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: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210526152457.335210-1-yhs@fb.com

Documentation/bpf/index.rst

@@ -84,6 +84,7 @@ Other
    :maxdepth: 1
 
    ringbuf
+   llvm_reloc
 
 .. Links:
 .. _networking-filter: ../networking/filter.rst

tools/testing/selftests/bpf/README.rst

@@ -202,3 +202,22 @@ generate valid BTF information for weak variables. Please make sure you use
 Clang that contains the fix.
 
 __ 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