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Merge branch 'bpf: Update doc about calling kernel function'

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Martin KaFai says:

====================

This set updates the document about the bpf program calling kernel
function.  In particular, updates are regarding to the clang
requirement in selftests and kfunc-call not an ABI.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Alexei Starovoitov 2021-03-30 07:31:01 -07:00
commit b83fd195c2
2 changed files with 29 additions and 0 deletions
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15
Documentation/bpf/bpf_design_QA.rst
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@ -258,3 +258,18 @@ Q: Can BPF functionality such as new program or map types, new
helpers, etc be added out of kernel module code?
A: NO.
Q: Directly calling kernel function is an ABI?
----------------------------------------------
Q: Some kernel functions (e.g. tcp_slow_start) can be called
by BPF programs. Do these kernel functions become an ABI?
A: NO.
The kernel function protos will change and the bpf programs will be
rejected by the verifier. Also, for example, some of the bpf-callable
kernel functions have already been used by other kernel tcp
cc (congestion-control) implementations. If any of these kernel
functions has changed, both the in-tree and out-of-tree kernel tcp cc
implementations have to be changed. The same goes for the bpf
programs and they have to be adjusted accordingly.

14
tools/testing/selftests/bpf/README.rst
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@ -179,3 +179,17 @@ types, which was introduced in `Clang 13`__. The older Clang versions will
either crash when compiling these tests, or generate an incorrect BTF.
__ https://reviews.llvm.org/D83289
Kernel function call test and Clang version
===========================================
Some selftests (e.g. kfunc_call and bpf_tcp_ca) require a LLVM support
to generate extern function in BTF. It was introduced in `Clang 13`__.
Without it, the error from compiling bpf selftests looks like:
.. code-block:: console
libbpf: failed to find BTF for extern 'tcp_slow_start' [25] section: -2
__ https://reviews.llvm.org/D93563