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Diffstat (limited to 'Documentation/bpf/btf.rst')
| -rw-r--r-- | Documentation/bpf/btf.rst | 108 |
1 files changed, 53 insertions, 55 deletions
diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst index 1d434c3a268d..1d761f1c5b2b 100644 --- a/Documentation/bpf/btf.rst +++ b/Documentation/bpf/btf.rst @@ -5,7 +5,7 @@ BPF Type Format (BTF) 1. Introduction *************** -BTF (BPF Type Format) is the meta data format which +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 @@ -40,8 +40,8 @@ details in :ref:`BTF_Type_String`. 2. BTF Type and String Encoding ******************************* -The file ``include/uapi/linux/btf.h`` provides high -level definition on how types/strings are encoded. +The file ``include/uapi/linux/btf.h`` provides high-level +definition of how types/strings are encoded. The beginning of data blob must be:: @@ -59,23 +59,23 @@ The beginning of data blob must be:: }; The magic is ``0xeB9F``, which has different encoding for big and little -endian system, 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 the data blob is generated. +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-treminated +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 the type id is assigned to +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:: @@ -122,9 +122,9 @@ Each type contains the following common data:: }; }; -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 details encoding of each kind. +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 ~~~~~~~~~~~~~~~~~~ @@ -136,7 +136,7 @@ The following details encoding of each kind. * ``info.vlen``: 0 * ``size``: the size of the int type in bytes. -``btf_type`` is followed by a ``u32`` with following bits arrangement:: +``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) @@ -148,7 +148,7 @@ The ``BTF_INT_ENCODING`` has the following attributes:: #define BTF_INT_CHAR (1 << 1) #define BTF_INT_BOOL (1 << 2) -The ``BTF_INT_ENCODING()`` provides extra information, signness, +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. @@ -161,8 +161,7 @@ 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 - +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`` @@ -179,7 +178,7 @@ access the same bits as the above: The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of bitfield encoding. -Currently, both llvm and pahole generates ``BTF_INT_OFFSET() = 0`` +Currently, both llvm and pahole generate ``BTF_INT_OFFSET() = 0`` for all int types. 2.2.2 BTF_KIND_PTR @@ -204,7 +203,7 @@ No additional type data follow ``btf_type``. * ``info.vlen``: 0 * ``size/type``: 0, not used -btf_type is followed by one "struct btf_array":: +``btf_type`` is followed by one ``struct btf_array``:: struct btf_array { __u32 type; @@ -217,27 +216,26 @@ The ``struct btf_array`` encoding: * ``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 types -(u8, u16, u32, u64, unsigned __int128). -The original design of including ``index_type`` follows dwarf -which has a ``index_type`` for its array type. +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 -multiple dimensional arrays. For example, ``int a[5][6]``, the following -type system illustrates the chaining: +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 multiple dimensional array -into one dimensional array, e.g., ``a[5][6]``, the btf_array.nelems -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 +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 -multiple dimensional arrays. +changed to generate proper chained representation for multidimensional arrays. 2.2.4 BTF_KIND_STRUCT ~~~~~~~~~~~~~~~~~~~~~ @@ -382,7 +380,7 @@ No additional type data follow ``btf_type``. No additional type data follow ``btf_type``. -A BTF_KIND_FUNC defines, not a type, but a subprogram (function) whose +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 @@ -459,10 +457,10 @@ The workflow typically looks like: 3.1 BPF_BTF_LOAD ================ -Load a blob of BTF data into kernel. A blob of data -described in :ref:`BTF_Type_String` +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`` returns to userspace. +A ``btf_fd`` is returned to a userspace. 3.2 BPF_MAP_CREATE ================== @@ -487,7 +485,7 @@ In libbpf, the map can be defined with extra annotation like below: 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 assigned them to BPF_MAP_CREATE attributes automatically. +and assign them to BPF_MAP_CREATE attributes automatically. .. _BPF_Prog_Load: @@ -532,7 +530,7 @@ Below are requirements for func_info: bpf func boundaries. Below are requirements for line_info: - * the first insn in each func must points to a line_info record. + * 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: @@ -544,26 +542,26 @@ For line_info, the line number and column number are defined as below: 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 life time of the program, map or btf. +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, -so the inspection tool can inspect all programs and maps. +program or maps, respectively, +so an inspection tool can inspect all programs and maps. 3.5 BPF_{PROG,MAP}_GET_FD_BY_ID -The introspection tool cannot use id to get details about program or maps. -A file descriptor needs to be obtained first for reference counting purpose. +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, the introspection tool can +Once a program/map fd is acquired, an introspection tool can get the detailed information from kernel about this fd, -some of which is btf related. For example, -``bpf_map_info`` returns ``btf_id``, key/value type id. -``bpf_prog_info`` returns ``btf_id``, func_info and line info +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 @@ -574,9 +572,9 @@ 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``, the introspection +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, dump line info along with byte/jit codes. +dump func signatures and line info, along with byte/jit codes. 4. ELF File Format Interface **************************** @@ -625,8 +623,8 @@ The func_info is organized as below.:: ... ``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure -when .BTF.ext is generated. btf_ext_info_sec, defined below, is -the func_info for each specific ELF section.:: +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 */ @@ -661,7 +659,7 @@ from the beginning of section (``btf_ext_info_sec->sec_name_off``). 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 you data structure +valuable for large structure or if your data structure has bitfields. For example, for the following map,:: enum A { A1, A2, A3, A4, A5 }; @@ -702,8 +700,8 @@ bpftool is able to pretty print like below: 5.2 bpftool prog dump ===================== -The following is an example to show func_info and line_info -can help prog dump with better kernel symbol name, function prototype +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 @@ -733,10 +731,10 @@ and line information.:: ; counts = bpf_map_lookup_elem(&btf_map, &key); [...] -5.3 verifier log +5.3 Verifier Log ================ -The following is an example how line_info can help verifier failure debug.:: +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. @@ -867,4 +865,4 @@ The assembly code (-S) is able to show the BTF encoding in assembly format.:: 7. Testing ********** -Kernel bpf selftest `test_btf.c` provides extensive set of BTF related tests. +Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests. |