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diff --git a/Documentation/bpf/llvm_reloc.rst b/Documentation/bpf/llvm_reloc.rst new file mode 100644 index 000000000000..44188e219d32 --- /dev/null +++ b/Documentation/bpf/llvm_reloc.rst @@ -0,0 +1,546 @@ +.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) + +==================== +BPF LLVM Relocations +==================== + +This document describes LLVM BPF backend relocation types. + +Relocation Record +================= + +LLVM BPF backend records each relocation with the following 16-byte +ELF structure:: + + typedef struct + { + Elf64_Addr r_offset; // Offset from the beginning of section. + Elf64_Xword r_info; // Relocation type and symbol index. + } Elf64_Rel; + +For example, for the following code:: + + int g1 __attribute__((section("sec"))); + int g2 __attribute__((section("sec"))); + static volatile int l1 __attribute__((section("sec"))); + static volatile int l2 __attribute__((section("sec"))); + int test() { + return g1 + g2 + l1 + l2; + } + +Compiled with ``clang --target=bpf -O2 -c test.c``, the following is +the code with ``llvm-objdump -dr test.o``:: + + 0: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll + 0000000000000000: R_BPF_64_64 g1 + 2: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0) + 3: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll + 0000000000000018: R_BPF_64_64 g2 + 5: 61 20 00 00 00 00 00 00 r0 = *(u32 *)(r2 + 0) + 6: 0f 10 00 00 00 00 00 00 r0 += r1 + 7: 18 01 00 00 08 00 00 00 00 00 00 00 00 00 00 00 r1 = 8 ll + 0000000000000038: R_BPF_64_64 sec + 9: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0) + 10: 0f 10 00 00 00 00 00 00 r0 += r1 + 11: 18 01 00 00 0c 00 00 00 00 00 00 00 00 00 00 00 r1 = 12 ll + 0000000000000058: R_BPF_64_64 sec + 13: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0) + 14: 0f 10 00 00 00 00 00 00 r0 += r1 + 15: 95 00 00 00 00 00 00 00 exit + +There are four relocations in the above for four ``LD_imm64`` instructions. +The following ``llvm-readelf -r test.o`` shows the binary values of the four +relocations:: + + Relocation section '.rel.text' at offset 0x190 contains 4 entries: + Offset Info Type Symbol's Value Symbol's Name + 0000000000000000 0000000600000001 R_BPF_64_64 0000000000000000 g1 + 0000000000000018 0000000700000001 R_BPF_64_64 0000000000000004 g2 + 0000000000000038 0000000400000001 R_BPF_64_64 0000000000000000 sec + 0000000000000058 0000000400000001 R_BPF_64_64 0000000000000000 sec + +Each relocation is represented by ``Offset`` (8 bytes) and ``Info`` (8 bytes). +For example, the first relocation corresponds to the first instruction +(Offset 0x0) and the corresponding ``Info`` indicates the relocation type +of ``R_BPF_64_64`` (type 1) and the entry in the symbol table (entry 6). +The following is the symbol table with ``llvm-readelf -s test.o``:: + + Symbol table '.symtab' contains 8 entries: + Num: Value Size Type Bind Vis Ndx Name + 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND + 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS test.c + 2: 0000000000000008 4 OBJECT LOCAL DEFAULT 4 l1 + 3: 000000000000000c 4 OBJECT LOCAL DEFAULT 4 l2 + 4: 0000000000000000 0 SECTION LOCAL DEFAULT 4 sec + 5: 0000000000000000 128 FUNC GLOBAL DEFAULT 2 test + 6: 0000000000000000 4 OBJECT GLOBAL DEFAULT 4 g1 + 7: 0000000000000004 4 OBJECT GLOBAL DEFAULT 4 g2 + +The 6th entry is global variable ``g1`` with value 0. + +Similarly, the second relocation is at ``.text`` offset ``0x18``, instruction 3, +has a type of ``R_BPF_64_64`` and refers to entry 7 in the symbol table. +The second relocation resolves to global variable ``g2`` which has a symbol +value 4. The symbol value represents the offset from the start of ``.data`` +section where the initial value of the global variable ``g2`` is stored. + +The third and fourth relocations refer to static variables ``l1`` +and ``l2``. From the ``.rel.text`` section above, it is not clear +to which symbols they really refer as they both refer to +symbol table entry 4, symbol ``sec``, which has ``STT_SECTION`` type +and represents a section. So for a static variable or function, +the section offset is written to the original insn +buffer, which is called ``A`` (addend). Looking at +above insn ``7`` and ``11``, they have section offset ``8`` and ``12``. +From symbol table, we can find that they correspond to entries ``2`` +and ``3`` for ``l1`` and ``l2``. + +In general, the ``A`` is 0 for global variables and functions, +and is the section offset or some computation result based on +section offset for static variables/functions. The non-section-offset +case refers to function calls. See below for more details. + +Different Relocation Types +========================== + +Six relocation types are supported. The following is an overview and +``S`` represents the value of the symbol in the symbol table:: + + Enum ELF Reloc Type Description BitSize Offset Calculation + 0 R_BPF_NONE None + 1 R_BPF_64_64 ld_imm64 insn 32 r_offset + 4 S + A + 2 R_BPF_64_ABS64 normal data 64 r_offset S + A + 3 R_BPF_64_ABS32 normal data 32 r_offset S + A + 4 R_BPF_64_NODYLD32 .BTF[.ext] data 32 r_offset S + A + 10 R_BPF_64_32 call insn 32 r_offset + 4 (S + A) / 8 - 1 + +For example, ``R_BPF_64_64`` relocation type is used for ``ld_imm64`` instruction. +The actual to-be-relocated data (0 or section offset) +is stored at ``r_offset + 4`` and the read/write +data bitsize is 32 (4 bytes). The relocation can be resolved with +the symbol value plus implicit addend. Note that the ``BitSize`` is 32 which +means the section offset must be less than or equal to ``UINT32_MAX`` and this +is enforced by LLVM BPF backend. + +In another case, ``R_BPF_64_ABS64`` relocation type is used for normal 64-bit data. +The actual to-be-relocated data is stored at ``r_offset`` and the read/write data +bitsize is 64 (8 bytes). The relocation can be resolved with +the symbol value plus implicit addend. + +Both ``R_BPF_64_ABS32`` and ``R_BPF_64_NODYLD32`` types are for 32-bit data. +But ``R_BPF_64_NODYLD32`` specifically refers to relocations in ``.BTF`` and +``.BTF.ext`` sections. For cases like bcc where llvm ``ExecutionEngine RuntimeDyld`` +is involved, ``R_BPF_64_NODYLD32`` types of relocations should not be resolved +to actual function/variable address. Otherwise, ``.BTF`` and ``.BTF.ext`` +become unusable by bcc and kernel. + +Type ``R_BPF_64_32`` is used for call instruction. The call target section +offset is stored at ``r_offset + 4`` (32bit) and calculated as +``(S + A) / 8 - 1``. + +Examples +======== + +Types ``R_BPF_64_64`` and ``R_BPF_64_32`` are used to resolve ``ld_imm64`` +and ``call`` instructions. For example:: + + __attribute__((noinline)) __attribute__((section("sec1"))) + int gfunc(int a, int b) { + return a * b; + } + static __attribute__((noinline)) __attribute__((section("sec1"))) + int lfunc(int a, int b) { + return a + b; + } + int global __attribute__((section("sec2"))); + int test(int a, int b) { + return gfunc(a, b) + lfunc(a, b) + global; + } + +Compiled with ``clang --target=bpf -O2 -c test.c``, we will have +following code with `llvm-objdump -dr test.o``:: + + Disassembly of section .text: + + 0000000000000000 <test>: + 0: bf 26 00 00 00 00 00 00 r6 = r2 + 1: bf 17 00 00 00 00 00 00 r7 = r1 + 2: 85 10 00 00 ff ff ff ff call -1 + 0000000000000010: R_BPF_64_32 gfunc + 3: bf 08 00 00 00 00 00 00 r8 = r0 + 4: bf 71 00 00 00 00 00 00 r1 = r7 + 5: bf 62 00 00 00 00 00 00 r2 = r6 + 6: 85 10 00 00 02 00 00 00 call 2 + 0000000000000030: R_BPF_64_32 sec1 + 7: 0f 80 00 00 00 00 00 00 r0 += r8 + 8: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll + 0000000000000040: R_BPF_64_64 global + 10: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0) + 11: 0f 10 00 00 00 00 00 00 r0 += r1 + 12: 95 00 00 00 00 00 00 00 exit + + Disassembly of section sec1: + + 0000000000000000 <gfunc>: + 0: bf 20 00 00 00 00 00 00 r0 = r2 + 1: 2f 10 00 00 00 00 00 00 r0 *= r1 + 2: 95 00 00 00 00 00 00 00 exit + + 0000000000000018 <lfunc>: + 3: bf 20 00 00 00 00 00 00 r0 = r2 + 4: 0f 10 00 00 00 00 00 00 r0 += r1 + 5: 95 00 00 00 00 00 00 00 exit + +The first relocation corresponds to ``gfunc(a, b)`` where ``gfunc`` has a value of 0, +so the ``call`` instruction offset is ``(0 + 0)/8 - 1 = -1``. +The second relocation corresponds to ``lfunc(a, b)`` where ``lfunc`` has a section +offset ``0x18``, so the ``call`` instruction offset is ``(0 + 0x18)/8 - 1 = 2``. +The third relocation corresponds to ld_imm64 of ``global``, which has a section +offset ``0``. + +The following is an example to show how R_BPF_64_ABS64 could be generated:: + + int global() { return 0; } + struct t { void *g; } gbl = { global }; + +Compiled with ``clang --target=bpf -O2 -g -c test.c``, we will see a +relocation below in ``.data`` section with command +``llvm-readelf -r test.o``:: + + Relocation section '.rel.data' at offset 0x458 contains 1 entries: + Offset Info Type Symbol's Value Symbol's Name + 0000000000000000 0000000700000002 R_BPF_64_ABS64 0000000000000000 global + +The relocation says the first 8-byte of ``.data`` section should be +filled with address of ``global`` variable. + +With ``llvm-readelf`` output, we can see that dwarf sections have a bunch of +``R_BPF_64_ABS32`` and ``R_BPF_64_ABS64`` relocations:: + + Relocation section '.rel.debug_info' at offset 0x468 contains 13 entries: + Offset Info Type Symbol's Value Symbol's Name + 0000000000000006 0000000300000003 R_BPF_64_ABS32 0000000000000000 .debug_abbrev + 000000000000000c 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str + 0000000000000012 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str + 0000000000000016 0000000600000003 R_BPF_64_ABS32 0000000000000000 .debug_line + 000000000000001a 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str + 000000000000001e 0000000200000002 R_BPF_64_ABS64 0000000000000000 .text + 000000000000002b 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str + 0000000000000037 0000000800000002 R_BPF_64_ABS64 0000000000000000 gbl + 0000000000000040 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str + ...... + +The .BTF/.BTF.ext sections has R_BPF_64_NODYLD32 relocations:: + + Relocation section '.rel.BTF' at offset 0x538 contains 1 entries: + Offset Info Type Symbol's Value Symbol's Name + 0000000000000084 0000000800000004 R_BPF_64_NODYLD32 0000000000000000 gbl + + Relocation section '.rel.BTF.ext' at offset 0x548 contains 2 entries: + Offset Info Type Symbol's Value Symbol's Name + 000000000000002c 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text + 0000000000000040 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text + +.. _btf-co-re-relocations: + +================= +CO-RE Relocations +================= + +From object file point of view CO-RE mechanism is implemented as a set +of CO-RE specific relocation records. These relocation records are not +related to ELF relocations and are encoded in .BTF.ext section. +See :ref:`Documentation/bpf/btf.rst <BTF_Ext_Section>` for more +information on .BTF.ext structure. + +CO-RE relocations are applied to BPF instructions to update immediate +or offset fields of the instruction at load time with information +relevant for target kernel. + +Field to patch is selected basing on the instruction class: + +* For BPF_ALU, BPF_ALU64, BPF_LD `immediate` field is patched; +* For BPF_LDX, BPF_STX, BPF_ST `offset` field is patched; +* BPF_JMP, BPF_JMP32 instructions **should not** be patched. + +Relocation kinds +================ + +There are several kinds of CO-RE relocations that could be split in +three groups: + +* Field-based - patch instruction with field related information, e.g. + change offset field of the BPF_LDX instruction to reflect offset + of a specific structure field in the target kernel. + +* Type-based - patch instruction with type related information, e.g. + change immediate field of the BPF_ALU move instruction to 0 or 1 to + reflect if specific type is present in the target kernel. + +* Enum-based - patch instruction with enum related information, e.g. + change immediate field of the BPF_LD_IMM64 instruction to reflect + value of a specific enum literal in the target kernel. + +The complete list of relocation kinds is represented by the following enum: + +.. code-block:: c + + enum bpf_core_relo_kind { + BPF_CORE_FIELD_BYTE_OFFSET = 0, /* field byte offset */ + BPF_CORE_FIELD_BYTE_SIZE = 1, /* field size in bytes */ + BPF_CORE_FIELD_EXISTS = 2, /* field existence in target kernel */ + BPF_CORE_FIELD_SIGNED = 3, /* field signedness (0 - unsigned, 1 - signed) */ + BPF_CORE_FIELD_LSHIFT_U64 = 4, /* bitfield-specific left bitshift */ + BPF_CORE_FIELD_RSHIFT_U64 = 5, /* bitfield-specific right bitshift */ + BPF_CORE_TYPE_ID_LOCAL = 6, /* type ID in local BPF object */ + BPF_CORE_TYPE_ID_TARGET = 7, /* type ID in target kernel */ + BPF_CORE_TYPE_EXISTS = 8, /* type existence in target kernel */ + BPF_CORE_TYPE_SIZE = 9, /* type size in bytes */ + BPF_CORE_ENUMVAL_EXISTS = 10, /* enum value existence in target kernel */ + BPF_CORE_ENUMVAL_VALUE = 11, /* enum value integer value */ + BPF_CORE_TYPE_MATCHES = 12, /* type match in target kernel */ + }; + +Notes: + +* ``BPF_CORE_FIELD_LSHIFT_U64`` and ``BPF_CORE_FIELD_RSHIFT_U64`` are + supposed to be used to read bitfield values using the following + algorithm: + + .. code-block:: c + + // To read bitfield ``f`` from ``struct s`` + is_signed = relo(s->f, BPF_CORE_FIELD_SIGNED) + off = relo(s->f, BPF_CORE_FIELD_BYTE_OFFSET) + sz = relo(s->f, BPF_CORE_FIELD_BYTE_SIZE) + l = relo(s->f, BPF_CORE_FIELD_LSHIFT_U64) + r = relo(s->f, BPF_CORE_FIELD_RSHIFT_U64) + // define ``v`` as signed or unsigned integer of size ``sz`` + v = *({s|u}<sz> *)((void *)s + off) + v <<= l + v >>= r + +* The ``BPF_CORE_TYPE_MATCHES`` queries matching relation, defined as + follows: + + * for integers: types match if size and signedness match; + * for arrays & pointers: target types are recursively matched; + * for structs & unions: + + * local members need to exist in target with the same name; + + * for each member we recursively check match unless it is already behind a + pointer, in which case we only check matching names and compatible kind; + + * for enums: + + * local variants have to have a match in target by symbolic name (but not + numeric value); + + * size has to match (but enum may match enum64 and vice versa); + + * for function pointers: + + * number and position of arguments in local type has to match target; + * for each argument and the return value we recursively check match. + +CO-RE Relocation Record +======================= + +Relocation record is encoded as the following structure: + +.. code-block:: c + + struct bpf_core_relo { + __u32 insn_off; + __u32 type_id; + __u32 access_str_off; + enum bpf_core_relo_kind kind; + }; + +* ``insn_off`` - instruction offset (in bytes) within a code section + associated with this relocation; + +* ``type_id`` - BTF type ID of the "root" (containing) entity of a + relocatable type or field; + +* ``access_str_off`` - offset into corresponding .BTF string section. + String interpretation depends on specific relocation kind: + + * for field-based relocations, string encodes an accessed field using + a sequence of field and array indices, separated by colon (:). It's + conceptually very close to LLVM's `getelementptr <GEP_>`_ instruction's + arguments for identifying offset to a field. For example, consider the + following C code: + + .. code-block:: c + + struct sample { + int a; + int b; + struct { int c[10]; }; + } __attribute__((preserve_access_index)); + struct sample *s; + + * Access to ``s[0].a`` would be encoded as ``0:0``: + + * ``0``: first element of ``s`` (as if ``s`` is an array); + * ``0``: index of field ``a`` in ``struct sample``. + + * Access to ``s->a`` would be encoded as ``0:0`` as well. + * Access to ``s->b`` would be encoded as ``0:1``: + + * ``0``: first element of ``s``; + * ``1``: index of field ``b`` in ``struct sample``. + + * Access to ``s[1].c[5]`` would be encoded as ``1:2:0:5``: + + * ``1``: second element of ``s``; + * ``2``: index of anonymous structure field in ``struct sample``; + * ``0``: index of field ``c`` in anonymous structure; + * ``5``: access to array element #5. + + * for type-based relocations, string is expected to be just "0"; + + * for enum value-based relocations, string contains an index of enum + value within its enum type; + +* ``kind`` - one of ``enum bpf_core_relo_kind``. + +.. _GEP: https://llvm.org/docs/LangRef.html#getelementptr-instruction + +.. _btf_co_re_relocation_examples: + +CO-RE Relocation Examples +========================= + +For the following C code: + +.. code-block:: c + + struct foo { + int a; + int b; + unsigned c:15; + } __attribute__((preserve_access_index)); + + enum bar { U, V }; + +With the following BTF definitions: + +.. code-block:: + + ... + [2] STRUCT 'foo' size=8 vlen=2 + 'a' type_id=3 bits_offset=0 + 'b' type_id=3 bits_offset=32 + 'c' type_id=4 bits_offset=64 bitfield_size=15 + [3] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED + [4] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none) + ... + [16] ENUM 'bar' encoding=UNSIGNED size=4 vlen=2 + 'U' val=0 + 'V' val=1 + +Field offset relocations are generated automatically when +``__attribute__((preserve_access_index))`` is used, for example: + +.. code-block:: c + + void alpha(struct foo *s, volatile unsigned long *g) { + *g = s->a; + s->a = 1; + } + + 00 <alpha>: + 0: r3 = *(s32 *)(r1 + 0x0) + 00: CO-RE <byte_off> [2] struct foo::a (0:0) + 1: *(u64 *)(r2 + 0x0) = r3 + 2: *(u32 *)(r1 + 0x0) = 0x1 + 10: CO-RE <byte_off> [2] struct foo::a (0:0) + 3: exit + + +All relocation kinds could be requested via built-in functions. +E.g. field-based relocations: + +.. code-block:: c + + void bravo(struct foo *s, volatile unsigned long *g) { + *g = __builtin_preserve_field_info(s->b, 0 /* field byte offset */); + *g = __builtin_preserve_field_info(s->b, 1 /* field byte size */); + *g = __builtin_preserve_field_info(s->b, 2 /* field existence */); + *g = __builtin_preserve_field_info(s->b, 3 /* field signedness */); + *g = __builtin_preserve_field_info(s->c, 4 /* bitfield left shift */); + *g = __builtin_preserve_field_info(s->c, 5 /* bitfield right shift */); + } + + 20 <bravo>: + 4: r1 = 0x4 + 20: CO-RE <byte_off> [2] struct foo::b (0:1) + 5: *(u64 *)(r2 + 0x0) = r1 + 6: r1 = 0x4 + 30: CO-RE <byte_sz> [2] struct foo::b (0:1) + 7: *(u64 *)(r2 + 0x0) = r1 + 8: r1 = 0x1 + 40: CO-RE <field_exists> [2] struct foo::b (0:1) + 9: *(u64 *)(r2 + 0x0) = r1 + 10: r1 = 0x1 + 50: CO-RE <signed> [2] struct foo::b (0:1) + 11: *(u64 *)(r2 + 0x0) = r1 + 12: r1 = 0x31 + 60: CO-RE <lshift_u64> [2] struct foo::c (0:2) + 13: *(u64 *)(r2 + 0x0) = r1 + 14: r1 = 0x31 + 70: CO-RE <rshift_u64> [2] struct foo::c (0:2) + 15: *(u64 *)(r2 + 0x0) = r1 + 16: exit + + +Type-based relocations: + +.. code-block:: c + + void charlie(struct foo *s, volatile unsigned long *g) { + *g = __builtin_preserve_type_info(*s, 0 /* type existence */); + *g = __builtin_preserve_type_info(*s, 1 /* type size */); + *g = __builtin_preserve_type_info(*s, 2 /* type matches */); + *g = __builtin_btf_type_id(*s, 0 /* type id in this object file */); + *g = __builtin_btf_type_id(*s, 1 /* type id in target kernel */); + } + + 88 <charlie>: + 17: r1 = 0x1 + 88: CO-RE <type_exists> [2] struct foo + 18: *(u64 *)(r2 + 0x0) = r1 + 19: r1 = 0xc + 98: CO-RE <type_size> [2] struct foo + 20: *(u64 *)(r2 + 0x0) = r1 + 21: r1 = 0x1 + a8: CO-RE <type_matches> [2] struct foo + 22: *(u64 *)(r2 + 0x0) = r1 + 23: r1 = 0x2 ll + b8: CO-RE <local_type_id> [2] struct foo + 25: *(u64 *)(r2 + 0x0) = r1 + 26: r1 = 0x2 ll + d0: CO-RE <target_type_id> [2] struct foo + 28: *(u64 *)(r2 + 0x0) = r1 + 29: exit + +Enum-based relocations: + +.. code-block:: c + + void delta(struct foo *s, volatile unsigned long *g) { + *g = __builtin_preserve_enum_value(*(enum bar *)U, 0 /* enum literal existence */); + *g = __builtin_preserve_enum_value(*(enum bar *)V, 1 /* enum literal value */); + } + + f0 <delta>: + 30: r1 = 0x1 ll + f0: CO-RE <enumval_exists> [16] enum bar::U = 0 + 32: *(u64 *)(r2 + 0x0) = r1 + 33: r1 = 0x1 ll + 108: CO-RE <enumval_value> [16] enum bar::V = 1 + 35: *(u64 *)(r2 + 0x0) = r1 + 36: exit |