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Date: Tue, 15 Jan 2019 16:58:40 -0800
From: Yonghong Song <yhs@...com>
To: <ast@...com>, <daniel@...earbox.net>, <netdev@...r.kernel.org>
CC: <kernel-team@...com>
Subject: [PATCH bpf-next] bpf: btf: add btf documentation
This patch added documentation for BTF (BPF Debug Format).
The document is placed under linux:Documentation/bpf directory.
Signed-off-by: Yonghong Song <yhs@...com>
---
Documentation/bpf/btf.rst | 787 ++++++++++++++++++++++++++++++++++++
Documentation/bpf/index.rst | 7 +
2 files changed, 794 insertions(+)
create mode 100644 Documentation/bpf/btf.rst
diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
new file mode 100644
index 000000000000..3dfa8edd22ac
--- /dev/null
+++ b/Documentation/bpf/btf.rst
@@ -0,0 +1,787 @@
+=====================
+BPF Type Format (BTF)
+=====================
+
+1. Introduction
+***************
+
+BTF (BPF Type Format) is the meta data 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 Section 2.
+
+2. BTF Type/String Encoding
+***************************
+
+The file ``include/uapi/linux/btf.h`` provides high
+level definition on 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 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 specifying
+the struct btf_header length when the data blob is generated.
+
+2.1 String Encoding
+===================
+
+The first byte of string section must be ``'\0'`` to represent a null string.
+The rest of string table is a cancatenation of other 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
+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 */
+
+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 details 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 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, signness,
+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_OFFSET()`` specifies the starting bit offset to
+calculate values for this int. Typically it should be 0 and
+currently both llvm and pahole generates ``BTF_INT_OFFSET() = 0``.
+
+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.
+
+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.
+
+For a multiple dimensional array, e.g., ``a[5][6]``, the btf_array.nelems
+equals ``30``. ``nelems = 0`` is also allowed.
+
+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``.
+
+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 to be redefined
+
+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 having volatile modifier
+
+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 having const modifier
+
+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 having restrict modifier
+
+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``.
+
+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 the variable arguments, the last parameter
+is encoded with ``name_off = 0`` and ``type = 0``.
+
+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 & BPF_PROG_LOAD
+ |
+ V
+ ......
+
+ Introspection tool:
+ ......
+ |
+ 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
+ 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 Section 2 can be directly loaded into the kernel.
+A ``btf_fd`` returns to 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 libbtf, if the map is specified like below in the bpf program:
+::
+
+ 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 assigned them to BPF_MAP_CREATE attributes automatically.
+
+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 points to a line_info record.
+ * 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_BTF_GET_FD_BY_ID
+========================
+
+ Given a btf id, a btf fd is returned.
+
+3.5 BPF_OBJ_GET_INFO_BY_FD
+==========================
+
+Users can get btf blob, bpf_map_info and bpf_prog_info.
+bpf_map_info returns btf_id, key/value type id.
+bpf_prog_info returns btf_id, func_info and line info
+for translated bpf byte codes, and jited_line_info.
+
+4. ELF File Format Interface
+****************************
+
+4.1 .BTF section
+================
+
+pahole currently generates .BTF section with the same format
+as described in Section 2. pahole doesn't generate
+BTF_KIND_FUNC yet.
+
+llvm generates two sections .BTF and .BTF.ext.
+The .BTF section has the same specification as in Section 2.
+The .BTF.ext section encodes func_info and line_info which
+needs loader manipulation before loading into the kernel.
+
+4.2 .BTF.ext section
+====================
+
+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.
+
+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
+the 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``).
+
+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 you 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 to show func_info and line_info
+can help prog dump with better ksym name, function prototype
+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 how line_info can help verifier failure debug.::
+
+ /* 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.
diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst
index 00a8450a602f..4e77932959cc 100644
--- a/Documentation/bpf/index.rst
+++ b/Documentation/bpf/index.rst
@@ -15,6 +15,13 @@ that goes into great technical depth about the BPF Architecture.
The primary info for the bpf syscall is available in the `man-pages`_
for `bpf(2)`_.
+BPF Type Format (BTF)
+=====================
+
+.. toctree::
+ :maxdepth: 1
+
+ btf
Frequently asked questions (FAQ)
--
2.17.1
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