docs/btf: reflow text to fill up to 78 characters
Reflow paragraphs to more fully and evenly fill 78 character lines. Signed-off-by: Andrii Nakryiko <andriin@fb.com> Acked-by: Yonghong Song <yhs@fb.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
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Andrii Nakryiko
Daniel Borkmann
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@ -5,43 +5,35 @@ BPF Type Format (BTF)
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1. Introduction
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***************
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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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function info for defined subroutines, and line info
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for source/line information.
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BTF (BPF Type Format) is the metadata format which encodes the debug info
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related to BPF program/map. The name BTF was used initially to describe data
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types. The BTF was later extended to include function info for defined
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subroutines, and line info for source/line information.
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The debug info is used for map pretty print, function
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signature, etc. The function signature enables better
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bpf program/function kernel symbol.
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The line info helps generate
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source annotated translated byte code, jited code
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and verifier log.
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The debug info is used for map pretty print, function signature, etc. The
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function signature enables better bpf program/function kernel symbol. The line
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info helps generate source annotated translated byte code, jited code and
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verifier log.
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The BTF specification contains two parts,
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* BTF kernel API
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* BTF ELF file format
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The kernel API is the contract between
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user space and kernel. The kernel verifies
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the BTF info before using it.
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The ELF file format is a user space contract
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between ELF file and libbpf loader.
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The kernel API is the contract between user space and kernel. The kernel
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verifies the BTF info before using it. The ELF file format is a user space
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contract between ELF file and libbpf loader.
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The type and string sections are part of the
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BTF kernel API, describing the debug info
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(mostly types related) referenced by the bpf program.
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These two sections are discussed in
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details in :ref:`BTF_Type_String`.
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The type and string sections are part of the BTF kernel API, describing the
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debug info (mostly types related) referenced by the bpf program. These two
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sections are discussed in details in :ref:`BTF_Type_String`.
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.. _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-level
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definition of how types/strings are encoded.
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The file ``include/uapi/linux/btf.h`` provides high-level definition of how
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types/strings are encoded.
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The beginning of data blob must be::
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@ -59,25 +51,23 @@ 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 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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endian systems, and can be used to test whether BTF is generated for big- or
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little-endian target. The ``btf_header`` is designed to be extensible with
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``hdr_len`` equal to ``sizeof(struct btf_header)`` when a data blob is
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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-terminated
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strings.
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The first string in the string section must be a null string. The rest of
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string table is a concatenation of other null-terminated 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 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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The type id ``0`` is reserved for ``void`` type. The type section is parsed
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sequentially and type id is assigned to each recognized type starting from id
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``1``. Currently, the following types are supported::
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#define BTF_KIND_INT 1 /* Integer */
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#define BTF_KIND_PTR 2 /* Pointer */
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@ -122,9 +112,9 @@ 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
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table. The following sections detail encoding of each kind.
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For certain kinds, the common data are followed by kind-specific data. The
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``name_off`` in ``struct btf_type`` specifies the offset in the string table.
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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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@ -148,38 +138,33 @@ 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: 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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The ``BTF_INT_ENCODING()`` provides extra information: signedness, char, or
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bool, for the int type. The char and bool encoding are mostly useful for
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pretty print. At most one encoding can be specified for the int type.
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The ``BTF_INT_BITS()`` specifies the number of actual bits held by
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this int type. For example, a 4-bit bitfield encodes
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``BTF_INT_BITS()`` equals to 4. The ``btf_type.size * 8``
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must be equal to or greater than ``BTF_INT_BITS()`` for the type.
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The maximum value of ``BTF_INT_BITS()`` is 128.
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The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
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type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
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The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
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for the type. 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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* 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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The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
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for this int. For example, a bitfield struct member has: * btf member bit
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offset 100 from the start of the structure, * btf member pointing to an int
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type, * the int type has ``BTF_INT_OFFSET() = 2`` and ``BTF_INT_BITS() = 4``
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Then in the struct memory layout, this member will occupy
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``4`` bits starting from bits ``100 + 2 = 102``.
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Then in the struct memory layout, this member will occupy ``4`` bits starting
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from bits ``100 + 2 = 102``.
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Alternatively, the bitfield struct member can be the following to
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access the same bits as the above:
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Alternatively, the bitfield struct member can be the following to access the
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same bits as the above:
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* btf member bit offset 102,
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* btf member pointing to an int type,
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* the int type has ``BTF_INT_OFFSET() = 0`` and ``BTF_INT_BITS() = 4``
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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 generate ``BTF_INT_OFFSET() = 0``
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for all int types.
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The original intention of ``BTF_INT_OFFSET()`` is to provide flexibility of
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bitfield encoding. Currently, both llvm and pahole generate
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``BTF_INT_OFFSET() = 0`` for all int types.
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2.2.2 BTF_KIND_PTR
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~~~~~~~~~~~~~~~~~~
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@ -216,26 +201,25 @@ 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 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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The ``index_type`` can be any regular int type (``u8``, ``u16``, ``u32``,
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``u64``, ``unsigned __int128``). The original design of including
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``index_type`` follows DWARF, 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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multidimensional arrays. For example, for ``int a[5][6]``, the following
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type information illustrates the chaining:
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multidimensional arrays. For example, for ``int a[5][6]``, the following type
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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 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 multidimensional arrays.
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Currently, both pahole and llvm collapse multidimensional array into
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one-dimensional array, e.g., for ``a[5][6]``, the ``btf_array.nelems`` is
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equal to ``30``. This is because the original use case is map pretty print
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where the whole array is dumped out so one-dimensional array is enough. As
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more BTF usage is explored, pahole and llvm can be changed to generate proper
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chained representation for multidimensional arrays.
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2.2.4 BTF_KIND_STRUCT
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~~~~~~~~~~~~~~~~~~~~~
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@ -262,28 +246,26 @@ changed to generate proper chained representation for multidimensional arrays.
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* ``type``: the member type
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* ``offset``: <see below>
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If the type info ``kind_flag`` is not set, the offset contains
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only bit offset of the member. Note that the base type of the
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bitfield can only be int or enum type. If the bitfield size
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is 32, the base type can be either int or enum type.
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If the bitfield size is not 32, the base type must be int,
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and int type ``BTF_INT_BITS()`` encodes the bitfield size.
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If the type info ``kind_flag`` is not set, the offset contains only bit offset
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of the member. Note that the base type of the bitfield can only be int or enum
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type. If the bitfield size is 32, the base type can be either int or enum
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type. If the bitfield size is not 32, the base type must be int, and int type
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``BTF_INT_BITS()`` encodes the bitfield size.
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If the ``kind_flag`` is set, the ``btf_member.offset``
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contains both member bitfield size and bit offset. The
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bitfield size and bit offset are calculated as below.::
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If the ``kind_flag`` is set, the ``btf_member.offset`` contains both member
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bitfield size and bit offset. The bitfield size and bit offset are calculated
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as below.::
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#define BTF_MEMBER_BITFIELD_SIZE(val) ((val) >> 24)
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#define BTF_MEMBER_BIT_OFFSET(val) ((val) & 0xffffff)
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In this case, if the base type is an int type, it must
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be a regular int type:
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In this case, if the base type is an int type, it must be a regular int type:
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* ``BTF_INT_OFFSET()`` must be 0.
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* ``BTF_INT_BITS()`` must be equal to ``{1,2,4,8,16} * 8``.
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The following kernel patch introduced ``kind_flag`` and
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explained why both modes exist:
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The following kernel patch introduced ``kind_flag`` and explained why both
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modes exist:
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https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3#diff-fa650a64fdd3968396883d2fe8215ff3
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@ -381,10 +363,10 @@ 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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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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:ref:`BPF_Prog_Load` (ABI).
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signature is defined by ``type``. The subprogram is thus an instance of that
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type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the
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:ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load`
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(ABI).
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2.2.13 BTF_KIND_FUNC_PROTO
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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@ -403,13 +385,13 @@ the :ref:`BTF_Ext_Section` (ELF) or in the arguments to
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__u32 type;
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};
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If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type,
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then ``btf_param.name_off`` must point to a valid C identifier
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except for the possible last argument representing the variable
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argument. The btf_param.type refers to parameter type.
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If a BTF_KIND_FUNC_PROTO type is referred by a BTF_KIND_FUNC type, then
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``btf_param.name_off`` must point to a valid C identifier except for the
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possible last argument representing the variable argument. The btf_param.type
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refers to parameter type.
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If the function has variable arguments, the last parameter
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is encoded with ``name_off = 0`` and ``type = 0``.
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If the function has variable arguments, the last parameter is encoded with
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``name_off = 0`` and ``type = 0``.
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3. BTF Kernel API
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*****************
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@ -457,10 +439,9 @@ 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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can be directly loaded into the kernel.
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A ``btf_fd`` is returned to a userspace.
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Load a blob of BTF data into kernel. A blob of data, described in
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:ref:`BTF_Type_String`, can be directly loaded into the kernel. A ``btf_fd``
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is returned to a userspace.
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3.2 BPF_MAP_CREATE
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==================
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@ -482,18 +463,18 @@ In libbpf, the map can be defined with extra annotation like below:
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};
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BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts);
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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 assign them to BPF_MAP_CREATE attributes automatically.
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Here, the parameters for macro BPF_ANNOTATE_KV_PAIR are map name, key and
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value types for the map. During ELF parsing, libbpf is able to extract
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key/value type_id's and assign them to BPF_MAP_CREATE attributes
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automatically.
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.. _BPF_Prog_Load:
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3.3 BPF_PROG_LOAD
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=================
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During prog_load, func_info and line_info can be passed to kernel with
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proper values for the following attributes:
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During prog_load, func_info and line_info can be passed to kernel with proper
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values for the following attributes:
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::
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__u32 insn_cnt;
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@ -520,9 +501,9 @@ The func_info and line_info are an array of below, respectively.::
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__u32 line_col; /* line number and column number */
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};
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func_info_rec_size is the size of each func_info record, and line_info_rec_size
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is the size of each line_info record. Passing the record size to kernel make
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it possible to extend the record itself in the future.
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func_info_rec_size is the size of each func_info record, and
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line_info_rec_size is the size of each line_info record. Passing the record
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size to kernel make it possible to extend the record itself in the future.
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Below are requirements for func_info:
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* func_info[0].insn_off must be 0.
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@ -541,13 +522,12 @@ 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 lifetime of a program, map, or btf.
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In kernel, every loaded program, map or btf has a unique id. The id won't
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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, respectively,
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so an inspection tool can inspect all programs and maps.
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The bpf syscall command BPF_{PROG,MAP}_GET_NEXT_ID returns all id's, one for
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each command, to user space, for bpf program or maps, respectively, so an
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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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@ -557,24 +537,23 @@ 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, an introspection tool can
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get the detailed information from kernel about this fd,
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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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Once a program/map fd is acquired, an introspection tool can get the detailed
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information from kernel about this fd, some of which are BTF-related. For
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example, ``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 for translated
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bpf byte codes, and jited_line_info.
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3.7 BPF_BTF_GET_FD_BY_ID
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========================
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With ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``,
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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 ``btf_id`` obtained in ``bpf_map_info`` and ``bpf_prog_info``, bpf
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syscall command BPF_BTF_GET_FD_BY_ID can retrieve a btf fd. Then, with
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command BPF_OBJ_GET_INFO_BY_FD, the btf blob, originally loaded into the
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kernel with BPF_BTF_LOAD, can be retrieved.
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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 and line info, along with byte/jit codes.
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tool has full btf knowledge and is able to pretty print map key/values, dump
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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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@ -582,19 +561,19 @@ dump func signatures and line info, along with byte/jit codes.
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4.1 .BTF section
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================
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||||
|
||||
The .BTF section contains type and string data. The format of this section
|
||||
is same as the one describe in :ref:`BTF_Type_String`.
|
||||
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 .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 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::
|
||||
|
||||
@ -611,9 +590,9 @@ The current header of .BTF.ext section::
|
||||
__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.
|
||||
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.::
|
||||
|
||||
@ -622,9 +601,9 @@ The func_info is organized as below.::
|
||||
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.::
|
||||
``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 */
|
||||
@ -642,14 +621,14 @@ The line_info is organized as below.::
|
||||
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.
|
||||
``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``).
|
||||
``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``).
|
||||
|
||||
5. Using BTF
|
||||
************
|
||||
@ -657,10 +636,9 @@ from the beginning of section (``btf_ext_info_sec->sec_name_off``).
|
||||
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,::
|
||||
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;
|
||||
@ -700,9 +678,9 @@ bpftool is able to pretty print like below:
|
||||
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.::
|
||||
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
|
||||
[...]
|
||||
@ -734,7 +712,8 @@ and line information.::
|
||||
5.3 Verifier Log
|
||||
================
|
||||
|
||||
The following is an example of how line_info can help debugging verification failure.::
|
||||
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.
|
||||
@ -763,8 +742,8 @@ 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,::
|
||||
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 {
|
||||
@ -781,8 +760,9 @@ and btf BTF_KIND_FUNC type yet. For example,::
|
||||
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.::
|
||||
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;
|
||||
|
||||
Loading…
Reference in New Issue
Block a user