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docs/btf: fix typos, improve wording
Fix various typos, some of the formatting and wording for Documentation/btf.rst. Signed-off-by: Andrii Nakryiko <andriin@fb.com> Acked-by: Yonghong Song <yhs@fb.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Andrii Nakryiko
Daniel Borkmann
parent
4b9113045b
commit
5efc529fb4
@ -5,7 +5,7 @@ BPF Type Format (BTF)
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1. Introduction
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***************
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BTF (BPF Type Format) is the meta data format which
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BTF (BPF Type Format) is the metadata format which
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encodes the debug info related to BPF program/map.
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The name BTF was used initially to describe
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data types. The BTF was later extended to include
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@ -40,8 +40,8 @@ details in :ref:`BTF_Type_String`.
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2. BTF Type and String Encoding
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*******************************
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The file ``include/uapi/linux/btf.h`` provides high
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level definition on how types/strings are encoded.
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The file ``include/uapi/linux/btf.h`` provides high-level
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definition of how types/strings are encoded.
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The beginning of data blob must be::
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@ -59,23 +59,23 @@ The beginning of data blob must be::
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};
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The magic is ``0xeB9F``, which has different encoding for big and little
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endian system, and can be used to test whether BTF is generated for
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big or little endian target.
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The btf_header is designed to be extensible with hdr_len equal to
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``sizeof(struct btf_header)`` when the data blob is generated.
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endian systems, and can be used to test whether BTF is generated for
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big- or little-endian target.
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The ``btf_header`` is designed to be extensible with ``hdr_len`` equal to
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``sizeof(struct btf_header)`` when a data blob is generated.
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2.1 String Encoding
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===================
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The first string in the string section must be a null string.
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The rest of string table is a concatenation of other null-treminated
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The rest of string table is a concatenation of other null-terminated
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strings.
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2.2 Type Encoding
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=================
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The type id ``0`` is reserved for ``void`` type.
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The type section is parsed sequentially and the type id is assigned to
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The type section is parsed sequentially and type id is assigned to
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each recognized type starting from id ``1``.
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Currently, the following types are supported::
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@ -122,9 +122,9 @@ Each type contains the following common data::
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};
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};
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For certain kinds, the common data are followed by kind specific data.
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The ``name_off`` in ``struct btf_type`` specifies the offset in the string table.
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The following details encoding of each kind.
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For certain kinds, the common data are followed by kind-specific data.
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The ``name_off`` in ``struct btf_type`` specifies the offset in the string
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table. The following sections detail encoding of each kind.
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2.2.1 BTF_KIND_INT
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~~~~~~~~~~~~~~~~~~
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@ -136,7 +136,7 @@ The following details encoding of each kind.
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* ``info.vlen``: 0
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* ``size``: the size of the int type in bytes.
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``btf_type`` is followed by a ``u32`` with following bits arrangement::
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``btf_type`` is followed by a ``u32`` with the following bits arrangement::
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#define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
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#define BTF_INT_OFFSET(VAL) (((VAL & 0x00ff0000)) >> 16)
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@ -148,7 +148,7 @@ The ``BTF_INT_ENCODING`` has the following attributes::
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#define BTF_INT_CHAR (1 << 1)
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#define BTF_INT_BOOL (1 << 2)
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The ``BTF_INT_ENCODING()`` provides extra information, signness,
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The ``BTF_INT_ENCODING()`` provides extra information: signedness,
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char, or bool, for the int type. The char and bool encoding
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are mostly useful for pretty print. At most one encoding can
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be specified for the int type.
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@ -161,8 +161,7 @@ The maximum value of ``BTF_INT_BITS()`` is 128.
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The ``BTF_INT_OFFSET()`` specifies the starting bit offset to
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calculate values for this int. For example, a bitfield struct
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member has
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member has:
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* btf member bit offset 100 from the start of the structure,
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* btf member pointing to an int type,
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* the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4``
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@ -179,7 +178,7 @@ access the same bits as the above:
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The original intention of ``BTF_INT_OFFSET()`` is to provide
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flexibility of bitfield encoding.
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Currently, both llvm and pahole generates ``BTF_INT_OFFSET() = 0``
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Currently, both llvm and pahole generate ``BTF_INT_OFFSET() = 0``
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for all int types.
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2.2.2 BTF_KIND_PTR
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@ -204,7 +203,7 @@ No additional type data follow ``btf_type``.
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* ``info.vlen``: 0
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* ``size/type``: 0, not used
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btf_type is followed by one "struct btf_array"::
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``btf_type`` is followed by one ``struct btf_array``::
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struct btf_array {
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__u32 type;
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@ -217,27 +216,26 @@ The ``struct btf_array`` encoding:
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* ``index_type``: the index type
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* ``nelems``: the number of elements for this array (``0`` is also allowed).
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The ``index_type`` can be any regular int types
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(u8, u16, u32, u64, unsigned __int128).
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The original design of including ``index_type`` follows dwarf
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which has a ``index_type`` for its array type.
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The ``index_type`` can be any regular int type
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(``u8``, ``u16``, ``u32``, ``u64``, ``unsigned __int128``).
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The original design of including ``index_type`` follows DWARF,
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which has an ``index_type`` for its array type.
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Currently in BTF, beyond type verification, the ``index_type`` is not used.
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The ``struct btf_array`` allows chaining through element type to represent
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multiple dimensional arrays. For example, ``int a[5][6]``, the following
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type system illustrates the chaining:
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multidimensional arrays. For example, for ``int a[5][6]``, the following
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type information illustrates the chaining:
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* [1]: int
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* [2]: array, ``btf_array.type = [1]``, ``btf_array.nelems = 6``
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* [3]: array, ``btf_array.type = [2]``, ``btf_array.nelems = 5``
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Currently, both pahole and llvm collapse multiple dimensional array
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into one dimensional array, e.g., ``a[5][6]``, the btf_array.nelems
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equal to ``30``. This is because the original use case is map pretty
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print where the whole array is dumped out so one dimensional array
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Currently, both pahole and llvm collapse multidimensional array
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into one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems``
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is equal to ``30``. This is because the original use case is map pretty
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print where the whole array is dumped out so one-dimensional array
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is enough. As more BTF usage is explored, pahole and llvm can be
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changed to generate proper chained representation for
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multiple dimensional arrays.
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changed to generate proper chained representation for multidimensional arrays.
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2.2.4 BTF_KIND_STRUCT
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~~~~~~~~~~~~~~~~~~~~~
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@ -382,7 +380,7 @@ No additional type data follow ``btf_type``.
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No additional type data follow ``btf_type``.
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A BTF_KIND_FUNC defines, not a type, but a subprogram (function) whose
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A BTF_KIND_FUNC defines not a type, but a subprogram (function) whose
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signature is defined by ``type``. The subprogram is thus an instance of
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that type. The BTF_KIND_FUNC may in turn be referenced by a func_info in
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the :ref:`BTF_Ext_Section` (ELF) or in the arguments to
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@ -459,10 +457,10 @@ The workflow typically looks like:
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3.1 BPF_BTF_LOAD
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================
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Load a blob of BTF data into kernel. A blob of data
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described in :ref:`BTF_Type_String`
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Load a blob of BTF data into kernel. A blob of data,
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described in :ref:`BTF_Type_String`,
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can be directly loaded into the kernel.
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A ``btf_fd`` returns to userspace.
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A ``btf_fd`` is returned to a userspace.
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3.2 BPF_MAP_CREATE
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==================
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@ -487,7 +485,7 @@ In libbpf, the map can be defined with extra annotation like below:
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Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name,
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key and value types for the map.
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During ELF parsing, libbpf is able to extract key/value type_id's
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and assigned them to BPF_MAP_CREATE attributes automatically.
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and assign them to BPF_MAP_CREATE attributes automatically.
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.. _BPF_Prog_Load:
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@ -532,7 +530,7 @@ Below are requirements for func_info:
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bpf func boundaries.
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Below are requirements for line_info:
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* the first insn in each func must points to a line_info record.
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* the first insn in each func must have a line_info record pointing to it.
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* the line_info insn_off is in strictly increasing order.
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For line_info, the line number and column number are defined as below:
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@ -544,26 +542,26 @@ For line_info, the line number and column number are defined as below:
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3.4 BPF_{PROG,MAP}_GET_NEXT_ID
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In kernel, every loaded program, map or btf has a unique id.
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The id won't change during the life time of the program, map or btf.
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The id won't change during the lifetime of a program, map, or btf.
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The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID
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returns all id's, one for each command, to user space, for bpf
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program or maps,
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so the inspection tool can inspect all programs and maps.
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program or maps, respectively,
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so an inspection tool can inspect all programs and maps.
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3.5 BPF_{PROG,MAP}_GET_FD_BY_ID
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The introspection tool cannot use id to get details about program or maps.
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A file descriptor needs to be obtained first for reference counting purpose.
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An introspection tool cannot use id to get details about program or maps.
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A file descriptor needs to be obtained first for reference-counting purpose.
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3.6 BPF_OBJ_GET_INFO_BY_FD
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==========================
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Once a program/map fd is acquired, the introspection tool can
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Once a program/map fd is acquired, an introspection tool can
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get the detailed information from kernel about this fd,
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some of which is btf related. For example,
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``bpf_map_info`` returns ``btf_id``, key/value type id.
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``bpf_prog_info`` returns ``btf_id``, func_info and line info
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some of which are BTF-related. For example,
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``bpf_map_info`` returns ``btf_id`` and key/value type ids.
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``bpf_prog_info`` returns ``btf_id``, func_info, and line info
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for translated bpf byte codes, and jited_line_info.
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3.7 BPF_BTF_GET_FD_BY_ID
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@ -574,9 +572,9 @@ bpf syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd.
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Then, with command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally
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loaded into the kernel with BPF_BTF_LOAD, can be retrieved.
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With the btf blob, ``bpf_map_info`` and ``bpf_prog_info``, the introspection
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With the btf blob, ``bpf_map_info``, and ``bpf_prog_info``, an introspection
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tool has full btf knowledge and is able to pretty print map key/values,
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dump func signatures, dump line info along with byte/jit codes.
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dump func signatures and line info, along with byte/jit codes.
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4. ELF File Format Interface
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****************************
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@ -625,8 +623,8 @@ The func_info is organized as below.::
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...
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``func_info_rec_size`` specifies the size of ``bpf_func_info`` structure
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when .BTF.ext is generated. btf_ext_info_sec, defined below, is
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the func_info for each specific ELF section.::
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when .BTF.ext is generated. ``btf_ext_info_sec``, defined below, is
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a collection of func_info for each specific ELF section.::
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struct btf_ext_info_sec {
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__u32 sec_name_off; /* offset to section name */
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@ -661,7 +659,7 @@ from the beginning of section (``btf_ext_info_sec->sec_name_off``).
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With BTF, the map key/value can be printed based on fields rather than
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simply raw bytes. This is especially
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valuable for large structure or if you data structure
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valuable for large structure or if your data structure
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has bitfields. For example, for the following map,::
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enum A { A1, A2, A3, A4, A5 };
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@ -702,8 +700,8 @@ bpftool is able to pretty print like below:
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5.2 bpftool prog dump
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=====================
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The following is an example to show func_info and line_info
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can help prog dump with better kernel symbol name, function prototype
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The following is an example showing how func_info and line_info
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can help prog dump with better kernel symbol names, function prototypes
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and line information.::
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$ bpftool prog dump jited pinned /sys/fs/bpf/test_btf_haskv
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@ -733,10 +731,10 @@ and line information.::
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; counts = bpf_map_lookup_elem(&btf_map, &key);
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[...]
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5.3 verifier log
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5.3 Verifier Log
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================
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The following is an example how line_info can help verifier failure debug.::
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The following is an example of how line_info can help debugging verification failure.::
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/* The code at tools/testing/selftests/bpf/test_xdp_noinline.c
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* is modified as below.
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@ -867,4 +865,4 @@ The assembly code (-S) is able to show the BTF encoding in assembly format.::
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7. Testing
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**********
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Kernel bpf selftest `test_btf.c` provides extensive set of BTF related tests.
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Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests.
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