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 */ 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 90Note that the type section encodes debug info, not just pure types. 91``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram. 92 93Each type contains the following common data:: 94 95 struct btf_type { 96 __u32 name_off; 97 /* "info" bits arrangement 98 * bits 0-15: vlen (e.g. # of struct's members) 99 * bits 16-23: unused 100 * bits 24-28: kind (e.g. int, ptr, array...etc) 101 * bits 29-30: unused 102 * bit 31: kind_flag, currently used by 103 * struct, union and fwd 104 */ 105 __u32 info; 106 /* "size" is used by INT, ENUM, STRUCT and UNION. 107 * "size" tells the size of the type it is describing. 108 * 109 * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT, 110 * FUNC, FUNC_PROTO and DECL_TAG. 111 * "type" is a type_id referring to another type. 112 */ 113 union { 114 __u32 size; 115 __u32 type; 116 }; 117 }; 118 119For certain kinds, the common data are followed by kind-specific data. The 120``name_off`` in ``struct btf_type`` specifies the offset in the string table. 121The following sections detail encoding of each kind. 122 1232.2.1 BTF_KIND_INT 124~~~~~~~~~~~~~~~~~~ 125 126``struct btf_type`` encoding requirement: 127 * ``name_off``: any valid offset 128 * ``info.kind_flag``: 0 129 * ``info.kind``: BTF_KIND_INT 130 * ``info.vlen``: 0 131 * ``size``: the size of the int type in bytes. 132 133``btf_type`` is followed by a ``u32`` with the following bits arrangement:: 134 135 #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24) 136 #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16) 137 #define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff) 138 139The ``BTF_INT_ENCODING`` has the following attributes:: 140 141 #define BTF_INT_SIGNED (1 << 0) 142 #define BTF_INT_CHAR (1 << 1) 143 #define BTF_INT_BOOL (1 << 2) 144 145The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or 146bool, for the int type. The char and bool encoding are mostly useful for 147pretty print. At most one encoding can be specified for the int type. 148 149The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int 150type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4. 151The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()`` 152for the type. The maximum value of ``BTF_INT_BITS()`` is 128. 153 154The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values 155for this int. For example, a bitfield struct member has: 156 157 * btf member bit offset 100 from the start of the structure, 158 * btf member pointing to an int type, 159 * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4`` 160 161Then in the struct memory layout, this member will occupy ``4`` bits starting 162from bits ``100 + 2 = 102``. 163 164Alternatively, the bitfield struct member can be the following to access the 165same bits as the above: 166 167 * btf member bit offset 102, 168 * btf member pointing to an int type, 169 * the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4`` 170 171The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of 172bitfield encoding. Currently, both llvm and pahole generate 173``BTF_INT_OFFSET() = 0`` for all int types. 174 1752.2.2 BTF_KIND_PTR 176~~~~~~~~~~~~~~~~~~ 177 178``struct btf_type`` encoding requirement: 179 * ``name_off``: 0 180 * ``info.kind_flag``: 0 181 * ``info.kind``: BTF_KIND_PTR 182 * ``info.vlen``: 0 183 * ``type``: the pointee type of the pointer 184 185No additional type data follow ``btf_type``. 186 1872.2.3 BTF_KIND_ARRAY 188~~~~~~~~~~~~~~~~~~~~ 189 190``struct btf_type`` encoding requirement: 191 * ``name_off``: 0 192 * ``info.kind_flag``: 0 193 * ``info.kind``: BTF_KIND_ARRAY 194 * ``info.vlen``: 0 195 * ``size/type``: 0, not used 196 197``btf_type`` is followed by one ``struct btf_array``:: 198 199 struct btf_array { 200 __u32 type; 201 __u32 index_type; 202 __u32 nelems; 203 }; 204 205The ``struct btf_array`` encoding: 206 * ``type``: the element type 207 * ``index_type``: the index type 208 * ``nelems``: the number of elements for this array (``0`` is also allowed). 209 210The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``, 211``u64``, ``unsigned __int128``). The original design of including 212``index_type`` follows DWARF, which has an ``index_type`` for its array type. 213Currently in BTF, beyond type verification, the ``index_type`` is not used. 214 215The ``struct btf_array`` allows chaining through element type to represent 216multidimensional arrays. For example, for ``int a[5][6]``, the following type 217information illustrates the chaining: 218 219 * [1]: int 220 * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6`` 221 * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5`` 222 223Currently, both pahole and llvm collapse multidimensional array into 224one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is 225equal to ``30``. This is because the original use case is map pretty print 226where the whole array is dumped out so one-dimensional array is enough. As 227more BTF usage is explored, pahole and llvm can be changed to generate proper 228chained representation for multidimensional arrays. 229 2302.2.4 BTF_KIND_STRUCT 231~~~~~~~~~~~~~~~~~~~~~ 2322.2.5 BTF_KIND_UNION 233~~~~~~~~~~~~~~~~~~~~ 234 235``struct btf_type`` encoding requirement: 236 * ``name_off``: 0 or offset to a valid C identifier 237 * ``info.kind_flag``: 0 or 1 238 * ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION 239 * ``info.vlen``: the number of struct/union members 240 * ``info.size``: the size of the struct/union in bytes 241 242``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.:: 243 244 struct btf_member { 245 __u32 name_off; 246 __u32 type; 247 __u32 offset; 248 }; 249 250``struct btf_member`` encoding: 251 * ``name_off``: offset to a valid C identifier 252 * ``type``: the member type 253 * ``offset``: <see below> 254 255If the type info ``kind_flag`` is not set, the offset contains only bit offset 256of the member. Note that the base type of the bitfield can only be int or enum 257type. If the bitfield size is 32, the base type can be either int or enum 258type. If the bitfield size is not 32, the base type must be int, and int type 259``BTF_INT_BITS()`` encodes the bitfield size. 260 261If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member 262bitfield size and bit offset. The bitfield size and bit offset are calculated 263as below.:: 264 265 #define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24) 266 #define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff) 267 268In this case, if the base type is an int type, it must be a regular int type: 269 270 * ``BTF_INT_OFFSET()`` must be 0. 271 * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``. 272 273The following kernel patch introduced ``kind_flag`` and explained why both 274modes exist: 275 276 https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3#diff-fa650a64fdd3968396883d2fe8215ff3 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``: 0 284 * ``info.kind``: BTF_KIND_ENUM 285 * ``info.vlen``: number of enum values 286 * ``size``: 4 287 288``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 2992.2.7 BTF_KIND_FWD 300~~~~~~~~~~~~~~~~~~ 301 302``struct btf_type`` encoding requirement: 303 * ``name_off``: offset to a valid C identifier 304 * ``info.kind_flag``: 0 for struct, 1 for union 305 * ``info.kind``: BTF_KIND_FWD 306 * ``info.vlen``: 0 307 * ``type``: 0 308 309No additional type data follow ``btf_type``. 310 3112.2.8 BTF_KIND_TYPEDEF 312~~~~~~~~~~~~~~~~~~~~~~ 313 314``struct btf_type`` encoding requirement: 315 * ``name_off``: offset to a valid C identifier 316 * ``info.kind_flag``: 0 317 * ``info.kind``: BTF_KIND_TYPEDEF 318 * ``info.vlen``: 0 319 * ``type``: the type which can be referred by name at ``name_off`` 320 321No additional type data follow ``btf_type``. 322 3232.2.9 BTF_KIND_VOLATILE 324~~~~~~~~~~~~~~~~~~~~~~~ 325 326``struct btf_type`` encoding requirement: 327 * ``name_off``: 0 328 * ``info.kind_flag``: 0 329 * ``info.kind``: BTF_KIND_VOLATILE 330 * ``info.vlen``: 0 331 * ``type``: the type with ``volatile`` qualifier 332 333No additional type data follow ``btf_type``. 334 3352.2.10 BTF_KIND_CONST 336~~~~~~~~~~~~~~~~~~~~~ 337 338``struct btf_type`` encoding requirement: 339 * ``name_off``: 0 340 * ``info.kind_flag``: 0 341 * ``info.kind``: BTF_KIND_CONST 342 * ``info.vlen``: 0 343 * ``type``: the type with ``const`` qualifier 344 345No additional type data follow ``btf_type``. 346 3472.2.11 BTF_KIND_RESTRICT 348~~~~~~~~~~~~~~~~~~~~~~~~ 349 350``struct btf_type`` encoding requirement: 351 * ``name_off``: 0 352 * ``info.kind_flag``: 0 353 * ``info.kind``: BTF_KIND_RESTRICT 354 * ``info.vlen``: 0 355 * ``type``: the type with ``restrict`` qualifier 356 357No additional type data follow ``btf_type``. 358 3592.2.12 BTF_KIND_FUNC 360~~~~~~~~~~~~~~~~~~~~ 361 362``struct btf_type`` encoding requirement: 363 * ``name_off``: offset to a valid C identifier 364 * ``info.kind_flag``: 0 365 * ``info.kind``: BTF_KIND_FUNC 366 * ``info.vlen``: 0 367 * ``type``: a BTF_KIND_FUNC_PROTO type 368 369No additional type data follow ``btf_type``. 370 371A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose 372signature is defined by ``type``. The subprogram is thus an instance of that 373type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the 374:ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load` 375(ABI). 376 3772.2.13 BTF_KIND_FUNC_PROTO 378~~~~~~~~~~~~~~~~~~~~~~~~~~ 379 380``struct btf_type`` encoding requirement: 381 * ``name_off``: 0 382 * ``info.kind_flag``: 0 383 * ``info.kind``: BTF_KIND_FUNC_PROTO 384 * ``info.vlen``: # of parameters 385 * ``type``: the return type 386 387``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.:: 388 389 struct btf_param { 390 __u32 name_off; 391 __u32 type; 392 }; 393 394If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then 395``btf_param.name_off`` must point to a valid C identifier except for the 396possible last argument representing the variable argument. The btf_param.type 397refers to parameter type. 398 399If the function has variable arguments, the last parameter is encoded with 400``name_off = 0`` and ``type = 0``. 401 4022.2.14 BTF_KIND_VAR 403~~~~~~~~~~~~~~~~~~~ 404 405``struct btf_type`` encoding requirement: 406 * ``name_off``: offset to a valid C identifier 407 * ``info.kind_flag``: 0 408 * ``info.kind``: BTF_KIND_VAR 409 * ``info.vlen``: 0 410 * ``type``: the type of the variable 411 412``btf_type`` is followed by a single ``struct btf_variable`` with the 413following data:: 414 415 struct btf_var { 416 __u32 linkage; 417 }; 418 419``struct btf_var`` encoding: 420 * ``linkage``: currently only static variable 0, or globally allocated 421 variable in ELF sections 1 422 423Not all type of global variables are supported by LLVM at this point. 424The following is currently available: 425 426 * static variables with or without section attributes 427 * global variables with section attributes 428 429The latter is for future extraction of map key/value type id's from a 430map definition. 431 4322.2.15 BTF_KIND_DATASEC 433~~~~~~~~~~~~~~~~~~~~~~~ 434 435``struct btf_type`` encoding requirement: 436 * ``name_off``: offset to a valid name associated with a variable or 437 one of .data/.bss/.rodata 438 * ``info.kind_flag``: 0 439 * ``info.kind``: BTF_KIND_DATASEC 440 * ``info.vlen``: # of variables 441 * ``size``: total section size in bytes (0 at compilation time, patched 442 to actual size by BPF loaders such as libbpf) 443 444``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.:: 445 446 struct btf_var_secinfo { 447 __u32 type; 448 __u32 offset; 449 __u32 size; 450 }; 451 452``struct btf_var_secinfo`` encoding: 453 * ``type``: the type of the BTF_KIND_VAR variable 454 * ``offset``: the in-section offset of the variable 455 * ``size``: the size of the variable in bytes 456 4572.2.16 BTF_KIND_FLOAT 458~~~~~~~~~~~~~~~~~~~~~ 459 460``struct btf_type`` encoding requirement: 461 * ``name_off``: any valid offset 462 * ``info.kind_flag``: 0 463 * ``info.kind``: BTF_KIND_FLOAT 464 * ``info.vlen``: 0 465 * ``size``: the size of the float type in bytes: 2, 4, 8, 12 or 16. 466 467No additional type data follow ``btf_type``. 468 4692.2.17 BTF_KIND_DECL_TAG 470~~~~~~~~~~~~~~~~~~~~~~~~ 471 472``struct btf_type`` encoding requirement: 473 * ``name_off``: offset to a non-empty string 474 * ``info.kind_flag``: 0 475 * ``info.kind``: BTF_KIND_DECL_TAG 476 * ``info.vlen``: 0 477 * ``type``: ``struct``, ``union``, ``func``, ``var`` or ``typedef`` 478 479``btf_type`` is followed by ``struct btf_decl_tag``.:: 480 481 struct btf_decl_tag { 482 __u32 component_idx; 483 }; 484 485The ``name_off`` encodes btf_decl_tag attribute string. 486The ``type`` should be ``struct``, ``union``, ``func``, ``var`` or ``typedef``. 487For ``var`` or ``typedef`` type, ``btf_decl_tag.component_idx`` must be ``-1``. 488For the other three types, if the btf_decl_tag attribute is 489applied to the ``struct``, ``union`` or ``func`` itself, 490``btf_decl_tag.component_idx`` must be ``-1``. Otherwise, 491the attribute is applied to a ``struct``/``union`` member or 492a ``func`` argument, and ``btf_decl_tag.component_idx`` should be a 493valid index (starting from 0) pointing to a member or an argument. 494 4953. BTF Kernel API 496***************** 497 498The following bpf syscall command involves BTF: 499 * BPF_BTF_LOAD: load a blob of BTF data into kernel 500 * BPF_MAP_CREATE: map creation with btf key and value type info. 501 * BPF_PROG_LOAD: prog load with btf function and line info. 502 * BPF_BTF_GET_FD_BY_ID: get a btf fd 503 * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info 504 and other btf related info are returned. 505 506The workflow typically looks like: 507:: 508 509 Application: 510 BPF_BTF_LOAD 511 | 512 v 513 BPF_MAP_CREATE and BPF_PROG_LOAD 514 | 515 V 516 ...... 517 518 Introspection tool: 519 ...... 520 BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's) 521 | 522 V 523 BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd) 524 | 525 V 526 BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id) 527 | | 528 V | 529 BPF_BTF_GET_FD_BY_ID (get btf_fd) | 530 | | 531 V | 532 BPF_OBJ_GET_INFO_BY_FD (get btf) | 533 | | 534 V V 535 pretty print types, dump func signatures and line info, etc. 536 537 5383.1 BPF_BTF_LOAD 539================ 540 541Load a blob of BTF data into kernel. A blob of data, described in 542:ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd`` 543is returned to a userspace. 544 5453.2 BPF_MAP_CREATE 546================== 547 548A map can be created with ``btf_fd`` and specified key/value type id.:: 549 550 __u32 btf_fd; /* fd pointing to a BTF type data */ 551 __u32 btf_key_type_id; /* BTF type_id of the key */ 552 __u32 btf_value_type_id; /* BTF type_id of the value */ 553 554In libbpf, the map can be defined with extra annotation like below: 555:: 556 557 struct bpf_map_def SEC("maps") btf_map = { 558 .type = BPF_MAP_TYPE_ARRAY, 559 .key_size = sizeof(int), 560 .value_size = sizeof(struct ipv_counts), 561 .max_entries = 4, 562 }; 563 BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts); 564 565Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name, key and 566value types for the map. During ELF parsing, libbpf is able to extract 567key/value type_id's and assign them to BPF_MAP_CREATE attributes 568automatically. 569 570.. _BPF_Prog_Load: 571 5723.3 BPF_PROG_LOAD 573================= 574 575During prog_load, func_info and line_info can be passed to kernel with proper 576values for the following attributes: 577:: 578 579 __u32 insn_cnt; 580 __aligned_u64 insns; 581 ...... 582 __u32 prog_btf_fd; /* fd pointing to BTF type data */ 583 __u32 func_info_rec_size; /* userspace bpf_func_info size */ 584 __aligned_u64 func_info; /* func info */ 585 __u32 func_info_cnt; /* number of bpf_func_info records */ 586 __u32 line_info_rec_size; /* userspace bpf_line_info size */ 587 __aligned_u64 line_info; /* line info */ 588 __u32 line_info_cnt; /* number of bpf_line_info records */ 589 590The func_info and line_info are an array of below, respectively.:: 591 592 struct bpf_func_info { 593 __u32 insn_off; /* [0, insn_cnt - 1] */ 594 __u32 type_id; /* pointing to a BTF_KIND_FUNC type */ 595 }; 596 struct bpf_line_info { 597 __u32 insn_off; /* [0, insn_cnt - 1] */ 598 __u32 file_name_off; /* offset to string table for the filename */ 599 __u32 line_off; /* offset to string table for the source line */ 600 __u32 line_col; /* line number and column number */ 601 }; 602 603func_info_rec_size is the size of each func_info record, and 604line_info_rec_size is the size of each line_info record. Passing the record 605size to kernel make it possible to extend the record itself in the future. 606 607Below are requirements for func_info: 608 * func_info[0].insn_off must be 0. 609 * the func_info insn_off is in strictly increasing order and matches 610 bpf func boundaries. 611 612Below are requirements for line_info: 613 * the first insn in each func must have a line_info record pointing to it. 614 * the line_info insn_off is in strictly increasing order. 615 616For line_info, the line number and column number are defined as below: 617:: 618 619 #define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10) 620 #define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff) 621 6223.4 BPF_{PROG,MAP}_GET_NEXT_ID 623============================== 624 625In kernel, every loaded program, map or btf has a unique id. The id won't 626change during the lifetime of a program, map, or btf. 627 628The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for 629each command, to user space, for bpf program or maps, respectively, so an 630inspection tool can inspect all programs and maps. 631 6323.5 BPF_{PROG,MAP}_GET_FD_BY_ID 633=============================== 634 635An introspection tool cannot use id to get details about program or maps. 636A file descriptor needs to be obtained first for reference-counting purpose. 637 6383.6 BPF_OBJ_GET_INFO_BY_FD 639========================== 640 641Once a program/map fd is acquired, an introspection tool can get the detailed 642information from kernel about this fd, some of which are BTF-related. For 643example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids. 644``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated 645bpf byte codes, and jited_line_info. 646 6473.7 BPF_BTF_GET_FD_BY_ID 648======================== 649 650With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf 651syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with 652command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the 653kernel with BPF_BTF_LOAD, can be retrieved. 654 655With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection 656tool has full btf knowledge and is able to pretty print map key/values, dump 657func signatures and line info, along with byte/jit codes. 658 6594. ELF File Format Interface 660**************************** 661 6624.1 .BTF section 663================ 664 665The .BTF section contains type and string data. The format of this section is 666same as the one describe in :ref:`BTF_Type_String`. 667 668.. _BTF_Ext_Section: 669 6704.2 .BTF.ext section 671==================== 672 673The .BTF.ext section encodes func_info and line_info which needs loader 674manipulation before loading into the kernel. 675 676The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h`` 677and ``tools/lib/bpf/btf.c``. 678 679The current header of .BTF.ext section:: 680 681 struct btf_ext_header { 682 __u16 magic; 683 __u8 version; 684 __u8 flags; 685 __u32 hdr_len; 686 687 /* All offsets are in bytes relative to the end of this header */ 688 __u32 func_info_off; 689 __u32 func_info_len; 690 __u32 line_info_off; 691 __u32 line_info_len; 692 }; 693 694It is very similar to .BTF section. Instead of type/string section, it 695contains func_info and line_info section. See :ref:`BPF_Prog_Load` for details 696about func_info and line_info record format. 697 698The func_info is organized as below.:: 699 700 func_info_rec_size 701 btf_ext_info_sec for section #1 /* func_info for section #1 */ 702 btf_ext_info_sec for section #2 /* func_info for section #2 */ 703 ... 704 705``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when 706.BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of 707func_info for each specific ELF section.:: 708 709 struct btf_ext_info_sec { 710 __u32 sec_name_off; /* offset to section name */ 711 __u32 num_info; 712 /* Followed by num_info * record_size number of bytes */ 713 __u8 data[0]; 714 }; 715 716Here, num_info must be greater than 0. 717 718The line_info is organized as below.:: 719 720 line_info_rec_size 721 btf_ext_info_sec for section #1 /* line_info for section #1 */ 722 btf_ext_info_sec for section #2 /* line_info for section #2 */ 723 ... 724 725``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when 726.BTF.ext is generated. 727 728The interpretation of ``bpf_func_info->insn_off`` and 729``bpf_line_info->insn_off`` is different between kernel API and ELF API. For 730kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct 731bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the 732beginning of section (``btf_ext_info_sec->sec_name_off``). 733 7344.2 .BTF_ids section 735==================== 736 737The .BTF_ids section encodes BTF ID values that are used within the kernel. 738 739This section is created during the kernel compilation with the help of 740macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can 741use them to create lists and sets (sorted lists) of BTF ID values. 742 743The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values, 744with following syntax:: 745 746 BTF_ID_LIST(list) 747 BTF_ID(type1, name1) 748 BTF_ID(type2, name2) 749 750resulting in following layout in .BTF_ids section:: 751 752 __BTF_ID__type1__name1__1: 753 .zero 4 754 __BTF_ID__type2__name2__2: 755 .zero 4 756 757The ``u32 list[];`` variable is defined to access the list. 758 759The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we 760want to define unused entry in BTF_ID_LIST, like:: 761 762 BTF_ID_LIST(bpf_skb_output_btf_ids) 763 BTF_ID(struct, sk_buff) 764 BTF_ID_UNUSED 765 BTF_ID(struct, task_struct) 766 767The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values 768and their count, with following syntax:: 769 770 BTF_SET_START(set) 771 BTF_ID(type1, name1) 772 BTF_ID(type2, name2) 773 BTF_SET_END(set) 774 775resulting in following layout in .BTF_ids section:: 776 777 __BTF_ID__set__set: 778 .zero 4 779 __BTF_ID__type1__name1__3: 780 .zero 4 781 __BTF_ID__type2__name2__4: 782 .zero 4 783 784The ``struct btf_id_set set;`` variable is defined to access the list. 785 786The ``typeX`` name can be one of following:: 787 788 struct, union, typedef, func 789 790and is used as a filter when resolving the BTF ID value. 791 792All the BTF ID lists and sets are compiled in the .BTF_ids section and 793resolved during the linking phase of kernel build by ``resolve_btfids`` tool. 794 7955. Using BTF 796************ 797 7985.1 bpftool map pretty print 799============================ 800 801With BTF, the map key/value can be printed based on fields rather than simply 802raw bytes. This is especially valuable for large structure or if your data 803structure has bitfields. For example, for the following map,:: 804 805 enum A { A1, A2, A3, A4, A5 }; 806 typedef enum A ___A; 807 struct tmp_t { 808 char a1:4; 809 int a2:4; 810 int :4; 811 __u32 a3:4; 812 int b; 813 ___A b1:4; 814 enum A b2:4; 815 }; 816 struct bpf_map_def SEC("maps") tmpmap = { 817 .type = BPF_MAP_TYPE_ARRAY, 818 .key_size = sizeof(__u32), 819 .value_size = sizeof(struct tmp_t), 820 .max_entries = 1, 821 }; 822 BPF_ANNOTATE_KV_PAIR(tmpmap, int, struct tmp_t); 823 824bpftool is able to pretty print like below: 825:: 826 827 [{ 828 "key": 0, 829 "value": { 830 "a1": 0x2, 831 "a2": 0x4, 832 "a3": 0x6, 833 "b": 7, 834 "b1": 0x8, 835 "b2": 0xa 836 } 837 } 838 ] 839 8405.2 bpftool prog dump 841===================== 842 843The following is an example showing how func_info and line_info can help prog 844dump with better kernel symbol names, function prototypes and line 845information.:: 846 847 $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv 848 [...] 849 int test_long_fname_2(struct dummy_tracepoint_args * arg): 850 bpf_prog_44a040bf25481309_test_long_fname_2: 851 ; static int test_long_fname_2(struct dummy_tracepoint_args *arg) 852 0: push %rbp 853 1: mov %rsp,%rbp 854 4: sub $0x30,%rsp 855 b: sub $0x28,%rbp 856 f: mov %rbx,0x0(%rbp) 857 13: mov %r13,0x8(%rbp) 858 17: mov %r14,0x10(%rbp) 859 1b: mov %r15,0x18(%rbp) 860 1f: xor %eax,%eax 861 21: mov %rax,0x20(%rbp) 862 25: xor %esi,%esi 863 ; int key = 0; 864 27: mov %esi,-0x4(%rbp) 865 ; if (!arg->sock) 866 2a: mov 0x8(%rdi),%rdi 867 ; if (!arg->sock) 868 2e: cmp $0x0,%rdi 869 32: je 0x0000000000000070 870 34: mov %rbp,%rsi 871 ; counts = bpf_map_lookup_elem(&btf_map, &key); 872 [...] 873 8745.3 Verifier Log 875================ 876 877The following is an example of how line_info can help debugging verification 878failure.:: 879 880 /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c 881 * is modified as below. 882 */ 883 data = (void *)(long)xdp->data; 884 data_end = (void *)(long)xdp->data_end; 885 /* 886 if (data + 4 > data_end) 887 return XDP_DROP; 888 */ 889 *(u32 *)data = dst->dst; 890 891 $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp 892 ; data = (void *)(long)xdp->data; 893 224: (79) r2 = *(u64 *)(r10 -112) 894 225: (61) r2 = *(u32 *)(r2 +0) 895 ; *(u32 *)data = dst->dst; 896 226: (63) *(u32 *)(r2 +0) = r1 897 invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0) 898 R2 offset is outside of the packet 899 9006. BTF Generation 901***************** 902 903You need latest pahole 904 905 https://git.kernel.org/pub/scm/devel/pahole/pahole.git/ 906 907or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't 908support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,:: 909 910 -bash-4.4$ cat t.c 911 struct t { 912 int a:2; 913 int b:3; 914 int c:2; 915 } g; 916 -bash-4.4$ gcc -c -O2 -g t.c 917 -bash-4.4$ pahole -JV t.o 918 File t.o: 919 [1] STRUCT t kind_flag=1 size=4 vlen=3 920 a type_id=2 bitfield_size=2 bits_offset=0 921 b type_id=2 bitfield_size=3 bits_offset=2 922 c type_id=2 bitfield_size=2 bits_offset=5 923 [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED 924 925The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target 926only. The assembly code (-S) is able to show the BTF encoding in assembly 927format.:: 928 929 -bash-4.4$ cat t2.c 930 typedef int __int32; 931 struct t2 { 932 int a2; 933 int (*f2)(char q1, __int32 q2, ...); 934 int (*f3)(); 935 } g2; 936 int main() { return 0; } 937 int test() { return 0; } 938 -bash-4.4$ clang -c -g -O2 -target bpf t2.c 939 -bash-4.4$ readelf -S t2.o 940 ...... 941 [ 8] .BTF PROGBITS 0000000000000000 00000247 942 000000000000016e 0000000000000000 0 0 1 943 [ 9] .BTF.ext PROGBITS 0000000000000000 000003b5 944 0000000000000060 0000000000000000 0 0 1 945 [10] .rel.BTF.ext REL 0000000000000000 000007e0 946 0000000000000040 0000000000000010 16 9 8 947 ...... 948 -bash-4.4$ clang -S -g -O2 -target bpf t2.c 949 -bash-4.4$ cat t2.s 950 ...... 951 .section .BTF,"",@progbits 952 .short 60319 # 0xeb9f 953 .byte 1 954 .byte 0 955 .long 24 956 .long 0 957 .long 220 958 .long 220 959 .long 122 960 .long 0 # BTF_KIND_FUNC_PROTO(id = 1) 961 .long 218103808 # 0xd000000 962 .long 2 963 .long 83 # BTF_KIND_INT(id = 2) 964 .long 16777216 # 0x1000000 965 .long 4 966 .long 16777248 # 0x1000020 967 ...... 968 .byte 0 # string offset=0 969 .ascii ".text" # string offset=1 970 .byte 0 971 .ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7 972 .byte 0 973 .ascii "int main() { return 0; }" # string offset=33 974 .byte 0 975 .ascii "int test() { return 0; }" # string offset=58 976 .byte 0 977 .ascii "int" # string offset=83 978 ...... 979 .section .BTF.ext,"",@progbits 980 .short 60319 # 0xeb9f 981 .byte 1 982 .byte 0 983 .long 24 984 .long 0 985 .long 28 986 .long 28 987 .long 44 988 .long 8 # FuncInfo 989 .long 1 # FuncInfo section string offset=1 990 .long 2 991 .long .Lfunc_begin0 992 .long 3 993 .long .Lfunc_begin1 994 .long 5 995 .long 16 # LineInfo 996 .long 1 # LineInfo section string offset=1 997 .long 2 998 .long .Ltmp0 999 .long 7 1000 .long 33 1001 .long 7182 # Line 7 Col 14 1002 .long .Ltmp3 1003 .long 7 1004 .long 58 1005 .long 8206 # Line 8 Col 14 1006 10077. Testing 1008********** 1009 1010Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests. 1011