Commit fc8c262e authored by Yonghong Song's avatar Yonghong Song Committed by Daniel Borkmann

bpf, docs: Add llvm_reloc.rst to explain llvm bpf relocations

LLVM upstream commit https://reviews.llvm.org/D102712 made some changes
to bpf relocations to make them llvm linker lld friendly. The scope of
existing relocations R_BPF_64_{64,32} is narrowed and new relocations
R_BPF_64_{ABS32,ABS64,NODYLD32} are introduced.

Let us add some documentation about llvm bpf relocations so people can
understand how to resolve them properly in their respective tools.
Signed-off-by: default avatarYonghong Song <yhs@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210526152457.335210-1-yhs@fb.com
parent d6a6a555
...@@ -84,6 +84,7 @@ Other ...@@ -84,6 +84,7 @@ Other
:maxdepth: 1 :maxdepth: 1
ringbuf ringbuf
llvm_reloc
.. Links: .. Links:
.. _networking-filter: ../networking/filter.rst .. _networking-filter: ../networking/filter.rst
......
This diff is collapsed.
...@@ -202,3 +202,22 @@ generate valid BTF information for weak variables. Please make sure you use ...@@ -202,3 +202,22 @@ generate valid BTF information for weak variables. Please make sure you use
Clang that contains the fix. Clang that contains the fix.
__ https://reviews.llvm.org/D100362 __ https://reviews.llvm.org/D100362
Clang relocation changes
========================
Clang 13 patch `clang reloc patch`_ made some changes on relocations such
that existing relocation types are broken into more types and
each new type corresponds to only one way to resolve relocation.
See `kernel llvm reloc`_ for more explanation and some examples.
Using clang 13 to compile old libbpf which has static linker support,
there will be a compilation failure::
libbpf: ELF relo #0 in section #6 has unexpected type 2 in .../bpf_tcp_nogpl.o
Here, ``type 2`` refers to new relocation type ``R_BPF_64_ABS64``.
To fix this issue, user newer libbpf.
.. Links
.. _clang reloc patch: https://reviews.llvm.org/D102712
.. _kernel llvm reloc: /Documentation/bpf/llvm_reloc.rst
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