@@ -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.