement:
432 * ``name_off``: offset to a valid name associated with a variable or
433 one of .data/.bss/.rodata
434 * ``info.kind_flag``: 0
435 * ``info.kind``: BTF_KIND_DATASEC
436 * ``info.vlen``: # of variables
437 * ``size``: total section size in bytes (0 at compilation time, patched
438 to actual size by BPF loaders such as libbpf)
440 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.::
442 struct btf_var_secinfo {
448 ``struct btf_var_secinfo`` encoding:
449 * ``type``: the type of the BTF_KIND_VAR variable
450 * ``offset``: the in-section offset of the variable
451 * ``size``: the size of the variable in bytes
456 The following bpf syscall command involves BTF:
457 * BPF_BTF_LOAD: load a blob of BTF data into kernel
458 * BPF_MAP_CREATE: map creation with btf key and value type info.
459 * BPF_PROG_LOAD: prog load with btf function and line info.
460 * BPF_BTF_GET_FD_BY_ID: get a btf fd
461 * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info
462 and other btf related info are returned.
464 The workflow typically looks like:
471 BPF_MAP_CREATE and BPF_PROG_LOAD
478 BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's)
481 BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd)
484 BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id)
487 BPF_BTF_GET_FD_BY_ID (get btf_fd) |
490 BPF_OBJ_GET_INFO_BY_FD (get btf) |
493 pretty print types, dump func signatures and line info, etc.
499 Load a blob of BTF data into kernel. A blob of data, described in
500 :ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd``
501 is returned to a userspace.
506 A map can be created with ``btf_fd`` and specified key/value type id.::
508 __u32 btf_fd; /* fd pointing to a BTF type data */
509 __u32 btf_key_type_id; /* BTF type_id of the key */
510 __u32 btf_value_type_id; /* BTF type_id of the value */
512 In libbpf, the map can be defined with extra annotation like below:
515 struct bpf_map_def SEC("maps") btf_map = {
516 .type = BPF_MAP_TYPE_ARRAY,
517 .key_size = sizeof(int),
518 .value_size = sizeof(struct ipv_counts),
521 BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts);
523 Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name, key and
524 value types for the map. During ELF parsing, libbpf is able to extract
525 key/value type_id's and assign them to BPF_MAP_CREATE attributes
533 During prog_load, func_info and line_info can be passed to kernel with proper
534 values for the following attributes:
540 __u32 prog_btf_fd; /* fd pointing to BTF type data */
541 __u32 func_info_rec_size; /* userspace bpf_func_info size */
542 __aligned_u64 func_info; /* func info */
543 __u32 func_info_cnt; /* number of bpf_func_info records */
544 __u32 line_info_rec_size; /* userspace bpf_line_info size */
545 __aligned_u64 line_info; /* line info */
546 __u32 line_info_cnt; /* number of bpf_line_info records */
548 The func_info and line_info are an array of below, respectively.::
550 struct bpf_func_info {
551 __u32 insn_off; /* [0, insn_cnt - 1] */
552 __u32 type_id; /* pointing to a BTF_KIND_FUNC type */
554 struct bpf_line_info {
555 __u32 insn_off; /* [0, insn_cnt - 1] */
556 __u32 file_name_off; /* offset to string table for the filename */
557 __u32 line_off; /* offset to string table for the source line */
558 __u32 line_col; /* line number and column number */
561 func_info_rec_size is the size of each func_info record, and
562 line_info_rec_size is the size of each line_info record. Passing the record
563 size to kernel make it possible to extend the record itself in the future.
565 Below are requirements for func_info:
566 * func_info[0].insn_off must be 0.
567 * the func_info insn_off is in strictly increasing order and matches
570 Below are requirements for line_info:
571 * the first insn in each func must have a line_info record pointing to it.
572 * the line_info insn_off is in strictly increasing order.
574 For line_info, the line number and column number are defined as below:
577 #define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10)
578 #define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff)
580 3.4 BPF_{PROG,MAP}_GET_NEXT_ID
581 ==============================
583 In kernel, every loaded program, map or btf has a unique id. The id won't
584 change during the lifetime of a program, map, or btf.
586 The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for
587 each command, to user space, for bpf program or maps, respectively, so an
588 inspection tool can inspect all programs and maps.
590 3.5 BPF_{PROG,MAP}_GET_FD_BY_ID
591 ===============================
593 An introspection tool cannot use id to get details about program or maps.
594 A file descriptor needs to be obtained first for reference-counting purpose.
596 3.6 BPF_OBJ_GET_INFO_BY_FD
597 ==========================
599 Once a program/map fd is acquired, an introspection tool can get the detailed
600 information from kernel about this fd, some of which are BTF-related. For
601 example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids.
602 ``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated
603 bpf byte codes, and jited_line_info.
605 3.7 BPF_BTF_GET_FD_BY_ID
606 ========================
608 With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf
609 syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with
610 command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the
611 kernel with BPF_BTF_LOAD, can be retrieved.
613 With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection
614 tool has full btf knowledge and is able to pretty print map key/values, dump
615 func signatures and line info, along with byte/jit codes.
617 4. ELF File Format Interface
618 ****************************
623 The .BTF section contains type and string data. The format of this section is
624 same as the one describe in :ref:`BTF_Type_String`.
631 The .BTF.ext section encodes func_info and line_info which needs loader
632 manipulation before loading into the kernel.
634 The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h``
635 and ``tools/lib/bpf/btf.c``.
637 The current header of .BTF.ext section::
639 struct btf_ext_header {
645 /* All offsets are in bytes relative to the end of this header */
652 It is very similar to .BTF section. Instead of type/string section, it
653 contains func_info and line_info section. See :ref:`BPF_Prog_Load` for details
654 about func_info and line_info record format.
656 The func_info is organized as below.::
659 btf_ext_info_sec for section #1 /* func_info for section #1 */
660 btf_ext_info_sec for section #2 /* func_info for section #2 */
663 ``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when
664 .BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of
665 func_info for each specific ELF section.::
667 struct btf_ext_info_sec {
668 __u32 sec_name_off; /* offset to section name */
670 /* Followed by num_info * record_size number of bytes */
674 Here, num_info must be greater than 0.
676 The line_info is organized as below.::
679 btf_ext_info_sec for section #1 /* line_info for section #1 */
680 btf_ext_info_sec for section #2 /* line_info for section #2 */
683 ``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when
684 .BTF.ext is generated.
686 The interpretation of ``bpf_func_info->insn_off`` and
687 ``bpf_line_info->insn_off`` is different between kernel API and ELF API. For
688 kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
689 bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
690 beginning of section (``btf_ext_info_sec->sec_name_off``).
695 5.1 bpftool map pretty print
696 ============================
698 With BTF, the map key/value can be printed based on fields rather than simply
699 raw bytes. This is especially valuable for large structure or if your data
700 structure has bitfields. For example, for the following map,::
702 enum A { A1, A2, A3, A4, A5 };
713 struct bpf_map_def SEC("maps") tmpmap = {
714 .type = BPF_MAP_TYPE_ARRAY,
715 .key_size = sizeof(__u32),
716 .value_size = sizeof(struct tmp_t),
719 BPF_ANNOTATE_KV_PAIR(tmpmap, int, struct tmp_t);
721 bpftool is able to pretty print like below:
737 5.2 bpftool prog dump
738 =====================
740 The following is an example showing how func_info and line_info can help prog
741 dump with better kernel symbol names, function prototypes and line
744 $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
746 int test_long_fname_2(struct dummy_tracepoint_args * arg):
747 bpf_prog_44a040bf25481309_test_long_fname_2:
748 ; static int test_long_fname_2(struct dummy_tracepoint_args *arg)
753 f: mov %rbx,0x0(%rbp)
754 13: mov %r13,0x8(%rbp)
755 17: mov %r14,0x10(%rbp)
756 1b: mov %r15,0x18(%rbp)
758 21: mov %rax,0x20(%rbp)
761 27: mov %esi,-0x4(%rbp)
763 2a: mov 0x8(%rdi),%rdi
766 32: je 0x0000000000000070
768 ; counts = bpf_map_lookup_elem(&btf_map, &key);
774 The following is an example of how line_info can help debugging verification
777 /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
778 * is modified as below.
780 data = (void *)(long)xdp->data;
781 data_end = (void *)(long)xdp->data_end;
783 if (data + 4 > data_end)
786 *(u32 *)data = dst->dst;
788 $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp
789 ; data = (void *)(long)xdp->data;
790 224: (79) r2 = *(u64 *)(r10 -112)
791 225: (61) r2 = *(u32 *)(r2 +0)
792 ; *(u32 *)data = dst->dst;
793 226: (63) *(u32 *)(r2 +0) = r1
794 invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0)
795 R2 offset is outside of the packet
800 You need latest pahole
802 https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
804 or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't
805 support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,::
813 -bash-4.4$ gcc -c -O2 -g t.c
814 -bash-4.4$ pahole -JV t.o
816 [1] STRUCT t kind_flag=1 size=4 vlen=3
817 a type_id=2 bitfield_size=2 bits_offset=0
818 b type_id=2 bitfield_size=3 bits_offset=2
819 c type_id=2 bitfield_size=2 bits_offset=5
820 [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
822 The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target
823 only. The assembly code (-S) is able to show the BTF encoding in assembly
830 int (*f2)(char q1, __int32 q2, ...);
833 int main() { return 0; }
834 int test() { return 0; }
835 -bash-4.4$ clang -c -g -O2 -target bpf t2.c
836 -bash-4.4$ readelf -S t2.o
838 [ 8] .BTF PROGBITS 0000000000000000 00000247
839 000000000000016e 0000000000000000 0 0 1
840 [ 9] .BTF.ext PROGBITS 0000000000000000 000003b5
841 0000000000000060 0000000000000000 0 0 1
842 [10] .rel.BTF.ext REL 0000000000000000 000007e0
843 0000000000000040 0000000000000010 16 9 8
845 -bash-4.4$ clang -S -g -O2 -target bpf t2.c
848 .section .BTF,"",@progbits
849 .short 60319 # 0xeb9f
857 .long 0 # BTF_KIND_FUNC_PROTO(id = 1)
858 .long 218103808 # 0xd000000
860 .long 83 # BTF_KIND_INT(id = 2)
861 .long 16777216 # 0x1000000
863 .long 16777248 # 0x1000020
865 .byte 0 # string offset=0
866 .ascii ".text" # string offset=1
868 .ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7
870 .ascii "int main() { return 0; }" # string offset=33
872 .ascii "int test() { return 0; }" # string offset=58
874 .ascii "int" # string offset=83
876 .section .BTF.ext,"",@progbits
877 .short 60319 # 0xeb9f
886 .long 1 # FuncInfo section string offset=1
893 .long 1 # LineInfo section string offset=1
898 .long 7182 # Line 7 Col 14
902 .long 8206 # Line 8 Col 14
907 Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests.