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 up to 32-bit values */
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 */
87 #define BTF_KIND_FLOAT 16 /* Floating point */
88 #define BTF_KIND_DECL_TAG 17 /* Decl Tag */
89 #define BTF_KIND_TYPE_TAG 18 /* Type Tag */
90 #define BTF_KIND_ENUM64 19 /* Enumeration up to 64-bit values */
92 Note that the type section encodes debug info, not just pure types.
93 ``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
95 Each type contains the following common data::
99 /* "info" bits arrangement
100 * bits 0-15: vlen (e.g. # of struct's members)
102 * bits 24-28: kind (e.g. int, ptr, array...etc)
104 * bit 31: kind_flag, currently used by
105 * struct, union, fwd, enum and enum64.
108 /* "size" is used by INT, ENUM, STRUCT, UNION and ENUM64.
109 * "size" tells the size of the type it is describing.
111 * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
112 * FUNC, FUNC_PROTO, DECL_TAG and TYPE_TAG.
113 * "type" is a type_id referring to another type.
121 For certain kinds, the common data are followed by kind-specific data. The
122 ``name_off`` in ``struct btf_type`` specifies the offset in the string table.
123 The following sections detail encoding of each kind.
128 ``struct btf_type`` encoding requirement:
129 * ``name_off``: any valid offset
130 * ``info.kind_flag``: 0
131 * ``info.kind``: BTF_KIND_INT
133 * ``size``: the size of the int type in bytes.
135 ``btf_type`` is followed by a ``u32`` with the following bits arrangement::
137 #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
138 #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
139 #define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)
141 The ``BTF_INT_ENCODING`` has the following attributes::
143 #define BTF_INT_SIGNED (1 << 0)
144 #define BTF_INT_CHAR (1 << 1)
145 #define BTF_INT_BOOL (1 << 2)
147 The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
148 bool, for the int type. The char and bool encoding are mostly useful for
149 pretty print. At most one encoding can be specified for the int type.
151 The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
152 type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
153 The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
154 for the type. The maximum value of ``BTF_INT_BITS()`` is 128.
156 The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
157 for this int. For example, a bitfield struct member has:
159 * btf member bit offset 100 from the start of the structure,
160 * btf member pointing to an int type,
161 * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4``
163 Then in the struct memory layout, this member will occupy ``4`` bits starting
164 from bits ``100 + 2 = 102``.
166 Alternatively, the bitfield struct member can be the following to access the
167 same bits as the above:
169 * btf member bit offset 102,
170 * btf member pointing to an int type,
171 * the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4``
173 The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of
174 bitfield encoding. Currently, both llvm and pahole generate
175 ``BTF_INT_OFFSET() = 0`` for all int types.
180 ``struct btf_type`` encoding requirement:
182 * ``info.kind_flag``: 0
183 * ``info.kind``: BTF_KIND_PTR
185 * ``type``: the pointee type of the pointer
187 No additional type data follow ``btf_type``.
192 ``struct btf_type`` encoding requirement:
194 * ``info.kind_flag``: 0
195 * ``info.kind``: BTF_KIND_ARRAY
197 * ``size/type``: 0, not used
199 ``btf_type`` is followed by one ``struct btf_array``::
207 The ``struct btf_array`` encoding:
208 * ``type``: the element type
209 * ``index_type``: the index type
210 * ``nelems``: the number of elements for this array (``0`` is also allowed).
212 The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``,
213 ``u64``, ``unsigned __int128``). The original design of including
214 ``index_type`` follows DWARF, which has an ``index_type`` for its array type.
215 Currently in BTF, beyond type verification, the ``index_type`` is not used.
217 The ``struct btf_array`` allows chaining through element type to represent
218 multidimensional arrays. For example, for ``int a[5][6]``, the following type
219 information illustrates the chaining:
222 * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6``
223 * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5``
225 Currently, both pahole and llvm collapse multidimensional array into
226 one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is
227 equal to ``30``. This is because the original use case is map pretty print
228 where the whole array is dumped out so one-dimensional array is enough. As
229 more BTF usage is explored, pahole and llvm can be changed to generate proper
230 chained representation for multidimensional arrays.
232 2.2.4 BTF_KIND_STRUCT
233 ~~~~~~~~~~~~~~~~~~~~~
237 ``struct btf_type`` encoding requirement:
238 * ``name_off``: 0 or offset to a valid C identifier
239 * ``info.kind_flag``: 0 or 1
240 * ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION
241 * ``info.vlen``: the number of struct/union members
242 * ``info.size``: the size of the struct/union in bytes
244 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.::
252 ``struct btf_member`` encoding:
253 * ``name_off``: offset to a valid C identifier
254 * ``type``: the member type
255 * ``offset``: <see below>
257 If the type info ``kind_flag`` is not set, the offset contains only bit offset
258 of the member. Note that the base type of the bitfield can only be int or enum
259 type. If the bitfield size is 32, the base type can be either int or enum
260 type. If the bitfield size is not 32, the base type must be int, and int type
261 ``BTF_INT_BITS()`` encodes the bitfield size.
263 If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member
264 bitfield size and bit offset. The bitfield size and bit offset are calculated
267 #define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24)
268 #define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff)
270 In this case, if the base type is an int type, it must be a regular int type:
272 * ``BTF_INT_OFFSET()`` must be 0.
273 * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``.
275 Commit 9d5f9f701b18 introduced ``kind_flag`` and explains why both modes
281 ``struct btf_type`` encoding requirement:
282 * ``name_off``: 0 or offset to a valid C identifier
283 * ``info.kind_flag``: 0 for unsigned, 1 for signed
284 * ``info.kind``: BTF_KIND_ENUM
285 * ``info.vlen``: number of enum values
288 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.::
295 The ``btf_enum`` encoding:
296 * ``name_off``: offset to a valid C identifier
299 If the original enum value is signed and the size is less than 4,
300 that value will be sign extended into 4 bytes. If the size is 8,
301 the value will be truncated into 4 bytes.
306 ``struct btf_type`` encoding requirement:
307 * ``name_off``: offset to a valid C identifier
308 * ``info.kind_flag``: 0 for struct, 1 for union
309 * ``info.kind``: BTF_KIND_FWD
313 No additional type data follow ``btf_type``.
315 2.2.8 BTF_KIND_TYPEDEF
316 ~~~~~~~~~~~~~~~~~~~~~~
318 ``struct btf_type`` encoding requirement:
319 * ``name_off``: offset to a valid C identifier
320 * ``info.kind_flag``: 0
321 * ``info.kind``: BTF_KIND_TYPEDEF
323 * ``type``: the type which can be referred by name at ``name_off``
325 No additional type data follow ``btf_type``.
327 2.2.9 BTF_KIND_VOLATILE
328 ~~~~~~~~~~~~~~~~~~~~~~~
330 ``struct btf_type`` encoding requirement:
332 * ``info.kind_flag``: 0
333 * ``info.kind``: BTF_KIND_VOLATILE
335 * ``type``: the type with ``volatile`` qualifier
337 No additional type data follow ``btf_type``.
339 2.2.10 BTF_KIND_CONST
340 ~~~~~~~~~~~~~~~~~~~~~
342 ``struct btf_type`` encoding requirement:
344 * ``info.kind_flag``: 0
345 * ``info.kind``: BTF_KIND_CONST
347 * ``type``: the type with ``const`` qualifier
349 No additional type data follow ``btf_type``.
351 2.2.11 BTF_KIND_RESTRICT
352 ~~~~~~~~~~~~~~~~~~~~~~~~
354 ``struct btf_type`` encoding requirement:
356 * ``info.kind_flag``: 0
357 * ``info.kind``: BTF_KIND_RESTRICT
359 * ``type``: the type with ``restrict`` qualifier
361 No additional type data follow ``btf_type``.
366 ``struct btf_type`` encoding requirement:
367 * ``name_off``: offset to a valid C identifier
368 * ``info.kind_flag``: 0
369 * ``info.kind``: BTF_KIND_FUNC
370 * ``info.vlen``: linkage information (BTF_FUNC_STATIC, BTF_FUNC_GLOBAL
372 * ``type``: a BTF_KIND_FUNC_PROTO type
374 No additional type data follow ``btf_type``.
376 A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose
377 signature is defined by ``type``. The subprogram is thus an instance of that
378 type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the
379 :ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load`
382 Currently, only linkage values of BTF_FUNC_STATIC and BTF_FUNC_GLOBAL are
383 supported in the kernel.
385 2.2.13 BTF_KIND_FUNC_PROTO
386 ~~~~~~~~~~~~~~~~~~~~~~~~~~
388 ``struct btf_type`` encoding requirement:
390 * ``info.kind_flag``: 0
391 * ``info.kind``: BTF_KIND_FUNC_PROTO
392 * ``info.vlen``: # of parameters
393 * ``type``: the return type
395 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.::
402 If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then
403 ``btf_param.name_off`` must point to a valid C identifier except for the
404 possible last argument representing the variable argument. The btf_param.type
405 refers to parameter type.
407 If the function has variable arguments, the last parameter is encoded with
408 ``name_off = 0`` and ``type = 0``.
413 ``struct btf_type`` encoding requirement:
414 * ``name_off``: offset to a valid C identifier
415 * ``info.kind_flag``: 0
416 * ``info.kind``: BTF_KIND_VAR
418 * ``type``: the type of the variable
420 ``btf_type`` is followed by a single ``struct btf_variable`` with the
427 ``struct btf_var`` encoding:
428 * ``linkage``: currently only static variable 0, or globally allocated
429 variable in ELF sections 1
431 Not all type of global variables are supported by LLVM at this point.
432 The following is currently available:
434 * static variables with or without section attributes
435 * global variables with section attributes
437 The latter is for future extraction of map key/value type id's from a
440 2.2.15 BTF_KIND_DATASEC
441 ~~~~~~~~~~~~~~~~~~~~~~~
443 ``struct btf_type`` encoding requirement:
444 * ``name_off``: offset to a valid name associated with a variable or
445 one of .data/.bss/.rodata
446 * ``info.kind_flag``: 0
447 * ``info.kind``: BTF_KIND_DATASEC
448 * ``info.vlen``: # of variables
449 * ``size``: total section size in bytes (0 at compilation time, patched
450 to actual size by BPF loaders such as libbpf)
452 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.::
454 struct btf_var_secinfo {
460 ``struct btf_var_secinfo`` encoding:
461 * ``type``: the type of the BTF_KIND_VAR variable
462 * ``offset``: the in-section offset of the variable
463 * ``size``: the size of the variable in bytes
465 2.2.16 BTF_KIND_FLOAT
466 ~~~~~~~~~~~~~~~~~~~~~
468 ``struct btf_type`` encoding requirement:
469 * ``name_off``: any valid offset
470 * ``info.kind_flag``: 0
471 * ``info.kind``: BTF_KIND_FLOAT
473 * ``size``: the size of the float type in bytes: 2, 4, 8, 12 or 16.
475 No additional type data follow ``btf_type``.
477 2.2.17 BTF_KIND_DECL_TAG
478 ~~~~~~~~~~~~~~~~~~~~~~~~
480 ``struct btf_type`` encoding requirement:
481 * ``name_off``: offset to a non-empty string
482 * ``info.kind_flag``: 0
483 * ``info.kind``: BTF_KIND_DECL_TAG
485 * ``type``: ``struct``, ``union``, ``func``, ``var`` or ``typedef``
487 ``btf_type`` is followed by ``struct btf_decl_tag``.::
489 struct btf_decl_tag {
493 The ``name_off`` encodes btf_decl_tag attribute string.
494 The ``type`` should be ``struct``, ``union``, ``func``, ``var`` or ``typedef``.
495 For ``var`` or ``typedef`` type, ``btf_decl_tag.component_idx`` must be ``-1``.
496 For the other three types, if the btf_decl_tag attribute is
497 applied to the ``struct``, ``union`` or ``func`` itself,
498 ``btf_decl_tag.component_idx`` must be ``-1``. Otherwise,
499 the attribute is applied to a ``struct``/``union`` member or
500 a ``func`` argument, and ``btf_decl_tag.component_idx`` should be a
501 valid index (starting from 0) pointing to a member or an argument.
503 2.2.18 BTF_KIND_TYPE_TAG
504 ~~~~~~~~~~~~~~~~~~~~~~~~
506 ``struct btf_type`` encoding requirement:
507 * ``name_off``: offset to a non-empty string
508 * ``info.kind_flag``: 0
509 * ``info.kind``: BTF_KIND_TYPE_TAG
511 * ``type``: the type with ``btf_type_tag`` attribute
513 Currently, ``BTF_KIND_TYPE_TAG`` is only emitted for pointer types.
514 It has the following btf type chain:
518 -> [const | volatile | restrict | typedef]*
521 Basically, a pointer type points to zero or more
522 type_tag, then zero or more const/volatile/restrict/typedef
523 and finally the base type. The base type is one of
524 int, ptr, array, struct, union, enum, func_proto and float types.
526 2.2.19 BTF_KIND_ENUM64
527 ~~~~~~~~~~~~~~~~~~~~~~
529 ``struct btf_type`` encoding requirement:
530 * ``name_off``: 0 or offset to a valid C identifier
531 * ``info.kind_flag``: 0 for unsigned, 1 for signed
532 * ``info.kind``: BTF_KIND_ENUM64
533 * ``info.vlen``: number of enum values
536 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum64``.::
544 The ``btf_enum64`` encoding:
545 * ``name_off``: offset to a valid C identifier
546 * ``val_lo32``: lower 32-bit value for a 64-bit value
547 * ``val_hi32``: high 32-bit value for a 64-bit value
549 If the original enum value is signed and the size is less than 8,
550 that value will be sign extended into 8 bytes.
555 The following bpf syscall command involves BTF:
556 * BPF_BTF_LOAD: load a blob of BTF data into kernel
557 * BPF_MAP_CREATE: map creation with btf key and value type info.
558 * BPF_PROG_LOAD: prog load with btf function and line info.
559 * BPF_BTF_GET_FD_BY_ID: get a btf fd
560 * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info
561 and other btf related info are returned.
563 The workflow typically looks like:
570 BPF_MAP_CREATE and BPF_PROG_LOAD
577 BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's)
580 BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd)
583 BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id)
586 BPF_BTF_GET_FD_BY_ID (get btf_fd) |
589 BPF_OBJ_GET_INFO_BY_FD (get btf) |
592 pretty print types, dump func signatures and line info, etc.
598 Load a blob of BTF data into kernel. A blob of data, described in
599 :ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd``
600 is returned to a userspace.
605 A map can be created with ``btf_fd`` and specified key/value type id.::
607 __u32 btf_fd; /* fd pointing to a BTF type data */
608 __u32 btf_key_type_id; /* BTF type_id of the key */
609 __u32 btf_value_type_id; /* BTF type_id of the value */
611 In libbpf, the map can be defined with extra annotation like below:
615 __uint(type, BPF_MAP_TYPE_ARRAY);
617 __type(value, struct ipv_counts);
618 __uint(max_entries, 4);
619 } btf_map SEC(".maps");
621 During ELF parsing, libbpf is able to extract key/value type_id's and assign
622 them to BPF_MAP_CREATE attributes automatically.
629 During prog_load, func_info and line_info can be passed to kernel with proper
630 values for the following attributes:
636 __u32 prog_btf_fd; /* fd pointing to BTF type data */
637 __u32 func_info_rec_size; /* userspace bpf_func_info size */
638 __aligned_u64 func_info; /* func info */
639 __u32 func_info_cnt; /* number of bpf_func_info records */
640 __u32 line_info_rec_size; /* userspace bpf_line_info size */
641 __aligned_u64 line_info; /* line info */
642 __u32 line_info_cnt; /* number of bpf_line_info records */
644 The func_info and line_info are an array of below, respectively.::
646 struct bpf_func_info {
647 __u32 insn_off; /* [0, insn_cnt - 1] */
648 __u32 type_id; /* pointing to a BTF_KIND_FUNC type */
650 struct bpf_line_info {
651 __u32 insn_off; /* [0, insn_cnt - 1] */
652 __u32 file_name_off; /* offset to string table for the filename */
653 __u32 line_off; /* offset to string table for the source line */
654 __u32 line_col; /* line number and column number */
657 func_info_rec_size is the size of each func_info record, and
658 line_info_rec_size is the size of each line_info record. Passing the record
659 size to kernel make it possible to extend the record itself in the future.
661 Below are requirements for func_info:
662 * func_info[0].insn_off must be 0.
663 * the func_info insn_off is in strictly increasing order and matches
666 Below are requirements for line_info:
667 * the first insn in each func must have a line_info record pointing to it.
668 * the line_info insn_off is in strictly increasing order.
670 For line_info, the line number and column number are defined as below:
673 #define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10)
674 #define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff)
676 3.4 BPF_{PROG,MAP}_GET_NEXT_ID
677 ------------------------------
679 In kernel, every loaded program, map or btf has a unique id. The id won't
680 change during the lifetime of a program, map, or btf.
682 The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for
683 each command, to user space, for bpf program or maps, respectively, so an
684 inspection tool can inspect all programs and maps.
686 3.5 BPF_{PROG,MAP}_GET_FD_BY_ID
687 -------------------------------
689 An introspection tool cannot use id to get details about program or maps.
690 A file descriptor needs to be obtained first for reference-counting purpose.
692 3.6 BPF_OBJ_GET_INFO_BY_FD
693 --------------------------
695 Once a program/map fd is acquired, an introspection tool can get the detailed
696 information from kernel about this fd, some of which are BTF-related. For
697 example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids.
698 ``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated
699 bpf byte codes, and jited_line_info.
701 3.7 BPF_BTF_GET_FD_BY_ID
702 ------------------------
704 With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf
705 syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with
706 command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the
707 kernel with BPF_BTF_LOAD, can be retrieved.
709 With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection
710 tool has full btf knowledge and is able to pretty print map key/values, dump
711 func signatures and line info, along with byte/jit codes.
713 4. ELF File Format Interface
714 ============================
719 The .BTF section contains type and string data. The format of this section is
720 same as the one describe in :ref:`BTF_Type_String`.
727 The .BTF.ext section encodes func_info, line_info and CO-RE relocations
728 which needs loader manipulation before loading into the kernel.
730 The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h``
731 and ``tools/lib/bpf/btf.c``.
733 The current header of .BTF.ext section::
735 struct btf_ext_header {
741 /* All offsets are in bytes relative to the end of this header */
747 /* optional part of .BTF.ext header */
752 It is very similar to .BTF section. Instead of type/string section, it
753 contains func_info, line_info and core_relo sub-sections.
754 See :ref:`BPF_Prog_Load` for details about func_info and line_info
757 The func_info is organized as below.::
759 func_info_rec_size /* __u32 value */
760 btf_ext_info_sec for section #1 /* func_info for section #1 */
761 btf_ext_info_sec for section #2 /* func_info for section #2 */
764 ``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when
765 .BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of
766 func_info for each specific ELF section.::
768 struct btf_ext_info_sec {
769 __u32 sec_name_off; /* offset to section name */
771 /* Followed by num_info * record_size number of bytes */
775 Here, num_info must be greater than 0.
777 The line_info is organized as below.::
779 line_info_rec_size /* __u32 value */
780 btf_ext_info_sec for section #1 /* line_info for section #1 */
781 btf_ext_info_sec for section #2 /* line_info for section #2 */
784 ``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when
785 .BTF.ext is generated.
787 The interpretation of ``bpf_func_info->insn_off`` and
788 ``bpf_line_info->insn_off`` is different between kernel API and ELF API. For
789 kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
790 bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
791 beginning of section (``btf_ext_info_sec->sec_name_off``).
793 The core_relo is organized as below.::
795 core_relo_rec_size /* __u32 value */
796 btf_ext_info_sec for section #1 /* core_relo for section #1 */
797 btf_ext_info_sec for section #2 /* core_relo for section #2 */
799 ``core_relo_rec_size`` specifies the size of ``bpf_core_relo``
800 structure when .BTF.ext is generated. All ``bpf_core_relo`` structures
801 within a single ``btf_ext_info_sec`` describe relocations applied to
802 section named by ``btf_ext_info_sec->sec_name_off``.
804 See :ref:`Documentation/bpf/llvm_reloc.rst <btf-co-re-relocations>`
805 for more information on CO-RE relocations.
810 The .BTF_ids section encodes BTF ID values that are used within the kernel.
812 This section is created during the kernel compilation with the help of
813 macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can
814 use them to create lists and sets (sorted lists) of BTF ID values.
816 The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values,
817 with following syntax::
823 resulting in following layout in .BTF_ids section::
825 __BTF_ID__type1__name1__1:
827 __BTF_ID__type2__name2__2:
830 The ``u32 list[];`` variable is defined to access the list.
832 The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we
833 want to define unused entry in BTF_ID_LIST, like::
835 BTF_ID_LIST(bpf_skb_output_btf_ids)
836 BTF_ID(struct, sk_buff)
838 BTF_ID(struct, task_struct)
840 The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values
841 and their count, with following syntax::
848 resulting in following layout in .BTF_ids section::
852 __BTF_ID__type1__name1__3:
854 __BTF_ID__type2__name2__4:
857 The ``struct btf_id_set set;`` variable is defined to access the list.
859 The ``typeX`` name can be one of following::
861 struct, union, typedef, func
863 and is used as a filter when resolving the BTF ID value.
865 All the BTF ID lists and sets are compiled in the .BTF_ids section and
866 resolved during the linking phase of kernel build by ``resolve_btfids`` tool.
871 5.1 bpftool map pretty print
872 ----------------------------
874 With BTF, the map key/value can be printed based on fields rather than simply
875 raw bytes. This is especially valuable for large structure or if your data
876 structure has bitfields. For example, for the following map,::
878 enum A { A1, A2, A3, A4, A5 };
890 __uint(type, BPF_MAP_TYPE_ARRAY);
892 __type(value, struct tmp_t);
893 __uint(max_entries, 1);
894 } tmpmap SEC(".maps");
896 bpftool is able to pretty print like below:
912 5.2 bpftool prog dump
913 ---------------------
915 The following is an example showing how func_info and line_info can help prog
916 dump with better kernel symbol names, function prototypes and line
919 $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
921 int test_long_fname_2(struct dummy_tracepoint_args * arg):
922 bpf_prog_44a040bf25481309_test_long_fname_2:
923 ; static int test_long_fname_2(struct dummy_tracepoint_args *arg)
928 f: mov %rbx,0x0(%rbp)
929 13: mov %r13,0x8(%rbp)
930 17: mov %r14,0x10(%rbp)
931 1b: mov %r15,0x18(%rbp)
933 21: mov %rax,0x20(%rbp)
936 27: mov %esi,-0x4(%rbp)
938 2a: mov 0x8(%rdi),%rdi
941 32: je 0x0000000000000070
943 ; counts = bpf_map_lookup_elem(&btf_map, &key);
949 The following is an example of how line_info can help debugging verification
952 /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
953 * is modified as below.
955 data = (void *)(long)xdp->data;
956 data_end = (void *)(long)xdp->data_end;
958 if (data + 4 > data_end)
961 *(u32 *)data = dst->dst;
963 $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp
964 ; data = (void *)(long)xdp->data;
965 224: (79) r2 = *(u64 *)(r10 -112)
966 225: (61) r2 = *(u32 *)(r2 +0)
967 ; *(u32 *)data = dst->dst;
968 226: (63) *(u32 *)(r2 +0) = r1
969 invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0)
970 R2 offset is outside of the packet
975 You need latest pahole
977 https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
979 or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't
980 support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,::
988 -bash-4.4$ gcc -c -O2 -g t.c
989 -bash-4.4$ pahole -JV t.o
991 [1] STRUCT t kind_flag=1 size=4 vlen=3
992 a type_id=2 bitfield_size=2 bits_offset=0
993 b type_id=2 bitfield_size=3 bits_offset=2
994 c type_id=2 bitfield_size=2 bits_offset=5
995 [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
997 The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target
998 only. The assembly code (-S) is able to show the BTF encoding in assembly
1002 typedef int __int32;
1005 int (*f2)(char q1, __int32 q2, ...);
1008 int main() { return 0; }
1009 int test() { return 0; }
1010 -bash-4.4$ clang -c -g -O2 --target=bpf t2.c
1011 -bash-4.4$ readelf -S t2.o
1013 [ 8] .BTF PROGBITS 0000000000000000 00000247
1014 000000000000016e 0000000000000000 0 0 1
1015 [ 9] .BTF.ext PROGBITS 0000000000000000 000003b5
1016 0000000000000060 0000000000000000 0 0 1
1017 [10] .rel.BTF.ext REL 0000000000000000 000007e0
1018 0000000000000040 0000000000000010 16 9 8
1020 -bash-4.4$ clang -S -g -O2 --target=bpf t2.c
1023 .section .BTF,"",@progbits
1024 .short 60319 # 0xeb9f
1032 .long 0 # BTF_KIND_FUNC_PROTO(id = 1)
1033 .long 218103808 # 0xd000000
1035 .long 83 # BTF_KIND_INT(id = 2)
1036 .long 16777216 # 0x1000000
1038 .long 16777248 # 0x1000020
1040 .byte 0 # string offset=0
1041 .ascii ".text" # string offset=1
1043 .ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7
1045 .ascii "int main() { return 0; }" # string offset=33
1047 .ascii "int test() { return 0; }" # string offset=58
1049 .ascii "int" # string offset=83
1051 .section .BTF.ext,"",@progbits
1052 .short 60319 # 0xeb9f
1061 .long 1 # FuncInfo section string offset=1
1068 .long 1 # LineInfo section string offset=1
1073 .long 7182 # Line 7 Col 14
1077 .long 8206 # Line 8 Col 14
1082 The kernel BPF selftest `tools/testing/selftests/bpf/prog_tests/btf.c`_
1083 provides an extensive set of BTF-related tests.
1086 .. _tools/testing/selftests/bpf/prog_tests/btf.c:
1087 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/btf.c