241 lines
11 KiB
ReStructuredText
241 lines
11 KiB
ReStructuredText
.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
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====================
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BPF LLVM Relocations
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====================
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This document describes LLVM BPF backend relocation types.
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Relocation Record
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=================
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LLVM BPF backend records each relocation with the following 16-byte
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ELF structure::
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typedef struct
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{
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Elf64_Addr r_offset; // Offset from the beginning of section.
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Elf64_Xword r_info; // Relocation type and symbol index.
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} Elf64_Rel;
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For example, for the following code::
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int g1 __attribute__((section("sec")));
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int g2 __attribute__((section("sec")));
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static volatile int l1 __attribute__((section("sec")));
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static volatile int l2 __attribute__((section("sec")));
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int test() {
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return g1 + g2 + l1 + l2;
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}
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Compiled with ``clang -target bpf -O2 -c test.c``, the following is
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the code with ``llvm-objdump -dr test.o``::
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0: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
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0000000000000000: R_BPF_64_64 g1
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2: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
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3: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll
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0000000000000018: R_BPF_64_64 g2
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5: 61 20 00 00 00 00 00 00 r0 = *(u32 *)(r2 + 0)
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6: 0f 10 00 00 00 00 00 00 r0 += r1
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7: 18 01 00 00 08 00 00 00 00 00 00 00 00 00 00 00 r1 = 8 ll
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0000000000000038: R_BPF_64_64 sec
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9: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
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10: 0f 10 00 00 00 00 00 00 r0 += r1
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11: 18 01 00 00 0c 00 00 00 00 00 00 00 00 00 00 00 r1 = 12 ll
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0000000000000058: R_BPF_64_64 sec
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13: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
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14: 0f 10 00 00 00 00 00 00 r0 += r1
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15: 95 00 00 00 00 00 00 00 exit
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There are four relations in the above for four ``LD_imm64`` instructions.
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The following ``llvm-readelf -r test.o`` shows the binary values of the four
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relocations::
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Relocation section '.rel.text' at offset 0x190 contains 4 entries:
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Offset Info Type Symbol's Value Symbol's Name
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0000000000000000 0000000600000001 R_BPF_64_64 0000000000000000 g1
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0000000000000018 0000000700000001 R_BPF_64_64 0000000000000004 g2
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0000000000000038 0000000400000001 R_BPF_64_64 0000000000000000 sec
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0000000000000058 0000000400000001 R_BPF_64_64 0000000000000000 sec
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Each relocation is represented by ``Offset`` (8 bytes) and ``Info`` (8 bytes).
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For example, the first relocation corresponds to the first instruction
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(Offset 0x0) and the corresponding ``Info`` indicates the relocation type
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of ``R_BPF_64_64`` (type 1) and the entry in the symbol table (entry 6).
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The following is the symbol table with ``llvm-readelf -s test.o``::
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Symbol table '.symtab' contains 8 entries:
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Num: Value Size Type Bind Vis Ndx Name
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0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
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1: 0000000000000000 0 FILE LOCAL DEFAULT ABS test.c
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2: 0000000000000008 4 OBJECT LOCAL DEFAULT 4 l1
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3: 000000000000000c 4 OBJECT LOCAL DEFAULT 4 l2
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4: 0000000000000000 0 SECTION LOCAL DEFAULT 4 sec
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5: 0000000000000000 128 FUNC GLOBAL DEFAULT 2 test
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6: 0000000000000000 4 OBJECT GLOBAL DEFAULT 4 g1
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7: 0000000000000004 4 OBJECT GLOBAL DEFAULT 4 g2
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The 6th entry is global variable ``g1`` with value 0.
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Similarly, the second relocation is at ``.text`` offset ``0x18``, instruction 3,
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for global variable ``g2`` which has a symbol value 4, the offset
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from the start of ``.data`` section.
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The third and fourth relocations refers to static variables ``l1``
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and ``l2``. From ``.rel.text`` section above, it is not clear
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which symbols they really refers to as they both refers to
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symbol table entry 4, symbol ``sec``, which has ``STT_SECTION`` type
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and represents a section. So for static variable or function,
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the section offset is written to the original insn
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buffer, which is called ``A`` (addend). Looking at
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above insn ``7`` and ``11``, they have section offset ``8`` and ``12``.
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From symbol table, we can find that they correspond to entries ``2``
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and ``3`` for ``l1`` and ``l2``.
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In general, the ``A`` is 0 for global variables and functions,
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and is the section offset or some computation result based on
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section offset for static variables/functions. The non-section-offset
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case refers to function calls. See below for more details.
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Different Relocation Types
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==========================
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Six relocation types are supported. The following is an overview and
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``S`` represents the value of the symbol in the symbol table::
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Enum ELF Reloc Type Description BitSize Offset Calculation
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0 R_BPF_NONE None
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1 R_BPF_64_64 ld_imm64 insn 32 r_offset + 4 S + A
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2 R_BPF_64_ABS64 normal data 64 r_offset S + A
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3 R_BPF_64_ABS32 normal data 32 r_offset S + A
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4 R_BPF_64_NODYLD32 .BTF[.ext] data 32 r_offset S + A
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10 R_BPF_64_32 call insn 32 r_offset + 4 (S + A) / 8 - 1
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For example, ``R_BPF_64_64`` relocation type is used for ``ld_imm64`` instruction.
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The actual to-be-relocated data (0 or section offset)
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is stored at ``r_offset + 4`` and the read/write
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data bitsize is 32 (4 bytes). The relocation can be resolved with
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the symbol value plus implicit addend. Note that the ``BitSize`` is 32 which
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means the section offset must be less than or equal to ``UINT32_MAX`` and this
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is enforced by LLVM BPF backend.
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In another case, ``R_BPF_64_ABS64`` relocation type is used for normal 64-bit data.
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The actual to-be-relocated data is stored at ``r_offset`` and the read/write data
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bitsize is 64 (8 bytes). The relocation can be resolved with
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the symbol value plus implicit addend.
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Both ``R_BPF_64_ABS32`` and ``R_BPF_64_NODYLD32`` types are for 32-bit data.
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But ``R_BPF_64_NODYLD32`` specifically refers to relocations in ``.BTF`` and
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``.BTF.ext`` sections. For cases like bcc where llvm ``ExecutionEngine RuntimeDyld``
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is involved, ``R_BPF_64_NODYLD32`` types of relocations should not be resolved
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to actual function/variable address. Otherwise, ``.BTF`` and ``.BTF.ext``
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become unusable by bcc and kernel.
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Type ``R_BPF_64_32`` is used for call instruction. The call target section
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offset is stored at ``r_offset + 4`` (32bit) and calculated as
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``(S + A) / 8 - 1``.
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Examples
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========
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Types ``R_BPF_64_64`` and ``R_BPF_64_32`` are used to resolve ``ld_imm64``
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and ``call`` instructions. For example::
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__attribute__((noinline)) __attribute__((section("sec1")))
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int gfunc(int a, int b) {
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return a * b;
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}
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static __attribute__((noinline)) __attribute__((section("sec1")))
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int lfunc(int a, int b) {
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return a + b;
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}
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int global __attribute__((section("sec2")));
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int test(int a, int b) {
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return gfunc(a, b) + lfunc(a, b) + global;
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}
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Compiled with ``clang -target bpf -O2 -c test.c``, we will have
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following code with `llvm-objdump -dr test.o``::
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Disassembly of section .text:
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0000000000000000 <test>:
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0: bf 26 00 00 00 00 00 00 r6 = r2
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1: bf 17 00 00 00 00 00 00 r7 = r1
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2: 85 10 00 00 ff ff ff ff call -1
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0000000000000010: R_BPF_64_32 gfunc
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3: bf 08 00 00 00 00 00 00 r8 = r0
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4: bf 71 00 00 00 00 00 00 r1 = r7
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5: bf 62 00 00 00 00 00 00 r2 = r6
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6: 85 10 00 00 02 00 00 00 call 2
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0000000000000030: R_BPF_64_32 sec1
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7: 0f 80 00 00 00 00 00 00 r0 += r8
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8: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
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0000000000000040: R_BPF_64_64 global
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10: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
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11: 0f 10 00 00 00 00 00 00 r0 += r1
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12: 95 00 00 00 00 00 00 00 exit
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Disassembly of section sec1:
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0000000000000000 <gfunc>:
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0: bf 20 00 00 00 00 00 00 r0 = r2
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1: 2f 10 00 00 00 00 00 00 r0 *= r1
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2: 95 00 00 00 00 00 00 00 exit
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0000000000000018 <lfunc>:
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3: bf 20 00 00 00 00 00 00 r0 = r2
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4: 0f 10 00 00 00 00 00 00 r0 += r1
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5: 95 00 00 00 00 00 00 00 exit
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The first relocation corresponds to ``gfunc(a, b)`` where ``gfunc`` has a value of 0,
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so the ``call`` instruction offset is ``(0 + 0)/8 - 1 = -1``.
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The second relocation corresponds to ``lfunc(a, b)`` where ``lfunc`` has a section
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offset ``0x18``, so the ``call`` instruction offset is ``(0 + 0x18)/8 - 1 = 2``.
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The third relocation corresponds to ld_imm64 of ``global``, which has a section
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offset ``0``.
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The following is an example to show how R_BPF_64_ABS64 could be generated::
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int global() { return 0; }
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struct t { void *g; } gbl = { global };
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Compiled with ``clang -target bpf -O2 -g -c test.c``, we will see a
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relocation below in ``.data`` section with command
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``llvm-readelf -r test.o``::
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Relocation section '.rel.data' at offset 0x458 contains 1 entries:
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Offset Info Type Symbol's Value Symbol's Name
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0000000000000000 0000000700000002 R_BPF_64_ABS64 0000000000000000 global
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The relocation says the first 8-byte of ``.data`` section should be
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filled with address of ``global`` variable.
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With ``llvm-readelf`` output, we can see that dwarf sections have a bunch of
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``R_BPF_64_ABS32`` and ``R_BPF_64_ABS64`` relocations::
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Relocation section '.rel.debug_info' at offset 0x468 contains 13 entries:
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Offset Info Type Symbol's Value Symbol's Name
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0000000000000006 0000000300000003 R_BPF_64_ABS32 0000000000000000 .debug_abbrev
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000000000000000c 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
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0000000000000012 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
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0000000000000016 0000000600000003 R_BPF_64_ABS32 0000000000000000 .debug_line
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000000000000001a 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
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000000000000001e 0000000200000002 R_BPF_64_ABS64 0000000000000000 .text
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000000000000002b 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
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0000000000000037 0000000800000002 R_BPF_64_ABS64 0000000000000000 gbl
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0000000000000040 0000000400000003 R_BPF_64_ABS32 0000000000000000 .debug_str
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......
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The .BTF/.BTF.ext sections has R_BPF_64_NODYLD32 relocations::
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Relocation section '.rel.BTF' at offset 0x538 contains 1 entries:
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Offset Info Type Symbol's Value Symbol's Name
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0000000000000084 0000000800000004 R_BPF_64_NODYLD32 0000000000000000 gbl
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Relocation section '.rel.BTF.ext' at offset 0x548 contains 2 entries:
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Offset Info Type Symbol's Value Symbol's Name
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000000000000002c 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text
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0000000000000040 0000000200000004 R_BPF_64_NODYLD32 0000000000000000 .text
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