Documentation/bpf/llvm_reloc.rst

Source file repositories/reference/linux-study-clean/Documentation/bpf/llvm_reloc.rst

File Facts

System: Linux kernel
Corpus path: Documentation/bpf/llvm_reloc.rst
Extension: .rst
Size: 21501 bytes
Lines: 547
Domain: Support Tooling And Documentation
Bucket: Documentation
Inferred role: Support Tooling And Documentation: documentation
Status: atlas-only

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
Defines or uses C structs; map object ownership, embedded links, reference counts, and lock ownership.

Dependency Surface

No C-style include directives detected by the generator.

Detected Declarations

struct t
struct bpf_core_relo
struct sample
struct foo
enum bpf_core_relo_kind
enum bar
function test
function __attribute__
function __attribute__
function test
function global
function alpha
function bravo
function charlie
function delta

Annotated Snippet

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

Annotation

Detected declarations: `struct t`, `struct bpf_core_relo`, `struct sample`, `struct foo`, `enum bpf_core_relo_kind`, `enum bar`, `function test`, `function __attribute__`, `function __attribute__`, `function test`.
Atlas domain: Support Tooling And Documentation / Documentation.
Implementation status: atlas-only.

Implementation Notes

This generated page is the file-by-file coverage layer; curated subsystem chapters should link here when they synthesize a multi-file control flow.
Core OS pages should be promoted from atlas-only to deep-reviewed when they explain data structures, invariants, locking, lifecycle, and C implementation snippets.
Driver-family pages are intentionally pattern-oriented unless they are part of the selected PCIe/NVMe representative device path.