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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-11 08:27:49 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-11 08:27:49 +0000
commitace9429bb58fd418f0c81d4c2835699bddf6bde6 (patch)
treeb2d64bc10158fdd5497876388cd68142ca374ed3 /Documentation/bpf/llvm_reloc.rst
parentInitial commit. (diff)
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Adding upstream version 6.6.15.upstream/6.6.15
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+.. 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