Commit b83fd195 authored Mar 30, 2021 by Alexei Starovoitov

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



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>

parents 7aae231a 2ba4badc

Documentation/bpf/bpf_design_QA.rst

+15 −0

Original line number	Diff line number	Diff line
		@@ -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.

tools/testing/selftests/bpf/README.rst

+14 −0

Original line number	Diff line number	Diff line
		@@ -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