Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 546 insertions, 0 deletions

diff --git a/Documentation/bpf/llvm_reloc.rst b/Documentation/bpf/llvm_reloc.rst
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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