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 89Note that the type section encodes debug info, not just pure types. 90``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram. 91 92Each type contains the following common data:: 93 94 struct btf_type { 95 __u32 name_off; 96 /* "info" bits arrangement 97 * bits 0-15: vlen (e.g. # of struct's members) 98 * bits 16-23: unused 99 * bits 24-28: kind (e.g. int, ptr, array...etc) 100 * bits 29-30: unused 101 * bit 31: kind_flag, currently used by 102 * struct, union and fwd 103 */ 104 __u32 info; 105 /* "size" is used by INT, ENUM, STRUCT and UNION. 106 * "size" tells the size of the type it is describing. 107 * 108 * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT, 109 * FUNC and FUNC_PROTO. 110 * "type" is a type_id referring to another type. 111 */ 112 union { 113 __u32 size; 114 __u32 type; 115 }; 116 }; 117 118For certain kinds, the common data are followed by kind-specific data. The 119``name_off`` in ``struct btf_type`` specifies the offset in the string table. 120The following sections detail encoding of each kind. 121 1222.2.1 BTF_KIND_INT 123~~~~~~~~~~~~~~~~~~ 124 125``struct btf_type`` encoding requirement: 126 * ``name_off``: any valid offset 127 * ``info.kind_flag``: 0 128 * ``info.kind``: BTF_KIND_INT 129 * ``info.vlen``: 0 130 * ``size``: the size of the int type in bytes. 131 132``btf_type`` is followed by a ``u32`` with the following bits arrangement:: 133 134 #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24) 135 #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16) 136 #define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff) 137 138The ``BTF_INT_ENCODING`` has the following attributes:: 139 140 #define BTF_INT_SIGNED (1 << 0) 141 #define BTF_INT_CHAR (1 << 1) 142 #define BTF_INT_BOOL (1 << 2) 143 144The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or 145bool, for the int type. The char and bool encoding are mostly useful for 146pretty print. At most one encoding can be specified for the int type. 147 148The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int 149type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4. 150The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()`` 151for the type. The maximum value of ``BTF_INT_BITS()`` is 128. 152 153The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values 154for this int. For example, a bitfield struct member has: 155 156 * btf member bit offset 100 from the start of the structure, 157 * btf member pointing to an int type, 158 * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4`` 159 160Then in the struct memory layout, this member will occupy ``4`` bits starting 161from bits ``100 + 2 = 102``. 162 163Alternatively, the bitfield struct member can be the following to access the 164same bits as the above: 165 166 * btf member bit offset 102, 167 * btf member pointing to an int type, 168 * the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4`` 169 170The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of 171bitfield encoding. Currently, both llvm and pahole generate 172``BTF_INT_OFFSET() = 0`` for all int types. 173 1742.2.2 BTF_KIND_PTR 175~~~~~~~~~~~~~~~~~~ 176 177``struct btf_type`` encoding requirement: 178 * ``name_off``: 0 179 * ``info.kind_flag``: 0 180 * ``info.kind``: BTF_KIND_PTR 181 * ``info.vlen``: 0 182 * ``type``: the pointee type of the pointer 183 184No additional type data follow ``btf_type``. 185 1862.2.3 BTF_KIND_ARRAY 187~~~~~~~~~~~~~~~~~~~~ 188 189``struct btf_type`` encoding requirement: 190 * ``name_off``: 0 191 * ``info.kind_flag``: 0 192 * ``info.kind``: BTF_KIND_ARRAY 193 * ``info.vlen``: 0 194 * ``size/type``: 0, not used 195 196``btf_type`` is followed by one ``struct btf_array``:: 197 198 struct btf_array { 199 __u32 type; 200 __u32 index_type; 201 __u32 nelems; 202 }; 203 204The ``struct btf_array`` encoding: 205 * ``type``: the element type 206 * ``index_type``: the index type 207 * ``nelems``: the number of elements for this array (``0`` is also allowed). 208 209The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``, 210``u64``, ``unsigned __int128``). The original design of including 211``index_type`` follows DWARF, which has an ``index_type`` for its array type. 212Currently in BTF, beyond type verification, the ``index_type`` is not used. 213 214The ``struct btf_array`` allows chaining through element type to represent 215multidimensional arrays. For example, for ``int a[5][6]``, the following type 216information illustrates the chaining: 217 218 * [1]: int 219 * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6`` 220 * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5`` 221 222Currently, both pahole and llvm collapse multidimensional array into 223one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is 224equal to ``30``. This is because the original use case is map pretty print 225where the whole array is dumped out so one-dimensional array is enough. As 226more BTF usage is explored, pahole and llvm can be changed to generate proper 227chained representation for multidimensional arrays. 228 2292.2.4 BTF_KIND_STRUCT 230~~~~~~~~~~~~~~~~~~~~~ 2312.2.5 BTF_KIND_UNION 232~~~~~~~~~~~~~~~~~~~~ 233 234``struct btf_type`` encoding requirement: 235 * ``name_off``: 0 or offset to a valid C identifier 236 * ``info.kind_flag``: 0 or 1 237 * ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION 238 * ``info.vlen``: the number of struct/union members 239 * ``info.size``: the size of the struct/union in bytes 240 241``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.:: 242 243 struct btf_member { 244 __u32 name_off; 245 __u32 type; 246 __u32 offset; 247 }; 248 249``struct btf_member`` encoding: 250 * ``name_off``: offset to a valid C identifier 251 * ``type``: the member type 252 * ``offset``: <see below> 253 254If the type info ``kind_flag`` is not set, the offset contains only bit offset 255of the member. Note that the base type of the bitfield can only be int or enum 256type. If the bitfield size is 32, the base type can be either int or enum 257type. If the bitfield size is not 32, the base type must be int, and int type 258``BTF_INT_BITS()`` encodes the bitfield size. 259 260If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member 261bitfield size and bit offset. The bitfield size and bit offset are calculated 262as below.:: 263 264 #define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24) 265 #define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff) 266 267In this case, if the base type is an int type, it must be a regular int type: 268 269 * ``BTF_INT_OFFSET()`` must be 0. 270 * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``. 271 272The following kernel patch introduced ``kind_flag`` and explained why both 273modes exist: 274 275 https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3#diff-fa650a64fdd3968396883d2fe8215ff3 276 2772.2.6 BTF_KIND_ENUM 278~~~~~~~~~~~~~~~~~~~ 279 280``struct btf_type`` encoding requirement: 281 * ``name_off``: 0 or offset to a valid C identifier 282 * ``info.kind_flag``: 0 283 * ``info.kind``: BTF_KIND_ENUM 284 * ``info.vlen``: number of enum values 285 * ``size``: 4 286 287``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.:: 288 289 struct btf_enum { 290 __u32 name_off; 291 __s32 val; 292 }; 293 294The ``btf_enum`` encoding: 295 * ``name_off``: offset to a valid C identifier 296 * ``val``: any value 297 2982.2.7 BTF_KIND_FWD 299~~~~~~~~~~~~~~~~~~ 300 301``struct btf_type`` encoding requirement: 302 * ``name_off``: offset to a valid C identifier 303 * ``info.kind_flag``: 0 for struct, 1 for union 304 * ``info.kind``: BTF_KIND_FWD 305 * ``info.vlen``: 0 306 * ``type``: 0 307 308No additional type data follow ``btf_type``. 309 3102.2.8 BTF_KIND_TYPEDEF 311~~~~~~~~~~~~~~~~~~~~~~ 312 313``struct btf_type`` encoding requirement: 314 * ``name_off``: offset to a valid C identifier 315 * ``info.kind_flag``: 0 316 * ``info.kind``: BTF_KIND_TYPEDEF 317 * ``info.vlen``: 0 318 * ``type``: the type which can be referred by name at ``name_off`` 319 320No additional type data follow ``btf_type``. 321 3222.2.9 BTF_KIND_VOLATILE 323~~~~~~~~~~~~~~~~~~~~~~~ 324 325``struct btf_type`` encoding requirement: 326 * ``name_off``: 0 327 * ``info.kind_flag``: 0 328 * ``info.kind``: BTF_KIND_VOLATILE 329 * ``info.vlen``: 0 330 * ``type``: the type with ``volatile`` qualifier 331 332No additional type data follow ``btf_type``. 333 3342.2.10 BTF_KIND_CONST 335~~~~~~~~~~~~~~~~~~~~~ 336 337``struct btf_type`` encoding requirement: 338 * ``name_off``: 0 339 * ``info.kind_flag``: 0 340 * ``info.kind``: BTF_KIND_CONST 341 * ``info.vlen``: 0 342 * ``type``: the type with ``const`` qualifier 343 344No additional type data follow ``btf_type``. 345 3462.2.11 BTF_KIND_RESTRICT 347~~~~~~~~~~~~~~~~~~~~~~~~ 348 349``struct btf_type`` encoding requirement: 350 * ``name_off``: 0 351 * ``info.kind_flag``: 0 352 * ``info.kind``: BTF_KIND_RESTRICT 353 * ``info.vlen``: 0 354 * ``type``: the type with ``restrict`` qualifier 355 356No additional type data follow ``btf_type``. 357 3582.2.12 BTF_KIND_FUNC 359~~~~~~~~~~~~~~~~~~~~ 360 361``struct btf_type`` encoding requirement: 362 * ``name_off``: offset to a valid C identifier 363 * ``info.kind_flag``: 0 364 * ``info.kind``: BTF_KIND_FUNC 365 * ``info.vlen``: 0 366 * ``type``: a BTF_KIND_FUNC_PROTO type 367 368No additional type data follow ``btf_type``. 369 370A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose 371signature is defined by ``type``. The subprogram is thus an instance of that 372type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the 373:ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load` 374(ABI). 375 3762.2.13 BTF_KIND_FUNC_PROTO 377~~~~~~~~~~~~~~~~~~~~~~~~~~ 378 379``struct btf_type`` encoding requirement: 380 * ``name_off``: 0 381 * ``info.kind_flag``: 0 382 * ``info.kind``: BTF_KIND_FUNC_PROTO 383 * ``info.vlen``: # of parameters 384 * ``type``: the return type 385 386``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.:: 387 388 struct btf_param { 389 __u32 name_off; 390 __u32 type; 391 }; 392 393If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then 394``btf_param.name_off`` must point to a valid C identifier except for the 395possible last argument representing the variable argument. The btf_param.type 396refers to parameter type. 397 398If the function has variable arguments, the last parameter is encoded with 399``name_off = 0`` and ``type = 0``. 400 4012.2.14 BTF_KIND_VAR 402~~~~~~~~~~~~~~~~~~~ 403 404``struct btf_type`` encoding requirement: 405 * ``name_off``: offset to a valid C identifier 406 * ``info.kind_flag``: 0 407 * ``info.kind``: BTF_KIND_VAR 408 * ``info.vlen``: 0 409 * ``type``: the type of the variable 410 411``btf_type`` is followed by a single ``struct btf_variable`` with the 412following data:: 413 414 struct btf_var { 415 __u32 linkage; 416 }; 417 418``struct btf_var`` encoding: 419 * ``linkage``: currently only static variable 0, or globally allocated 420 variable in ELF sections 1 421 422Not all type of global variables are supported by LLVM at this point. 423The following is currently available: 424 425 * static variables with or without section attributes 426 * global variables with section attributes 427 428The latter is for future extraction of map key/value type id's from a 429map definition. 430 4312.2.15 BTF_KIND_DATASEC 432~~~~~~~~~~~~~~~~~~~~~~~ 433 434``struct btf_type`` encoding requirement: 435 * ``name_off``: offset to a valid name associated with a variable or 436 one of .data/.bss/.rodata 437 * ``info.kind_flag``: 0 438 * ``info.kind``: BTF_KIND_DATASEC 439 * ``info.vlen``: # of variables 440 * ``size``: total section size in bytes (0 at compilation time, patched 441 to actual size by BPF loaders such as libbpf) 442 443``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.:: 444 445 struct btf_var_secinfo { 446 __u32 type; 447 __u32 offset; 448 __u32 size; 449 }; 450 451``struct btf_var_secinfo`` encoding: 452 * ``type``: the type of the BTF_KIND_VAR variable 453 * ``offset``: the in-section offset of the variable 454 * ``size``: the size of the variable in bytes 455 4562.2.16 BTF_KIND_FLOAT 457~~~~~~~~~~~~~~~~~~~~~ 458 459``struct btf_type`` encoding requirement: 460 * ``name_off``: any valid offset 461 * ``info.kind_flag``: 0 462 * ``info.kind``: BTF_KIND_FLOAT 463 * ``info.vlen``: 0 464 * ``size``: the size of the float type in bytes: 2, 4, 8, 12 or 16. 465 466No additional type data follow ``btf_type``. 467 4683. BTF Kernel API 469***************** 470 471The following bpf syscall command involves BTF: 472 * BPF_BTF_LOAD: load a blob of BTF data into kernel 473 * BPF_MAP_CREATE: map creation with btf key and value type info. 474 * BPF_PROG_LOAD: prog load with btf function and line info. 475 * BPF_BTF_GET_FD_BY_ID: get a btf fd 476 * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info 477 and other btf related info are returned. 478 479The workflow typically looks like: 480:: 481 482 Application: 483 BPF_BTF_LOAD 484 | 485 v 486 BPF_MAP_CREATE and BPF_PROG_LOAD 487 | 488 V 489 ...... 490 491 Introspection tool: 492 ...... 493 BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's) 494 | 495 V 496 BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd) 497 | 498 V 499 BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id) 500 | | 501 V | 502 BPF_BTF_GET_FD_BY_ID (get btf_fd) | 503 | | 504 V | 505 BPF_OBJ_GET_INFO_BY_FD (get btf) | 506 | | 507 V V 508 pretty print types, dump func signatures and line info, etc. 509 510 5113.1 BPF_BTF_LOAD 512================ 513 514Load a blob of BTF data into kernel. A blob of data, described in 515:ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd`` 516is returned to a userspace. 517 5183.2 BPF_MAP_CREATE 519================== 520 521A map can be created with ``btf_fd`` and specified key/value type id.:: 522 523 __u32 btf_fd; /* fd pointing to a BTF type data */ 524 __u32 btf_key_type_id; /* BTF type_id of the key */ 525 __u32 btf_value_type_id; /* BTF type_id of the value */ 526 527In libbpf, the map can be defined with extra annotation like below: 528:: 529 530 struct bpf_map_def SEC("maps") btf_map = { 531 .type = BPF_MAP_TYPE_ARRAY, 532 .key_size = sizeof(int), 533 .value_size = sizeof(struct ipv_counts), 534 .max_entries = 4, 535 }; 536 BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts); 537 538Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name, key and 539value types for the map. During ELF parsing, libbpf is able to extract 540key/value type_id's and assign them to BPF_MAP_CREATE attributes 541automatically. 542 543.. _BPF_Prog_Load: 544 5453.3 BPF_PROG_LOAD 546================= 547 548During prog_load, func_info and line_info can be passed to kernel with proper 549values for the following attributes: 550:: 551 552 __u32 insn_cnt; 553 __aligned_u64 insns; 554 ...... 555 __u32 prog_btf_fd; /* fd pointing to BTF type data */ 556 __u32 func_info_rec_size; /* userspace bpf_func_info size */ 557 __aligned_u64 func_info; /* func info */ 558 __u32 func_info_cnt; /* number of bpf_func_info records */ 559 __u32 line_info_rec_size; /* userspace bpf_line_info size */ 560 __aligned_u64 line_info; /* line info */ 561 __u32 line_info_cnt; /* number of bpf_line_info records */ 562 563The func_info and line_info are an array of below, respectively.:: 564 565 struct bpf_func_info { 566 __u32 insn_off; /* [0, insn_cnt - 1] */ 567 __u32 type_id; /* pointing to a BTF_KIND_FUNC type */ 568 }; 569 struct bpf_line_info { 570 __u32 insn_off; /* [0, insn_cnt - 1] */ 571 __u32 file_name_off; /* offset to string table for the filename */ 572 __u32 line_off; /* offset to string table for the source line */ 573 __u32 line_col; /* line number and column number */ 574 }; 575 576func_info_rec_size is the size of each func_info record, and 577line_info_rec_size is the size of each line_info record. Passing the record 578size to kernel make it possible to extend the record itself in the future. 579 580Below are requirements for func_info: 581 * func_info[0].insn_off must be 0. 582 * the func_info insn_off is in strictly increasing order and matches 583 bpf func boundaries. 584 585Below are requirements for line_info: 586 * the first insn in each func must have a line_info record pointing to it. 587 * the line_info insn_off is in strictly increasing order. 588 589For line_info, the line number and column number are defined as below: 590:: 591 592 #define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10) 593 #define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff) 594 5953.4 BPF_{PROG,MAP}_GET_NEXT_ID 596============================== 597 598In kernel, every loaded program, map or btf has a unique id. The id won't 599change during the lifetime of a program, map, or btf. 600 601The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for 602each command, to user space, for bpf program or maps, respectively, so an 603inspection tool can inspect all programs and maps. 604 6053.5 BPF_{PROG,MAP}_GET_FD_BY_ID 606=============================== 607 608An introspection tool cannot use id to get details about program or maps. 609A file descriptor needs to be obtained first for reference-counting purpose. 610 6113.6 BPF_OBJ_GET_INFO_BY_FD 612========================== 613 614Once a program/map fd is acquired, an introspection tool can get the detailed 615information from kernel about this fd, some of which are BTF-related. For 616example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids. 617``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated 618bpf byte codes, and jited_line_info. 619 6203.7 BPF_BTF_GET_FD_BY_ID 621======================== 622 623With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf 624syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with 625command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the 626kernel with BPF_BTF_LOAD, can be retrieved. 627 628With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection 629tool has full btf knowledge and is able to pretty print map key/values, dump 630func signatures and line info, along with byte/jit codes. 631 6324. ELF File Format Interface 633**************************** 634 6354.1 .BTF section 636================ 637 638The .BTF section contains type and string data. The format of this section is 639same as the one describe in :ref:`BTF_Type_String`. 640 641.. _BTF_Ext_Section: 642 6434.2 .BTF.ext section 644==================== 645 646The .BTF.ext section encodes func_info and line_info which needs loader 647manipulation before loading into the kernel. 648 649The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h`` 650and ``tools/lib/bpf/btf.c``. 651 652The current header of .BTF.ext section:: 653 654 struct btf_ext_header { 655 __u16 magic; 656 __u8 version; 657 __u8 flags; 658 __u32 hdr_len; 659 660 /* All offsets are in bytes relative to the end of this header */ 661 __u32 func_info_off; 662 __u32 func_info_len; 663 __u32 line_info_off; 664 __u32 line_info_len; 665 }; 666 667It is very similar to .BTF section. Instead of type/string section, it 668contains func_info and line_info section. See :ref:`BPF_Prog_Load` for details 669about func_info and line_info record format. 670 671The func_info is organized as below.:: 672 673 func_info_rec_size 674 btf_ext_info_sec for section #1 /* func_info for section #1 */ 675 btf_ext_info_sec for section #2 /* func_info for section #2 */ 676 ... 677 678``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when 679.BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of 680func_info for each specific ELF section.:: 681 682 struct btf_ext_info_sec { 683 __u32 sec_name_off; /* offset to section name */ 684 __u32 num_info; 685 /* Followed by num_info * record_size number of bytes */ 686 __u8 data[0]; 687 }; 688 689Here, num_info must be greater than 0. 690 691The line_info is organized as below.:: 692 693 line_info_rec_size 694 btf_ext_info_sec for section #1 /* line_info for section #1 */ 695 btf_ext_info_sec for section #2 /* line_info for section #2 */ 696 ... 697 698``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when 699.BTF.ext is generated. 700 701The interpretation of ``bpf_func_info->insn_off`` and 702``bpf_line_info->insn_off`` is different between kernel API and ELF API. For 703kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct 704bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the 705beginning of section (``btf_ext_info_sec->sec_name_off``). 706 7074.2 .BTF_ids section 708==================== 709 710The .BTF_ids section encodes BTF ID values that are used within the kernel. 711 712This section is created during the kernel compilation with the help of 713macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can 714use them to create lists and sets (sorted lists) of BTF ID values. 715 716The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values, 717with following syntax:: 718 719 BTF_ID_LIST(list) 720 BTF_ID(type1, name1) 721 BTF_ID(type2, name2) 722 723resulting in following layout in .BTF_ids section:: 724 725 __BTF_ID__type1__name1__1: 726 .zero 4 727 __BTF_ID__type2__name2__2: 728 .zero 4 729 730The ``u32 list[];`` variable is defined to access the list. 731 732The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we 733want to define unused entry in BTF_ID_LIST, like:: 734 735 BTF_ID_LIST(bpf_skb_output_btf_ids) 736 BTF_ID(struct, sk_buff) 737 BTF_ID_UNUSED 738 BTF_ID(struct, task_struct) 739 740The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values 741and their count, with following syntax:: 742 743 BTF_SET_START(set) 744 BTF_ID(type1, name1) 745 BTF_ID(type2, name2) 746 BTF_SET_END(set) 747 748resulting in following layout in .BTF_ids section:: 749 750 __BTF_ID__set__set: 751 .zero 4 752 __BTF_ID__type1__name1__3: 753 .zero 4 754 __BTF_ID__type2__name2__4: 755 .zero 4 756 757The ``struct btf_id_set set;`` variable is defined to access the list. 758 759The ``typeX`` name can be one of following:: 760 761 struct, union, typedef, func 762 763and is used as a filter when resolving the BTF ID value. 764 765All the BTF ID lists and sets are compiled in the .BTF_ids section and 766resolved during the linking phase of kernel build by ``resolve_btfids`` tool. 767 7685. Using BTF 769************ 770 7715.1 bpftool map pretty print 772============================ 773 774With BTF, the map key/value can be printed based on fields rather than simply 775raw bytes. This is especially valuable for large structure or if your data 776structure has bitfields. For example, for the following map,:: 777 778 enum A { A1, A2, A3, A4, A5 }; 779 typedef enum A ___A; 780 struct tmp_t { 781 char a1:4; 782 int a2:4; 783 int :4; 784 __u32 a3:4; 785 int b; 786 ___A b1:4; 787 enum A b2:4; 788 }; 789 struct bpf_map_def SEC("maps") tmpmap = { 790 .type = BPF_MAP_TYPE_ARRAY, 791 .key_size = sizeof(__u32), 792 .value_size = sizeof(struct tmp_t), 793 .max_entries = 1, 794 }; 795 BPF_ANNOTATE_KV_PAIR(tmpmap, int, struct tmp_t); 796 797bpftool is able to pretty print like below: 798:: 799 800 [{ 801 "key": 0, 802 "value": { 803 "a1": 0x2, 804 "a2": 0x4, 805 "a3": 0x6, 806 "b": 7, 807 "b1": 0x8, 808 "b2": 0xa 809 } 810 } 811 ] 812 8135.2 bpftool prog dump 814===================== 815 816The following is an example showing how func_info and line_info can help prog 817dump with better kernel symbol names, function prototypes and line 818information.:: 819 820 $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv 821 [...] 822 int test_long_fname_2(struct dummy_tracepoint_args * arg): 823 bpf_prog_44a040bf25481309_test_long_fname_2: 824 ; static int test_long_fname_2(struct dummy_tracepoint_args *arg) 825 0: push %rbp 826 1: mov %rsp,%rbp 827 4: sub $0x30,%rsp 828 b: sub $0x28,%rbp 829 f: mov %rbx,0x0(%rbp) 830 13: mov %r13,0x8(%rbp) 831 17: mov %r14,0x10(%rbp) 832 1b: mov %r15,0x18(%rbp) 833 1f: xor %eax,%eax 834 21: mov %rax,0x20(%rbp) 835 25: xor %esi,%esi 836 ; int key = 0; 837 27: mov %esi,-0x4(%rbp) 838 ; if (!arg->sock) 839 2a: mov 0x8(%rdi),%rdi 840 ; if (!arg->sock) 841 2e: cmp $0x0,%rdi 842 32: je 0x0000000000000070 843 34: mov %rbp,%rsi 844 ; counts = bpf_map_lookup_elem(&btf_map, &key); 845 [...] 846 8475.3 Verifier Log 848================ 849 850The following is an example of how line_info can help debugging verification 851failure.:: 852 853 /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c 854 * is modified as below. 855 */ 856 data = (void *)(long)xdp->data; 857 data_end = (void *)(long)xdp->data_end; 858 /* 859 if (data + 4 > data_end) 860 return XDP_DROP; 861 */ 862 *(u32 *)data = dst->dst; 863 864 $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp 865 ; data = (void *)(long)xdp->data; 866 224: (79) r2 = *(u64 *)(r10 -112) 867 225: (61) r2 = *(u32 *)(r2 +0) 868 ; *(u32 *)data = dst->dst; 869 226: (63) *(u32 *)(r2 +0) = r1 870 invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0) 871 R2 offset is outside of the packet 872 8736. BTF Generation 874***************** 875 876You need latest pahole 877 878 https://git.kernel.org/pub/scm/devel/pahole/pahole.git/ 879 880or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't 881support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,:: 882 883 -bash-4.4$ cat t.c 884 struct t { 885 int a:2; 886 int b:3; 887 int c:2; 888 } g; 889 -bash-4.4$ gcc -c -O2 -g t.c 890 -bash-4.4$ pahole -JV t.o 891 File t.o: 892 [1] STRUCT t kind_flag=1 size=4 vlen=3 893 a type_id=2 bitfield_size=2 bits_offset=0 894 b type_id=2 bitfield_size=3 bits_offset=2 895 c type_id=2 bitfield_size=2 bits_offset=5 896 [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED 897 898The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target 899only. The assembly code (-S) is able to show the BTF encoding in assembly 900format.:: 901 902 -bash-4.4$ cat t2.c 903 typedef int __int32; 904 struct t2 { 905 int a2; 906 int (*f2)(char q1, __int32 q2, ...); 907 int (*f3)(); 908 } g2; 909 int main() { return 0; } 910 int test() { return 0; } 911 -bash-4.4$ clang -c -g -O2 -target bpf t2.c 912 -bash-4.4$ readelf -S t2.o 913 ...... 914 [ 8] .BTF PROGBITS 0000000000000000 00000247 915 000000000000016e 0000000000000000 0 0 1 916 [ 9] .BTF.ext PROGBITS 0000000000000000 000003b5 917 0000000000000060 0000000000000000 0 0 1 918 [10] .rel.BTF.ext REL 0000000000000000 000007e0 919 0000000000000040 0000000000000010 16 9 8 920 ...... 921 -bash-4.4$ clang -S -g -O2 -target bpf t2.c 922 -bash-4.4$ cat t2.s 923 ...... 924 .section .BTF,"",@progbits 925 .short 60319 # 0xeb9f 926 .byte 1 927 .byte 0 928 .long 24 929 .long 0 930 .long 220 931 .long 220 932 .long 122 933 .long 0 # BTF_KIND_FUNC_PROTO(id = 1) 934 .long 218103808 # 0xd000000 935 .long 2 936 .long 83 # BTF_KIND_INT(id = 2) 937 .long 16777216 # 0x1000000 938 .long 4 939 .long 16777248 # 0x1000020 940 ...... 941 .byte 0 # string offset=0 942 .ascii ".text" # string offset=1 943 .byte 0 944 .ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7 945 .byte 0 946 .ascii "int main() { return 0; }" # string offset=33 947 .byte 0 948 .ascii "int test() { return 0; }" # string offset=58 949 .byte 0 950 .ascii "int" # string offset=83 951 ...... 952 .section .BTF.ext,"",@progbits 953 .short 60319 # 0xeb9f 954 .byte 1 955 .byte 0 956 .long 24 957 .long 0 958 .long 28 959 .long 28 960 .long 44 961 .long 8 # FuncInfo 962 .long 1 # FuncInfo section string offset=1 963 .long 2 964 .long .Lfunc_begin0 965 .long 3 966 .long .Lfunc_begin1 967 .long 5 968 .long 16 # LineInfo 969 .long 1 # LineInfo section string offset=1 970 .long 2 971 .long .Ltmp0 972 .long 7 973 .long 33 974 .long 7182 # Line 7 Col 14 975 .long .Ltmp3 976 .long 7 977 .long 58 978 .long 8206 # Line 8 Col 14 979 9807. Testing 981********** 982 983Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests. 984