| ===================== |
| BPF Type Format (BTF) |
| ===================== |
| |
| 1. Introduction |
| *************** |
| |
| BTF (BPF Type Format) is the metadata format which encodes the debug info |
| related to BPF program/map. The name BTF was used initially to describe data |
| types. The BTF was later extended to include function info for defined |
| subroutines, and line info for source/line information. |
| |
| The debug info is used for map pretty print, function signature, etc. The |
| function signature enables better bpf program/function kernel symbol. The line |
| info helps generate source annotated translated byte code, jited code and |
| verifier log. |
| |
| The BTF specification contains two parts, |
| * BTF kernel API |
| * BTF ELF file format |
| |
| The kernel API is the contract between user space and kernel. The kernel |
| verifies the BTF info before using it. The ELF file format is a user space |
| contract between ELF file and libbpf loader. |
| |
| The type and string sections are part of the BTF kernel API, describing the |
| debug info (mostly types related) referenced by the bpf program. These two |
| sections are discussed in details in :ref:`BTF_Type_String`. |
| |
| .. _BTF_Type_String: |
| |
| 2. BTF Type and String Encoding |
| ******************************* |
| |
| The file ``include/uapi/linux/btf.h`` provides high-level definition of how |
| types/strings are encoded. |
| |
| The beginning of data blob must be:: |
| |
| struct btf_header { |
| __u16 magic; |
| __u8 version; |
| __u8 flags; |
| __u32 hdr_len; |
| |
| /* All offsets are in bytes relative to the end of this header */ |
| __u32 type_off; /* offset of type section */ |
| __u32 type_len; /* length of type section */ |
| __u32 str_off; /* offset of string section */ |
| __u32 str_len; /* length of string section */ |
| }; |
| |
| The magic is ``0xeB9F``, which has different encoding for big and little |
| endian systems, and can be used to test whether BTF is generated for big- or |
| little-endian target. The ``btf_header`` is designed to be extensible with |
| ``hdr_len`` equal to ``sizeof(struct btf_header)`` when a data blob is |
| generated. |
| |
| 2.1 String Encoding |
| =================== |
| |
| The first string in the string section must be a null string. The rest of |
| string table is a concatenation of other null-terminated strings. |
| |
| 2.2 Type Encoding |
| ================= |
| |
| The type id ``0`` is reserved for ``void`` type. The type section is parsed |
| sequentially and type id is assigned to each recognized type starting from id |
| ``1``. Currently, the following types are supported:: |
| |
| #define BTF_KIND_INT 1 /* Integer */ |
| #define BTF_KIND_PTR 2 /* Pointer */ |
| #define BTF_KIND_ARRAY 3 /* Array */ |
| #define BTF_KIND_STRUCT 4 /* Struct */ |
| #define BTF_KIND_UNION 5 /* Union */ |
| #define BTF_KIND_ENUM 6 /* Enumeration */ |
| #define BTF_KIND_FWD 7 /* Forward */ |
| #define BTF_KIND_TYPEDEF 8 /* Typedef */ |
| #define BTF_KIND_VOLATILE 9 /* Volatile */ |
| #define BTF_KIND_CONST 10 /* Const */ |
| #define BTF_KIND_RESTRICT 11 /* Restrict */ |
| #define BTF_KIND_FUNC 12 /* Function */ |
| #define BTF_KIND_FUNC_PROTO 13 /* Function Proto */ |
| #define BTF_KIND_VAR 14 /* Variable */ |
| #define BTF_KIND_DATASEC 15 /* Section */ |
| |
| Note that the type section encodes debug info, not just pure types. |
| ``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram. |
| |
| Each type contains the following common data:: |
| |
| struct btf_type { |
| __u32 name_off; |
| /* "info" bits arrangement |
| * bits 0-15: vlen (e.g. # of struct's members) |
| * bits 16-23: unused |
| * bits 24-27: kind (e.g. int, ptr, array...etc) |
| * bits 28-30: unused |
| * bit 31: kind_flag, currently used by |
| * struct, union and fwd |
| */ |
| __u32 info; |
| /* "size" is used by INT, ENUM, STRUCT and UNION. |
| * "size" tells the size of the type it is describing. |
| * |
| * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT, |
| * FUNC and FUNC_PROTO. |
| * "type" is a type_id referring to another type. |
| */ |
| union { |
| __u32 size; |
| __u32 type; |
| }; |
| }; |
| |
| For certain kinds, the common data are followed by kind-specific data. The |
| ``name_off`` in ``struct btf_type`` specifies the offset in the string table. |
| The following sections detail encoding of each kind. |
| |
| 2.2.1 BTF_KIND_INT |
| ~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: any valid offset |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_INT |
| * ``info.vlen``: 0 |
| * ``size``: the size of the int type in bytes. |
| |
| ``btf_type`` is followed by a ``u32`` with the following bits arrangement:: |
| |
| #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24) |
| #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16) |
| #define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff) |
| |
| The ``BTF_INT_ENCODING`` has the following attributes:: |
| |
| #define BTF_INT_SIGNED (1 << 0) |
| #define BTF_INT_CHAR (1 << 1) |
| #define BTF_INT_BOOL (1 << 2) |
| |
| The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or |
| bool, for the int type. The char and bool encoding are mostly useful for |
| pretty print. At most one encoding can be specified for the int type. |
| |
| The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int |
| type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4. |
| The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()`` |
| for the type. The maximum value of ``BTF_INT_BITS()`` is 128. |
| |
| The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values |
| for this int. For example, a bitfield struct member has: |
| |
| * btf member bit offset 100 from the start of the structure, |
| * btf member pointing to an int type, |
| * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4`` |
| |
| Then in the struct memory layout, this member will occupy ``4`` bits starting |
| from bits ``100 + 2 = 102``. |
| |
| Alternatively, the bitfield struct member can be the following to access the |
| same bits as the above: |
| |
| * btf member bit offset 102, |
| * btf member pointing to an int type, |
| * the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4`` |
| |
| The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of |
| bitfield encoding. Currently, both llvm and pahole generate |
| ``BTF_INT_OFFSET() = 0`` for all int types. |
| |
| 2.2.2 BTF_KIND_PTR |
| ~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: 0 |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_PTR |
| * ``info.vlen``: 0 |
| * ``type``: the pointee type of the pointer |
| |
| No additional type data follow ``btf_type``. |
| |
| 2.2.3 BTF_KIND_ARRAY |
| ~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: 0 |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_ARRAY |
| * ``info.vlen``: 0 |
| * ``size/type``: 0, not used |
| |
| ``btf_type`` is followed by one ``struct btf_array``:: |
| |
| struct btf_array { |
| __u32 type; |
| __u32 index_type; |
| __u32 nelems; |
| }; |
| |
| The ``struct btf_array`` encoding: |
| * ``type``: the element type |
| * ``index_type``: the index type |
| * ``nelems``: the number of elements for this array (``0`` is also allowed). |
| |
| The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``, |
| ``u64``, ``unsigned __int128``). The original design of including |
| ``index_type`` follows DWARF, which has an ``index_type`` for its array type. |
| Currently in BTF, beyond type verification, the ``index_type`` is not used. |
| |
| The ``struct btf_array`` allows chaining through element type to represent |
| multidimensional arrays. For example, for ``int a[5][6]``, the following type |
| information illustrates the chaining: |
| |
| * [1]: int |
| * [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6`` |
| * [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5`` |
| |
| Currently, both pahole and llvm collapse multidimensional array into |
| one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is |
| equal to ``30``. This is because the original use case is map pretty print |
| where the whole array is dumped out so one-dimensional array is enough. As |
| more BTF usage is explored, pahole and llvm can be changed to generate proper |
| chained representation for multidimensional arrays. |
| |
| 2.2.4 BTF_KIND_STRUCT |
| ~~~~~~~~~~~~~~~~~~~~~ |
| 2.2.5 BTF_KIND_UNION |
| ~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: 0 or offset to a valid C identifier |
| * ``info.kind_flag``: 0 or 1 |
| * ``info.kind``: BTF_KIND_STRUCT or BTF_KIND_UNION |
| * ``info.vlen``: the number of struct/union members |
| * ``info.size``: the size of the struct/union in bytes |
| |
| ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_member``.:: |
| |
| struct btf_member { |
| __u32 name_off; |
| __u32 type; |
| __u32 offset; |
| }; |
| |
| ``struct btf_member`` encoding: |
| * ``name_off``: offset to a valid C identifier |
| * ``type``: the member type |
| * ``offset``: <see below> |
| |
| If the type info ``kind_flag`` is not set, the offset contains only bit offset |
| of the member. Note that the base type of the bitfield can only be int or enum |
| type. If the bitfield size is 32, the base type can be either int or enum |
| type. If the bitfield size is not 32, the base type must be int, and int type |
| ``BTF_INT_BITS()`` encodes the bitfield size. |
| |
| If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member |
| bitfield size and bit offset. The bitfield size and bit offset are calculated |
| as below.:: |
| |
| #define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24) |
| #define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff) |
| |
| In this case, if the base type is an int type, it must be a regular int type: |
| |
| * ``BTF_INT_OFFSET()`` must be 0. |
| * ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``. |
| |
| The following kernel patch introduced ``kind_flag`` and explained why both |
| modes exist: |
| |
| https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3#diff-fa650a64fdd3968396883d2fe8215ff3 |
| |
| 2.2.6 BTF_KIND_ENUM |
| ~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: 0 or offset to a valid C identifier |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_ENUM |
| * ``info.vlen``: number of enum values |
| * ``size``: 4 |
| |
| ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.:: |
| |
| struct btf_enum { |
| __u32 name_off; |
| __s32 val; |
| }; |
| |
| The ``btf_enum`` encoding: |
| * ``name_off``: offset to a valid C identifier |
| * ``val``: any value |
| |
| 2.2.7 BTF_KIND_FWD |
| ~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: offset to a valid C identifier |
| * ``info.kind_flag``: 0 for struct, 1 for union |
| * ``info.kind``: BTF_KIND_FWD |
| * ``info.vlen``: 0 |
| * ``type``: 0 |
| |
| No additional type data follow ``btf_type``. |
| |
| 2.2.8 BTF_KIND_TYPEDEF |
| ~~~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: offset to a valid C identifier |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_TYPEDEF |
| * ``info.vlen``: 0 |
| * ``type``: the type which can be referred by name at ``name_off`` |
| |
| No additional type data follow ``btf_type``. |
| |
| 2.2.9 BTF_KIND_VOLATILE |
| ~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: 0 |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_VOLATILE |
| * ``info.vlen``: 0 |
| * ``type``: the type with ``volatile`` qualifier |
| |
| No additional type data follow ``btf_type``. |
| |
| 2.2.10 BTF_KIND_CONST |
| ~~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: 0 |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_CONST |
| * ``info.vlen``: 0 |
| * ``type``: the type with ``const`` qualifier |
| |
| No additional type data follow ``btf_type``. |
| |
| 2.2.11 BTF_KIND_RESTRICT |
| ~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: 0 |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_RESTRICT |
| * ``info.vlen``: 0 |
| * ``type``: the type with ``restrict`` qualifier |
| |
| No additional type data follow ``btf_type``. |
| |
| 2.2.12 BTF_KIND_FUNC |
| ~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: offset to a valid C identifier |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_FUNC |
| * ``info.vlen``: 0 |
| * ``type``: a BTF_KIND_FUNC_PROTO type |
| |
| No additional type data follow ``btf_type``. |
| |
| A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose |
| signature is defined by ``type``. The subprogram is thus an instance of that |
| type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the |
| :ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load` |
| (ABI). |
| |
| 2.2.13 BTF_KIND_FUNC_PROTO |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: 0 |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_FUNC_PROTO |
| * ``info.vlen``: # of parameters |
| * ``type``: the return type |
| |
| ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_param``.:: |
| |
| struct btf_param { |
| __u32 name_off; |
| __u32 type; |
| }; |
| |
| If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then |
| ``btf_param.name_off`` must point to a valid C identifier except for the |
| possible last argument representing the variable argument. The btf_param.type |
| refers to parameter type. |
| |
| If the function has variable arguments, the last parameter is encoded with |
| ``name_off = 0`` and ``type = 0``. |
| |
| 2.2.14 BTF_KIND_VAR |
| ~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: offset to a valid C identifier |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_VAR |
| * ``info.vlen``: 0 |
| * ``type``: the type of the variable |
| |
| ``btf_type`` is followed by a single ``struct btf_variable`` with the |
| following data:: |
| |
| struct btf_var { |
| __u32 linkage; |
| }; |
| |
| ``struct btf_var`` encoding: |
| * ``linkage``: currently only static variable 0, or globally allocated |
| variable in ELF sections 1 |
| |
| Not all type of global variables are supported by LLVM at this point. |
| The following is currently available: |
| |
| * static variables with or without section attributes |
| * global variables with section attributes |
| |
| The latter is for future extraction of map key/value type id's from a |
| map definition. |
| |
| 2.2.15 BTF_KIND_DATASEC |
| ~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| ``struct btf_type`` encoding requirement: |
| * ``name_off``: offset to a valid name associated with a variable or |
| one of .data/.bss/.rodata |
| * ``info.kind_flag``: 0 |
| * ``info.kind``: BTF_KIND_DATASEC |
| * ``info.vlen``: # of variables |
| * ``size``: total section size in bytes (0 at compilation time, patched |
| to actual size by BPF loaders such as libbpf) |
| |
| ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.:: |
| |
| struct btf_var_secinfo { |
| __u32 type; |
| __u32 offset; |
| __u32 size; |
| }; |
| |
| ``struct btf_var_secinfo`` encoding: |
| * ``type``: the type of the BTF_KIND_VAR variable |
| * ``offset``: the in-section offset of the variable |
| * ``size``: the size of the variable in bytes |
| |
| 3. BTF Kernel API |
| ***************** |
| |
| The following bpf syscall command involves BTF: |
| * BPF_BTF_LOAD: load a blob of BTF data into kernel |
| * BPF_MAP_CREATE: map creation with btf key and value type info. |
| * BPF_PROG_LOAD: prog load with btf function and line info. |
| * BPF_BTF_GET_FD_BY_ID: get a btf fd |
| * BPF_OBJ_GET_INFO_BY_FD: btf, func_info, line_info |
| and other btf related info are returned. |
| |
| The workflow typically looks like: |
| :: |
| |
| Application: |
| BPF_BTF_LOAD |
| | |
| v |
| BPF_MAP_CREATE and BPF_PROG_LOAD |
| | |
| V |
| ...... |
| |
| Introspection tool: |
| ...... |
| BPF_{PROG,MAP}_GET_NEXT_ID (get prog/map id's) |
| | |
| V |
| BPF_{PROG,MAP}_GET_FD_BY_ID (get a prog/map fd) |
| | |
| V |
| BPF_OBJ_GET_INFO_BY_FD (get bpf_prog_info/bpf_map_info with btf_id) |
| | | |
| V | |
| BPF_BTF_GET_FD_BY_ID (get btf_fd) | |
| | | |
| V | |
| BPF_OBJ_GET_INFO_BY_FD (get btf) | |
| | | |
| V V |
| pretty print types, dump func signatures and line info, etc. |
| |
| |
| 3.1 BPF_BTF_LOAD |
| ================ |
| |
| Load a blob of BTF data into kernel. A blob of data, described in |
| :ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd`` |
| is returned to a userspace. |
| |
| 3.2 BPF_MAP_CREATE |
| ================== |
| |
| A map can be created with ``btf_fd`` and specified key/value type id.:: |
| |
| __u32 btf_fd; /* fd pointing to a BTF type data */ |
| __u32 btf_key_type_id; /* BTF type_id of the key */ |
| __u32 btf_value_type_id; /* BTF type_id of the value */ |
| |
| In libbpf, the map can be defined with extra annotation like below: |
| :: |
| |
| struct bpf_map_def SEC("maps") btf_map = { |
| .type = BPF_MAP_TYPE_ARRAY, |
| .key_size = sizeof(int), |
| .value_size = sizeof(struct ipv_counts), |
| .max_entries = 4, |
| }; |
| BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts); |
| |
| Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name, key and |
| value types for the map. During ELF parsing, libbpf is able to extract |
| key/value type_id's and assign them to BPF_MAP_CREATE attributes |
| automatically. |
| |
| .. _BPF_Prog_Load: |
| |
| 3.3 BPF_PROG_LOAD |
| ================= |
| |
| During prog_load, func_info and line_info can be passed to kernel with proper |
| values for the following attributes: |
| :: |
| |
| __u32 insn_cnt; |
| __aligned_u64 insns; |
| ...... |
| __u32 prog_btf_fd; /* fd pointing to BTF type data */ |
| __u32 func_info_rec_size; /* userspace bpf_func_info size */ |
| __aligned_u64 func_info; /* func info */ |
| __u32 func_info_cnt; /* number of bpf_func_info records */ |
| __u32 line_info_rec_size; /* userspace bpf_line_info size */ |
| __aligned_u64 line_info; /* line info */ |
| __u32 line_info_cnt; /* number of bpf_line_info records */ |
| |
| The func_info and line_info are an array of below, respectively.:: |
| |
| struct bpf_func_info { |
| __u32 insn_off; /* [0, insn_cnt - 1] */ |
| __u32 type_id; /* pointing to a BTF_KIND_FUNC type */ |
| }; |
| struct bpf_line_info { |
| __u32 insn_off; /* [0, insn_cnt - 1] */ |
| __u32 file_name_off; /* offset to string table for the filename */ |
| __u32 line_off; /* offset to string table for the source line */ |
| __u32 line_col; /* line number and column number */ |
| }; |
| |
| func_info_rec_size is the size of each func_info record, and |
| line_info_rec_size is the size of each line_info record. Passing the record |
| size to kernel make it possible to extend the record itself in the future. |
| |
| Below are requirements for func_info: |
| * func_info[0].insn_off must be 0. |
| * the func_info insn_off is in strictly increasing order and matches |
| bpf func boundaries. |
| |
| Below are requirements for line_info: |
| * the first insn in each func must have a line_info record pointing to it. |
| * the line_info insn_off is in strictly increasing order. |
| |
| For line_info, the line number and column number are defined as below: |
| :: |
| |
| #define BPF_LINE_INFO_LINE_NUM(line_col) ((line_col) >> 10) |
| #define BPF_LINE_INFO_LINE_COL(line_col) ((line_col) & 0x3ff) |
| |
| 3.4 BPF_{PROG,MAP}_GET_NEXT_ID |
| ============================== |
| |
| In kernel, every loaded program, map or btf has a unique id. The id won't |
| change during the lifetime of a program, map, or btf. |
| |
| The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for |
| each command, to user space, for bpf program or maps, respectively, so an |
| inspection tool can inspect all programs and maps. |
| |
| 3.5 BPF_{PROG,MAP}_GET_FD_BY_ID |
| =============================== |
| |
| An introspection tool cannot use id to get details about program or maps. |
| A file descriptor needs to be obtained first for reference-counting purpose. |
| |
| 3.6 BPF_OBJ_GET_INFO_BY_FD |
| ========================== |
| |
| Once a program/map fd is acquired, an introspection tool can get the detailed |
| information from kernel about this fd, some of which are BTF-related. For |
| example, ``bpf_map_info`` returns ``btf_id`` and key/value type ids. |
| ``bpf_prog_info`` returns ``btf_id``, func_info, and line info for translated |
| bpf byte codes, and jited_line_info. |
| |
| 3.7 BPF_BTF_GET_FD_BY_ID |
| ======================== |
| |
| With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf |
| syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with |
| command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the |
| kernel with BPF_BTF_LOAD, can be retrieved. |
| |
| With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection |
| tool has full btf knowledge and is able to pretty print map key/values, dump |
| func signatures and line info, along with byte/jit codes. |
| |
| 4. ELF File Format Interface |
| **************************** |
| |
| 4.1 .BTF section |
| ================ |
| |
| The .BTF section contains type and string data. The format of this section is |
| same as the one describe in :ref:`BTF_Type_String`. |
| |
| .. _BTF_Ext_Section: |
| |
| 4.2 .BTF.ext section |
| ==================== |
| |
| The .BTF.ext section encodes func_info and line_info which needs loader |
| manipulation before loading into the kernel. |
| |
| The specification for .BTF.ext section is defined at ``tools/lib/bpf/btf.h`` |
| and ``tools/lib/bpf/btf.c``. |
| |
| The current header of .BTF.ext section:: |
| |
| struct btf_ext_header { |
| __u16 magic; |
| __u8 version; |
| __u8 flags; |
| __u32 hdr_len; |
| |
| /* All offsets are in bytes relative to the end of this header */ |
| __u32 func_info_off; |
| __u32 func_info_len; |
| __u32 line_info_off; |
| __u32 line_info_len; |
| }; |
| |
| It is very similar to .BTF section. Instead of type/string section, it |
| contains func_info and line_info section. See :ref:`BPF_Prog_Load` for details |
| about func_info and line_info record format. |
| |
| The func_info is organized as below.:: |
| |
| func_info_rec_size |
| btf_ext_info_sec for section #1 /* func_info for section #1 */ |
| btf_ext_info_sec for section #2 /* func_info for section #2 */ |
| ... |
| |
| ``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure when |
| .BTF.ext is generated. ``btf_ext_info_sec``, defined below, is a collection of |
| func_info for each specific ELF section.:: |
| |
| struct btf_ext_info_sec { |
| __u32 sec_name_off; /* offset to section name */ |
| __u32 num_info; |
| /* Followed by num_info * record_size number of bytes */ |
| __u8 data[0]; |
| }; |
| |
| Here, num_info must be greater than 0. |
| |
| The line_info is organized as below.:: |
| |
| line_info_rec_size |
| btf_ext_info_sec for section #1 /* line_info for section #1 */ |
| btf_ext_info_sec for section #2 /* line_info for section #2 */ |
| ... |
| |
| ``line_info_rec_size`` specifies the size of ``bpf_line_info`` structure when |
| .BTF.ext is generated. |
| |
| The interpretation of ``bpf_func_info->insn_off`` and |
| ``bpf_line_info->insn_off`` is different between kernel API and ELF API. For |
| kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct |
| bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the |
| beginning of section (``btf_ext_info_sec->sec_name_off``). |
| |
| 4.2 .BTF_ids section |
| ==================== |
| |
| The .BTF_ids section encodes BTF ID values that are used within the kernel. |
| |
| This section is created during the kernel compilation with the help of |
| macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can |
| use them to create lists and sets (sorted lists) of BTF ID values. |
| |
| The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values, |
| with following syntax:: |
| |
| BTF_ID_LIST(list) |
| BTF_ID(type1, name1) |
| BTF_ID(type2, name2) |
| |
| resulting in following layout in .BTF_ids section:: |
| |
| __BTF_ID__type1__name1__1: |
| .zero 4 |
| __BTF_ID__type2__name2__2: |
| .zero 4 |
| |
| The ``u32 list[];`` variable is defined to access the list. |
| |
| The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we |
| want to define unused entry in BTF_ID_LIST, like:: |
| |
| BTF_ID_LIST(bpf_skb_output_btf_ids) |
| BTF_ID(struct, sk_buff) |
| BTF_ID_UNUSED |
| BTF_ID(struct, task_struct) |
| |
| The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values |
| and their count, with following syntax:: |
| |
| BTF_SET_START(set) |
| BTF_ID(type1, name1) |
| BTF_ID(type2, name2) |
| BTF_SET_END(set) |
| |
| resulting in following layout in .BTF_ids section:: |
| |
| __BTF_ID__set__set: |
| .zero 4 |
| __BTF_ID__type1__name1__3: |
| .zero 4 |
| __BTF_ID__type2__name2__4: |
| .zero 4 |
| |
| The ``struct btf_id_set set;`` variable is defined to access the list. |
| |
| The ``typeX`` name can be one of following:: |
| |
| struct, union, typedef, func |
| |
| and is used as a filter when resolving the BTF ID value. |
| |
| All the BTF ID lists and sets are compiled in the .BTF_ids section and |
| resolved during the linking phase of kernel build by ``resolve_btfids`` tool. |
| |
| 5. Using BTF |
| ************ |
| |
| 5.1 bpftool map pretty print |
| ============================ |
| |
| With BTF, the map key/value can be printed based on fields rather than simply |
| raw bytes. This is especially valuable for large structure or if your data |
| structure has bitfields. For example, for the following map,:: |
| |
| enum A { A1, A2, A3, A4, A5 }; |
| typedef enum A ___A; |
| struct tmp_t { |
| char a1:4; |
| int a2:4; |
| int :4; |
| __u32 a3:4; |
| int b; |
| ___A b1:4; |
| enum A b2:4; |
| }; |
| struct bpf_map_def SEC("maps") tmpmap = { |
| .type = BPF_MAP_TYPE_ARRAY, |
| .key_size = sizeof(__u32), |
| .value_size = sizeof(struct tmp_t), |
| .max_entries = 1, |
| }; |
| BPF_ANNOTATE_KV_PAIR(tmpmap, int, struct tmp_t); |
| |
| bpftool is able to pretty print like below: |
| :: |
| |
| [{ |
| "key": 0, |
| "value": { |
| "a1": 0x2, |
| "a2": 0x4, |
| "a3": 0x6, |
| "b": 7, |
| "b1": 0x8, |
| "b2": 0xa |
| } |
| } |
| ] |
| |
| 5.2 bpftool prog dump |
| ===================== |
| |
| The following is an example showing how func_info and line_info can help prog |
| dump with better kernel symbol names, function prototypes and line |
| information.:: |
| |
| $ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv |
| [...] |
| int test_long_fname_2(struct dummy_tracepoint_args * arg): |
| bpf_prog_44a040bf25481309_test_long_fname_2: |
| ; static int test_long_fname_2(struct dummy_tracepoint_args *arg) |
| 0: push %rbp |
| 1: mov %rsp,%rbp |
| 4: sub $0x30,%rsp |
| b: sub $0x28,%rbp |
| f: mov %rbx,0x0(%rbp) |
| 13: mov %r13,0x8(%rbp) |
| 17: mov %r14,0x10(%rbp) |
| 1b: mov %r15,0x18(%rbp) |
| 1f: xor %eax,%eax |
| 21: mov %rax,0x20(%rbp) |
| 25: xor %esi,%esi |
| ; int key = 0; |
| 27: mov %esi,-0x4(%rbp) |
| ; if (!arg->sock) |
| 2a: mov 0x8(%rdi),%rdi |
| ; if (!arg->sock) |
| 2e: cmp $0x0,%rdi |
| 32: je 0x0000000000000070 |
| 34: mov %rbp,%rsi |
| ; counts = bpf_map_lookup_elem(&btf_map, &key); |
| [...] |
| |
| 5.3 Verifier Log |
| ================ |
| |
| The following is an example of how line_info can help debugging verification |
| failure.:: |
| |
| /* The code at tools/testing/selftests/bpf/test_xdp_noinline.c |
| * is modified as below. |
| */ |
| data = (void *)(long)xdp->data; |
| data_end = (void *)(long)xdp->data_end; |
| /* |
| if (data + 4 > data_end) |
| return XDP_DROP; |
| */ |
| *(u32 *)data = dst->dst; |
| |
| $ bpftool prog load ./test_xdp_noinline.o /sys/fs/bpf/test_xdp_noinline type xdp |
| ; data = (void *)(long)xdp->data; |
| 224: (79) r2 = *(u64 *)(r10 -112) |
| 225: (61) r2 = *(u32 *)(r2 +0) |
| ; *(u32 *)data = dst->dst; |
| 226: (63) *(u32 *)(r2 +0) = r1 |
| invalid access to packet, off=0 size=4, R2(id=0,off=0,r=0) |
| R2 offset is outside of the packet |
| |
| 6. BTF Generation |
| ***************** |
| |
| You need latest pahole |
| |
| https://git.kernel.org/pub/scm/devel/pahole/pahole.git/ |
| |
| or llvm (8.0 or later). The pahole acts as a dwarf2btf converter. It doesn't |
| support .BTF.ext and btf BTF_KIND_FUNC type yet. For example,:: |
| |
| -bash-4.4$ cat t.c |
| struct t { |
| int a:2; |
| int b:3; |
| int c:2; |
| } g; |
| -bash-4.4$ gcc -c -O2 -g t.c |
| -bash-4.4$ pahole -JV t.o |
| File t.o: |
| [1] STRUCT t kind_flag=1 size=4 vlen=3 |
| a type_id=2 bitfield_size=2 bits_offset=0 |
| b type_id=2 bitfield_size=3 bits_offset=2 |
| c type_id=2 bitfield_size=2 bits_offset=5 |
| [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED |
| |
| The llvm is able to generate .BTF and .BTF.ext directly with -g for bpf target |
| only. The assembly code (-S) is able to show the BTF encoding in assembly |
| format.:: |
| |
| -bash-4.4$ cat t2.c |
| typedef int __int32; |
| struct t2 { |
| int a2; |
| int (*f2)(char q1, __int32 q2, ...); |
| int (*f3)(); |
| } g2; |
| int main() { return 0; } |
| int test() { return 0; } |
| -bash-4.4$ clang -c -g -O2 -target bpf t2.c |
| -bash-4.4$ readelf -S t2.o |
| ...... |
| [ 8] .BTF PROGBITS 0000000000000000 00000247 |
| 000000000000016e 0000000000000000 0 0 1 |
| [ 9] .BTF.ext PROGBITS 0000000000000000 000003b5 |
| 0000000000000060 0000000000000000 0 0 1 |
| [10] .rel.BTF.ext REL 0000000000000000 000007e0 |
| 0000000000000040 0000000000000010 16 9 8 |
| ...... |
| -bash-4.4$ clang -S -g -O2 -target bpf t2.c |
| -bash-4.4$ cat t2.s |
| ...... |
| .section .BTF,"",@progbits |
| .short 60319 # 0xeb9f |
| .byte 1 |
| .byte 0 |
| .long 24 |
| .long 0 |
| .long 220 |
| .long 220 |
| .long 122 |
| .long 0 # BTF_KIND_FUNC_PROTO(id = 1) |
| .long 218103808 # 0xd000000 |
| .long 2 |
| .long 83 # BTF_KIND_INT(id = 2) |
| .long 16777216 # 0x1000000 |
| .long 4 |
| .long 16777248 # 0x1000020 |
| ...... |
| .byte 0 # string offset=0 |
| .ascii ".text" # string offset=1 |
| .byte 0 |
| .ascii "/home/yhs/tmp-pahole/t2.c" # string offset=7 |
| .byte 0 |
| .ascii "int main() { return 0; }" # string offset=33 |
| .byte 0 |
| .ascii "int test() { return 0; }" # string offset=58 |
| .byte 0 |
| .ascii "int" # string offset=83 |
| ...... |
| .section .BTF.ext,"",@progbits |
| .short 60319 # 0xeb9f |
| .byte 1 |
| .byte 0 |
| .long 24 |
| .long 0 |
| .long 28 |
| .long 28 |
| .long 44 |
| .long 8 # FuncInfo |
| .long 1 # FuncInfo section string offset=1 |
| .long 2 |
| .long .Lfunc_begin0 |
| .long 3 |
| .long .Lfunc_begin1 |
| .long 5 |
| .long 16 # LineInfo |
| .long 1 # LineInfo section string offset=1 |
| .long 2 |
| .long .Ltmp0 |
| .long 7 |
| .long 33 |
| .long 7182 # Line 7 Col 14 |
| .long .Ltmp3 |
| .long 7 |
| .long 58 |
| .long 8206 # Line 8 Col 14 |
| |
| 7. Testing |
| ********** |
| |
| Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests. |
