xref: /linux/Documentation/bpf/btf.rst (revision c532de5a67a70f8533d495f8f2aaa9a0491c3ad0)
1=====================
2BPF Type Format (BTF)
3=====================
4
51. Introduction
6===============
7
8BTF (BPF Type Format) is the metadata format which encodes the debug info
9related to BPF program/map. The name BTF was used initially to describe data
10types. The BTF was later extended to include function info for defined
11subroutines, and line info for source/line information.
12
13The debug info is used for map pretty print, function signature, etc. The
14function signature enables better bpf program/function kernel symbol. The line
15info helps generate source annotated translated byte code, jited code and
16verifier log.
17
18The BTF specification contains two parts,
19  * BTF kernel API
20  * BTF ELF file format
21
22The kernel API is the contract between user space and kernel. The kernel
23verifies the BTF info before using it. The ELF file format is a user space
24contract between ELF file and libbpf loader.
25
26The type and string sections are part of the BTF kernel API, describing the
27debug info (mostly types related) referenced by the bpf program. These two
28sections are discussed in details in :ref:`BTF_Type_String`.
29
30.. _BTF_Type_String:
31
322. BTF Type and String Encoding
33===============================
34
35The file ``include/uapi/linux/btf.h`` provides high-level definition of how
36types/strings are encoded.
37
38The beginning of data blob must be::
39
40    struct btf_header {
41        __u16   magic;
42        __u8    version;
43        __u8    flags;
44        __u32   hdr_len;
45
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     */
51    };
52
53The magic is ``0xeB9F``, which has different encoding for big and little
54endian systems, and can be used to test whether BTF is generated for big- or
55little-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
57generated.
58
592.1 String Encoding
60-------------------
61
62The first string in the string section must be a null string. The rest of
63string table is a concatenation of other null-terminated strings.
64
652.2 Type Encoding
66-----------------
67
68The type id ``0`` is reserved for ``void`` type. The type section is parsed
69sequentially and type id is assigned to each recognized type starting from id
70``1``. Currently, the following types are supported::
71
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 */
91
92Note 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.
94
95Each type contains the following common data::
96
97    struct btf_type {
98        __u32 name_off;
99        /* "info" bits arrangement
100         * bits  0-15: vlen (e.g. # of struct's members)
101         * bits 16-23: unused
102         * bits 24-28: kind (e.g. int, ptr, array...etc)
103         * bits 29-30: unused
104         * bit     31: kind_flag, currently used by
105         *             struct, union, fwd, enum and enum64.
106         */
107        __u32 info;
108        /* "size" is used by INT, ENUM, STRUCT, UNION and ENUM64.
109         * "size" tells the size of the type it is describing.
110         *
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.
114         */
115        union {
116                __u32 size;
117                __u32 type;
118        };
119    };
120
121For 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.
123The following sections detail encoding of each kind.
124
1252.2.1 BTF_KIND_INT
126~~~~~~~~~~~~~~~~~~
127
128``struct btf_type`` encoding requirement:
129 * ``name_off``: any valid offset
130 * ``info.kind_flag``: 0
131 * ``info.kind``: BTF_KIND_INT
132 * ``info.vlen``: 0
133 * ``size``: the size of the int type in bytes.
134
135``btf_type`` is followed by a ``u32`` with the following bits arrangement::
136
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)
140
141The ``BTF_INT_ENCODING`` has the following attributes::
142
143  #define BTF_INT_SIGNED  (1 << 0)
144  #define BTF_INT_CHAR    (1 << 1)
145  #define BTF_INT_BOOL    (1 << 2)
146
147The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
148bool, for the int type. The char and bool encoding are mostly useful for
149pretty print. At most one encoding can be specified for the int type.
150
151The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
152type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
153The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
154for the type. The maximum value of ``BTF_INT_BITS()`` is 128.
155
156The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
157for this int. For example, a bitfield struct member has:
158
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``
162
163Then in the struct memory layout, this member will occupy ``4`` bits starting
164from bits ``100 + 2 = 102``.
165
166Alternatively, the bitfield struct member can be the following to access the
167same bits as the above:
168
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``
172
173The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of
174bitfield encoding. Currently, both llvm and pahole generate
175``BTF_INT_OFFSET() = 0`` for all int types.
176
1772.2.2 BTF_KIND_PTR
178~~~~~~~~~~~~~~~~~~
179
180``struct btf_type`` encoding requirement:
181  * ``name_off``: 0
182  * ``info.kind_flag``: 0
183  * ``info.kind``: BTF_KIND_PTR
184  * ``info.vlen``: 0
185  * ``type``: the pointee type of the pointer
186
187No additional type data follow ``btf_type``.
188
1892.2.3 BTF_KIND_ARRAY
190~~~~~~~~~~~~~~~~~~~~
191
192``struct btf_type`` encoding requirement:
193  * ``name_off``: 0
194  * ``info.kind_flag``: 0
195  * ``info.kind``: BTF_KIND_ARRAY
196  * ``info.vlen``: 0
197  * ``size/type``: 0, not used
198
199``btf_type`` is followed by one ``struct btf_array``::
200
201    struct btf_array {
202        __u32   type;
203        __u32   index_type;
204        __u32   nelems;
205    };
206
207The ``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).
211
212The ``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.
215Currently in BTF, beyond type verification, the ``index_type`` is not used.
216
217The ``struct btf_array`` allows chaining through element type to represent
218multidimensional arrays. For example, for ``int a[5][6]``, the following type
219information illustrates the chaining:
220
221  * [1]: int
222  * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6``
223  * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5``
224
225Currently, both pahole and llvm collapse multidimensional array into
226one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is
227equal to ``30``. This is because the original use case is map pretty print
228where the whole array is dumped out so one-dimensional array is enough. As
229more BTF usage is explored, pahole and llvm can be changed to generate proper
230chained representation for multidimensional arrays.
231
2322.2.4 BTF_KIND_STRUCT
233~~~~~~~~~~~~~~~~~~~~~
2342.2.5 BTF_KIND_UNION
235~~~~~~~~~~~~~~~~~~~~
236
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
243
244``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.::
245
246    struct btf_member {
247        __u32   name_off;
248        __u32   type;
249        __u32   offset;
250    };
251
252``struct btf_member`` encoding:
253  * ``name_off``: offset to a valid C identifier
254  * ``type``: the member type
255  * ``offset``: <see below>
256
257If the type info ``kind_flag`` is not set, the offset contains only bit offset
258of the member. Note that the base type of the bitfield can only be int or enum
259type. If the bitfield size is 32, the base type can be either int or enum
260type. If the bitfield size is not 32, the base type must be int, and int type
261``BTF_INT_BITS()`` encodes the bitfield size.
262
263If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member
264bitfield size and bit offset. The bitfield size and bit offset are calculated
265as below.::
266
267  #define BTF_MEMBER_BITFIELD_SIZE(val)   ((val) >> 24)
268  #define BTF_MEMBER_BIT_OFFSET(val)      ((val) & 0xffffff)
269
270In this case, if the base type is an int type, it must be a regular int type:
271
272  * ``BTF_INT_OFFSET()`` must be 0.
273  * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``.
274
275Commit 9d5f9f701b18 introduced ``kind_flag`` and explains why both modes
276exist.
277
2782.2.6 BTF_KIND_ENUM
279~~~~~~~~~~~~~~~~~~~
280
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
286  * ``size``: 1/2/4/8
287
288``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.::
289
290    struct btf_enum {
291        __u32   name_off;
292        __s32   val;
293    };
294
295The ``btf_enum`` encoding:
296  * ``name_off``: offset to a valid C identifier
297  * ``val``: any value
298
299If the original enum value is signed and the size is less than 4,
300that value will be sign extended into 4 bytes. If the size is 8,
301the value will be truncated into 4 bytes.
302
3032.2.7 BTF_KIND_FWD
304~~~~~~~~~~~~~~~~~~
305
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
310  * ``info.vlen``: 0
311  * ``type``: 0
312
313No additional type data follow ``btf_type``.
314
3152.2.8 BTF_KIND_TYPEDEF
316~~~~~~~~~~~~~~~~~~~~~~
317
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
322  * ``info.vlen``: 0
323  * ``type``: the type which can be referred by name at ``name_off``
324
325No additional type data follow ``btf_type``.
326
3272.2.9 BTF_KIND_VOLATILE
328~~~~~~~~~~~~~~~~~~~~~~~
329
330``struct btf_type`` encoding requirement:
331  * ``name_off``: 0
332  * ``info.kind_flag``: 0
333  * ``info.kind``: BTF_KIND_VOLATILE
334  * ``info.vlen``: 0
335  * ``type``: the type with ``volatile`` qualifier
336
337No additional type data follow ``btf_type``.
338
3392.2.10 BTF_KIND_CONST
340~~~~~~~~~~~~~~~~~~~~~
341
342``struct btf_type`` encoding requirement:
343  * ``name_off``: 0
344  * ``info.kind_flag``: 0
345  * ``info.kind``: BTF_KIND_CONST
346  * ``info.vlen``: 0
347  * ``type``: the type with ``const`` qualifier
348
349No additional type data follow ``btf_type``.
350
3512.2.11 BTF_KIND_RESTRICT
352~~~~~~~~~~~~~~~~~~~~~~~~
353
354``struct btf_type`` encoding requirement:
355  * ``name_off``: 0
356  * ``info.kind_flag``: 0
357  * ``info.kind``: BTF_KIND_RESTRICT
358  * ``info.vlen``: 0
359  * ``type``: the type with ``restrict`` qualifier
360
361No additional type data follow ``btf_type``.
362
3632.2.12 BTF_KIND_FUNC
364~~~~~~~~~~~~~~~~~~~~
365
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
371                   or BTF_FUNC_EXTERN - see :ref:`BTF_Function_Linkage_Constants`)
372  * ``type``: a BTF_KIND_FUNC_PROTO type
373
374No additional type data follow ``btf_type``.
375
376A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose
377signature is defined by ``type``. The subprogram is thus an instance of that
378type. 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`
380(ABI).
381
382Currently, only linkage values of BTF_FUNC_STATIC and BTF_FUNC_GLOBAL are
383supported in the kernel.
384
3852.2.13 BTF_KIND_FUNC_PROTO
386~~~~~~~~~~~~~~~~~~~~~~~~~~
387
388``struct btf_type`` encoding requirement:
389  * ``name_off``: 0
390  * ``info.kind_flag``: 0
391  * ``info.kind``: BTF_KIND_FUNC_PROTO
392  * ``info.vlen``: # of parameters
393  * ``type``: the return type
394
395``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.::
396
397    struct btf_param {
398        __u32   name_off;
399        __u32   type;
400    };
401
402If 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
404possible last argument representing the variable argument. The btf_param.type
405refers to parameter type.
406
407If the function has variable arguments, the last parameter is encoded with
408``name_off = 0`` and ``type = 0``.
409
4102.2.14 BTF_KIND_VAR
411~~~~~~~~~~~~~~~~~~~
412
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
417  * ``info.vlen``: 0
418  * ``type``: the type of the variable
419
420``btf_type`` is followed by a single ``struct btf_variable`` with the
421following data::
422
423    struct btf_var {
424        __u32   linkage;
425    };
426
427``btf_var.linkage`` may take the values: BTF_VAR_STATIC, BTF_VAR_GLOBAL_ALLOCATED or BTF_VAR_GLOBAL_EXTERN -
428see :ref:`BTF_Var_Linkage_Constants`.
429
430Not all type of global variables are supported by LLVM at this point.
431The following is currently available:
432
433  * static variables with or without section attributes
434  * global variables with section attributes
435
436The latter is for future extraction of map key/value type id's from a
437map definition.
438
4392.2.15 BTF_KIND_DATASEC
440~~~~~~~~~~~~~~~~~~~~~~~
441
442``struct btf_type`` encoding requirement:
443  * ``name_off``: offset to a valid name associated with a variable or
444                  one of .data/.bss/.rodata
445  * ``info.kind_flag``: 0
446  * ``info.kind``: BTF_KIND_DATASEC
447  * ``info.vlen``: # of variables
448  * ``size``: total section size in bytes (0 at compilation time, patched
449              to actual size by BPF loaders such as libbpf)
450
451``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.::
452
453    struct btf_var_secinfo {
454        __u32   type;
455        __u32   offset;
456        __u32   size;
457    };
458
459``struct btf_var_secinfo`` encoding:
460  * ``type``: the type of the BTF_KIND_VAR variable
461  * ``offset``: the in-section offset of the variable
462  * ``size``: the size of the variable in bytes
463
4642.2.16 BTF_KIND_FLOAT
465~~~~~~~~~~~~~~~~~~~~~
466
467``struct btf_type`` encoding requirement:
468 * ``name_off``: any valid offset
469 * ``info.kind_flag``: 0
470 * ``info.kind``: BTF_KIND_FLOAT
471 * ``info.vlen``: 0
472 * ``size``: the size of the float type in bytes: 2, 4, 8, 12 or 16.
473
474No additional type data follow ``btf_type``.
475
4762.2.17 BTF_KIND_DECL_TAG
477~~~~~~~~~~~~~~~~~~~~~~~~
478
479``struct btf_type`` encoding requirement:
480 * ``name_off``: offset to a non-empty string
481 * ``info.kind_flag``: 0
482 * ``info.kind``: BTF_KIND_DECL_TAG
483 * ``info.vlen``: 0
484 * ``type``: ``struct``, ``union``, ``func``, ``var`` or ``typedef``
485
486``btf_type`` is followed by ``struct btf_decl_tag``.::
487
488    struct btf_decl_tag {
489        __u32   component_idx;
490    };
491
492The ``name_off`` encodes btf_decl_tag attribute string.
493The ``type`` should be ``struct``, ``union``, ``func``, ``var`` or ``typedef``.
494For ``var`` or ``typedef`` type, ``btf_decl_tag.component_idx`` must be ``-1``.
495For the other three types, if the btf_decl_tag attribute is
496applied to the ``struct``, ``union`` or ``func`` itself,
497``btf_decl_tag.component_idx`` must be ``-1``. Otherwise,
498the attribute is applied to a ``struct``/``union`` member or
499a ``func`` argument, and ``btf_decl_tag.component_idx`` should be a
500valid index (starting from 0) pointing to a member or an argument.
501
5022.2.18 BTF_KIND_TYPE_TAG
503~~~~~~~~~~~~~~~~~~~~~~~~
504
505``struct btf_type`` encoding requirement:
506 * ``name_off``: offset to a non-empty string
507 * ``info.kind_flag``: 0
508 * ``info.kind``: BTF_KIND_TYPE_TAG
509 * ``info.vlen``: 0
510 * ``type``: the type with ``btf_type_tag`` attribute
511
512Currently, ``BTF_KIND_TYPE_TAG`` is only emitted for pointer types.
513It has the following btf type chain:
514::
515
516  ptr -> [type_tag]*
517      -> [const | volatile | restrict | typedef]*
518      -> base_type
519
520Basically, a pointer type points to zero or more
521type_tag, then zero or more const/volatile/restrict/typedef
522and finally the base type. The base type is one of
523int, ptr, array, struct, union, enum, func_proto and float types.
524
5252.2.19 BTF_KIND_ENUM64
526~~~~~~~~~~~~~~~~~~~~~~
527
528``struct btf_type`` encoding requirement:
529  * ``name_off``: 0 or offset to a valid C identifier
530  * ``info.kind_flag``: 0 for unsigned, 1 for signed
531  * ``info.kind``: BTF_KIND_ENUM64
532  * ``info.vlen``: number of enum values
533  * ``size``: 1/2/4/8
534
535``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum64``.::
536
537    struct btf_enum64 {
538        __u32   name_off;
539        __u32   val_lo32;
540        __u32   val_hi32;
541    };
542
543The ``btf_enum64`` encoding:
544  * ``name_off``: offset to a valid C identifier
545  * ``val_lo32``: lower 32-bit value for a 64-bit value
546  * ``val_hi32``: high 32-bit value for a 64-bit value
547
548If the original enum value is signed and the size is less than 8,
549that value will be sign extended into 8 bytes.
550
5512.3 Constant Values
552-------------------
553
554.. _BTF_Function_Linkage_Constants:
555
5562.3.1 Function Linkage Constant Values
557~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
558.. table:: Function Linkage Values and Meanings
559
560  ===================  =====  ===========
561  kind                 value  description
562  ===================  =====  ===========
563  ``BTF_FUNC_STATIC``  0x0    definition of subprogram not visible outside containing compilation unit
564  ``BTF_FUNC_GLOBAL``  0x1    definition of subprogram visible outside containing compilation unit
565  ``BTF_FUNC_EXTERN``  0x2    declaration of a subprogram whose definition is outside the containing compilation unit
566  ===================  =====  ===========
567
568
569.. _BTF_Var_Linkage_Constants:
570
5712.3.2 Variable Linkage Constant Values
572~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
573.. table:: Variable Linkage Values and Meanings
574
575  ============================  =====  ===========
576  kind                          value  description
577  ============================  =====  ===========
578  ``BTF_VAR_STATIC``            0x0    definition of global variable not visible outside containing compilation unit
579  ``BTF_VAR_GLOBAL_ALLOCATED``  0x1    definition of global variable visible outside containing compilation unit
580  ``BTF_VAR_GLOBAL_EXTERN``     0x2    declaration of global variable whose definition is outside the containing compilation unit
581  ============================  =====  ===========
582
5833. BTF Kernel API
584=================
585
586The following bpf syscall command involves BTF:
587   * BPF_BTF_LOAD: load a blob of BTF data into kernel
588   * BPF_MAP_CREATE: map creation with btf key and value type info.
589   * BPF_PROG_LOAD: prog load with btf function and line info.
590   * BPF_BTF_GET_FD_BY_ID: get a btf fd
591   * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info
592     and other btf related info are returned.
593
594The workflow typically looks like:
595::
596
597  Application:
598      BPF_BTF_LOAD
599          |
600          v
601      BPF_MAP_CREATE and BPF_PROG_LOAD
602          |
603          V
604      ......
605
606  Introspection tool:
607      ......
608      BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's)
609          |
610          V
611      BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd)
612          |
613          V
614      BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id)
615          |                                     |
616          V                                     |
617      BPF_BTF_GET_FD_BY_ID (get btf_fd)         |
618          |                                     |
619          V                                     |
620      BPF_OBJ_GET_INFO_BY_FD (get btf)          |
621          |                                     |
622          V                                     V
623      pretty print types, dump func signatures and line info, etc.
624
625
6263.1 BPF_BTF_LOAD
627----------------
628
629Load a blob of BTF data into kernel. A blob of data, described in
630:ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd``
631is returned to a userspace.
632
6333.2 BPF_MAP_CREATE
634------------------
635
636A map can be created with ``btf_fd`` and specified key/value type id.::
637
638    __u32   btf_fd;         /* fd pointing to a BTF type data */
639    __u32   btf_key_type_id;        /* BTF type_id of the key */
640    __u32   btf_value_type_id;      /* BTF type_id of the value */
641
642In libbpf, the map can be defined with extra annotation like below:
643::
644
645    struct {
646        __uint(type, BPF_MAP_TYPE_ARRAY);
647        __type(key, int);
648        __type(value, struct ipv_counts);
649        __uint(max_entries, 4);
650    } btf_map SEC(".maps");
651
652During ELF parsing, libbpf is able to extract key/value type_id's and assign
653them to BPF_MAP_CREATE attributes automatically.
654
655.. _BPF_Prog_Load:
656
6573.3 BPF_PROG_LOAD
658-----------------
659
660During prog_load, func_info and line_info can be passed to kernel with proper
661values for the following attributes:
662::
663
664    __u32           insn_cnt;
665    __aligned_u64   insns;
666    ......
667    __u32           prog_btf_fd;    /* fd pointing to BTF type data */
668    __u32           func_info_rec_size;     /* userspace bpf_func_info size */
669    __aligned_u64   func_info;      /* func info */
670    __u32           func_info_cnt;  /* number of bpf_func_info records */
671    __u32           line_info_rec_size;     /* userspace bpf_line_info size */
672    __aligned_u64   line_info;      /* line info */
673    __u32           line_info_cnt;  /* number of bpf_line_info records */
674
675The func_info and line_info are an array of below, respectively.::
676
677    struct bpf_func_info {
678        __u32   insn_off; /* [0, insn_cnt - 1] */
679        __u32   type_id;  /* pointing to a BTF_KIND_FUNC type */
680    };
681    struct bpf_line_info {
682        __u32   insn_off; /* [0, insn_cnt - 1] */
683        __u32   file_name_off; /* offset to string table for the filename */
684        __u32   line_off; /* offset to string table for the source line */
685        __u32   line_col; /* line number and column number */
686    };
687
688func_info_rec_size is the size of each func_info record, and
689line_info_rec_size is the size of each line_info record. Passing the record
690size to kernel make it possible to extend the record itself in the future.
691
692Below are requirements for func_info:
693  * func_info[0].insn_off must be 0.
694  * the func_info insn_off is in strictly increasing order and matches
695    bpf func boundaries.
696
697Below are requirements for line_info:
698  * the first insn in each func must have a line_info record pointing to it.
699  * the line_info insn_off is in strictly increasing order.
700
701For line_info, the line number and column number are defined as below:
702::
703
704    #define BPF_LINE_INFO_LINE_NUM(line_col)        ((line_col) >> 10)
705    #define BPF_LINE_INFO_LINE_COL(line_col)        ((line_col) & 0x3ff)
706
7073.4 BPF_{PROG,MAP}_GET_NEXT_ID
708------------------------------
709
710In kernel, every loaded program, map or btf has a unique id. The id won't
711change during the lifetime of a program, map, or btf.
712
713The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for
714each command, to user space, for bpf program or maps, respectively, so an
715inspection tool can inspect all programs and maps.
716
7173.5 BPF_{PROG,MAP}_GET_FD_BY_ID
718-------------------------------
719
720An introspection tool cannot use id to get details about program or maps.
721A file descriptor needs to be obtained first for reference-counting purpose.
722
7233.6 BPF_OBJ_GET_INFO_BY_FD
724--------------------------
725
726Once a program/map fd is acquired, an introspection tool can get the detailed
727information from kernel about this fd, some of which are BTF-related. For
728example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids.
729``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated
730bpf byte codes, and jited_line_info.
731
7323.7 BPF_BTF_GET_FD_BY_ID
733------------------------
734
735With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf
736syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with
737command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the
738kernel with BPF_BTF_LOAD, can be retrieved.
739
740With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection
741tool has full btf knowledge and is able to pretty print map key/values, dump
742func signatures and line info, along with byte/jit codes.
743
7444. ELF File Format Interface
745============================
746
7474.1 .BTF section
748----------------
749
750The .BTF section contains type and string data. The format of this section is
751same as the one describe in :ref:`BTF_Type_String`.
752
753.. _BTF_Ext_Section:
754
7554.2 .BTF.ext section
756--------------------
757
758The .BTF.ext section encodes func_info, line_info and CO-RE relocations
759which needs loader manipulation before loading into the kernel.
760
761The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h``
762and ``tools/lib/bpf/btf.c``.
763
764The current header of .BTF.ext section::
765
766    struct btf_ext_header {
767        __u16   magic;
768        __u8    version;
769        __u8    flags;
770        __u32   hdr_len;
771
772        /* All offsets are in bytes relative to the end of this header */
773        __u32   func_info_off;
774        __u32   func_info_len;
775        __u32   line_info_off;
776        __u32   line_info_len;
777
778        /* optional part of .BTF.ext header */
779        __u32   core_relo_off;
780        __u32   core_relo_len;
781    };
782
783It is very similar to .BTF section. Instead of type/string section, it
784contains func_info, line_info and core_relo sub-sections.
785See :ref:`BPF_Prog_Load` for details about func_info and line_info
786record format.
787
788The func_info is organized as below.::
789
790     func_info_rec_size              /* __u32 value */
791     btf_ext_info_sec for section #1 /* func_info for section #1 */
792     btf_ext_info_sec for section #2 /* func_info for section #2 */
793     ...
794
795``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when
796.BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of
797func_info for each specific ELF section.::
798
799     struct btf_ext_info_sec {
800        __u32   sec_name_off; /* offset to section name */
801        __u32   num_info;
802        /* Followed by num_info * record_size number of bytes */
803        __u8    data[0];
804     };
805
806Here, num_info must be greater than 0.
807
808The line_info is organized as below.::
809
810     line_info_rec_size              /* __u32 value */
811     btf_ext_info_sec for section #1 /* line_info for section #1 */
812     btf_ext_info_sec for section #2 /* line_info for section #2 */
813     ...
814
815``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when
816.BTF.ext is generated.
817
818The interpretation of ``bpf_func_info->insn_off`` and
819``bpf_line_info->insn_off`` is different between kernel API and ELF API. For
820kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
821bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
822beginning of section (``btf_ext_info_sec->sec_name_off``).
823
824The core_relo is organized as below.::
825
826     core_relo_rec_size              /* __u32 value */
827     btf_ext_info_sec for section #1 /* core_relo for section #1 */
828     btf_ext_info_sec for section #2 /* core_relo for section #2 */
829
830``core_relo_rec_size`` specifies the size of ``bpf_core_relo``
831structure when .BTF.ext is generated. All ``bpf_core_relo`` structures
832within a single ``btf_ext_info_sec`` describe relocations applied to
833section named by ``btf_ext_info_sec->sec_name_off``.
834
835See :ref:`Documentation/bpf/llvm_reloc.rst <btf-co-re-relocations>`
836for more information on CO-RE relocations.
837
8384.2 .BTF_ids section
839--------------------
840
841The .BTF_ids section encodes BTF ID values that are used within the kernel.
842
843This section is created during the kernel compilation with the help of
844macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can
845use them to create lists and sets (sorted lists) of BTF ID values.
846
847The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values,
848with following syntax::
849
850  BTF_ID_LIST(list)
851  BTF_ID(type1, name1)
852  BTF_ID(type2, name2)
853
854resulting in following layout in .BTF_ids section::
855
856  __BTF_ID__type1__name1__1:
857  .zero 4
858  __BTF_ID__type2__name2__2:
859  .zero 4
860
861The ``u32 list[];`` variable is defined to access the list.
862
863The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we
864want to define unused entry in BTF_ID_LIST, like::
865
866      BTF_ID_LIST(bpf_skb_output_btf_ids)
867      BTF_ID(struct, sk_buff)
868      BTF_ID_UNUSED
869      BTF_ID(struct, task_struct)
870
871The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values
872and their count, with following syntax::
873
874  BTF_SET_START(set)
875  BTF_ID(type1, name1)
876  BTF_ID(type2, name2)
877  BTF_SET_END(set)
878
879resulting in following layout in .BTF_ids section::
880
881  __BTF_ID__set__set:
882  .zero 4
883  __BTF_ID__type1__name1__3:
884  .zero 4
885  __BTF_ID__type2__name2__4:
886  .zero 4
887
888The ``struct btf_id_set set;`` variable is defined to access the list.
889
890The ``typeX`` name can be one of following::
891
892   struct, union, typedef, func
893
894and is used as a filter when resolving the BTF ID value.
895
896All the BTF ID lists and sets are compiled in the .BTF_ids section and
897resolved during the linking phase of kernel build by ``resolve_btfids`` tool.
898
8995. Using BTF
900============
901
9025.1 bpftool map pretty print
903----------------------------
904
905With BTF, the map key/value can be printed based on fields rather than simply
906raw bytes. This is especially valuable for large structure or if your data
907structure has bitfields. For example, for the following map,::
908
909      enum A { A1, A2, A3, A4, A5 };
910      typedef enum A ___A;
911      struct tmp_t {
912           char a1:4;
913           int  a2:4;
914           int  :4;
915           __u32 a3:4;
916           int b;
917           ___A b1:4;
918           enum A b2:4;
919      };
920      struct {
921           __uint(type, BPF_MAP_TYPE_ARRAY);
922           __type(key, int);
923           __type(value, struct tmp_t);
924           __uint(max_entries, 1);
925      } tmpmap SEC(".maps");
926
927bpftool is able to pretty print like below:
928::
929
930      [{
931            "key": 0,
932            "value": {
933                "a1": 0x2,
934                "a2": 0x4,
935                "a3": 0x6,
936                "b": 7,
937                "b1": 0x8,
938                "b2": 0xa
939            }
940        }
941      ]
942
9435.2 bpftool prog dump
944---------------------
945
946The following is an example showing how func_info and line_info can help prog
947dump with better kernel symbol names, function prototypes and line
948information.::
949
950    $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
951    [...]
952    int test_long_fname_2(struct dummy_tracepoint_args * arg):
953    bpf_prog_44a040bf25481309_test_long_fname_2:
954    ; static int test_long_fname_2(struct dummy_tracepoint_args *arg)
955       0:   push   %rbp
956       1:   mov    %rsp,%rbp
957       4:   sub    $0x30,%rsp
958       b:   sub    $0x28,%rbp
959       f:   mov    %rbx,0x0(%rbp)
960      13:   mov    %r13,0x8(%rbp)
961      17:   mov    %r14,0x10(%rbp)
962      1b:   mov    %r15,0x18(%rbp)
963      1f:   xor    %eax,%eax
964      21:   mov    %rax,0x20(%rbp)
965      25:   xor    %esi,%esi
966    ; int key = 0;
967      27:   mov    %esi,-0x4(%rbp)
968    ; if (!arg->sock)
969      2a:   mov    0x8(%rdi),%rdi
970    ; if (!arg->sock)
971      2e:   cmp    $0x0,%rdi
972      32:   je     0x0000000000000070
973      34:   mov    %rbp,%rsi
974    ; counts = bpf_map_lookup_elem(&btf_map, &key);
975    [...]
976
9775.3 Verifier Log
978----------------
979
980The following is an example of how line_info can help debugging verification
981failure.::
982
983       /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
984        * is modified as below.
985        */
986       data = (void *)(long)xdp->data;
987       data_end = (void *)(long)xdp->data_end;
988       /*
989       if (data + 4 > data_end)
990               return XDP_DROP;
991       */
992       *(u32 *)data = dst->dst;
993
994    $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp
995        ; data = (void *)(long)xdp->data;
996        224: (79) r2 = *(u64 *)(r10 -112)
997        225: (61) r2 = *(u32 *)(r2 +0)
998        ; *(u32 *)data = dst->dst;
999        226: (63) *(u32 *)(r2 +0) = r1
1000        invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0)
1001        R2 offset is outside of the packet
1002
10036. BTF Generation
1004=================
1005
1006You need latest pahole
1007
1008  https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
1009
1010or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't
1011support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,::
1012
1013      -bash-4.4$ cat t.c
1014      struct t {
1015        int a:2;
1016        int b:3;
1017        int c:2;
1018      } g;
1019      -bash-4.4$ gcc -c -O2 -g t.c
1020      -bash-4.4$ pahole -JV t.o
1021      File t.o:
1022      [1] STRUCT t kind_flag=1 size=4 vlen=3
1023              a type_id=2 bitfield_size=2 bits_offset=0
1024              b type_id=2 bitfield_size=3 bits_offset=2
1025              c type_id=2 bitfield_size=2 bits_offset=5
1026      [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
1027
1028The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target
1029only. The assembly code (-S) is able to show the BTF encoding in assembly
1030format.::
1031
1032    -bash-4.4$ cat t2.c
1033    typedef int __int32;
1034    struct t2 {
1035      int a2;
1036      int (*f2)(char q1, __int32 q2, ...);
1037      int (*f3)();
1038    } g2;
1039    int main() { return 0; }
1040    int test() { return 0; }
1041    -bash-4.4$ clang -c -g -O2 --target=bpf t2.c
1042    -bash-4.4$ readelf -S t2.o
1043      ......
1044      [ 8] .BTF              PROGBITS         0000000000000000  00000247
1045           000000000000016e  0000000000000000           0     0     1
1046      [ 9] .BTF.ext          PROGBITS         0000000000000000  000003b5
1047           0000000000000060  0000000000000000           0     0     1
1048      [10] .rel.BTF.ext      REL              0000000000000000  000007e0
1049           0000000000000040  0000000000000010          16     9     8
1050      ......
1051    -bash-4.4$ clang -S -g -O2 --target=bpf t2.c
1052    -bash-4.4$ cat t2.s
1053      ......
1054            .section        .BTF,"",@progbits
1055            .short  60319                   # 0xeb9f
1056            .byte   1
1057            .byte   0
1058            .long   24
1059            .long   0
1060            .long   220
1061            .long   220
1062            .long   122
1063            .long   0                       # BTF_KIND_FUNC_PROTO(id = 1)
1064            .long   218103808               # 0xd000000
1065            .long   2
1066            .long   83                      # BTF_KIND_INT(id = 2)
1067            .long   16777216                # 0x1000000
1068            .long   4
1069            .long   16777248                # 0x1000020
1070      ......
1071            .byte   0                       # string offset=0
1072            .ascii  ".text"                 # string offset=1
1073            .byte   0
1074            .ascii  "/home/yhs/tmp-pahole/t2.c" # string offset=7
1075            .byte   0
1076            .ascii  "int main() { return 0; }" # string offset=33
1077            .byte   0
1078            .ascii  "int test() { return 0; }" # string offset=58
1079            .byte   0
1080            .ascii  "int"                   # string offset=83
1081      ......
1082            .section        .BTF.ext,"",@progbits
1083            .short  60319                   # 0xeb9f
1084            .byte   1
1085            .byte   0
1086            .long   24
1087            .long   0
1088            .long   28
1089            .long   28
1090            .long   44
1091            .long   8                       # FuncInfo
1092            .long   1                       # FuncInfo section string offset=1
1093            .long   2
1094            .long   .Lfunc_begin0
1095            .long   3
1096            .long   .Lfunc_begin1
1097            .long   5
1098            .long   16                      # LineInfo
1099            .long   1                       # LineInfo section string offset=1
1100            .long   2
1101            .long   .Ltmp0
1102            .long   7
1103            .long   33
1104            .long   7182                    # Line 7 Col 14
1105            .long   .Ltmp3
1106            .long   7
1107            .long   58
1108            .long   8206                    # Line 8 Col 14
1109
11107. Testing
1111==========
1112
1113The kernel BPF selftest `tools/testing/selftests/bpf/prog_tests/btf.c`_
1114provides an extensive set of BTF-related tests.
1115
1116.. Links
1117.. _tools/testing/selftests/bpf/prog_tests/btf.c:
1118   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/btf.c
1119