Re: [PATCH bpf-next v9 02/11] selftests/bpf: Fix test_kmods KDIR to honor O= and distro kernels

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Automatical fallback to host kernel headers (`/lib/modules/$(uname -r)/build`) for building in-tree BPF test modules will cause confusing build failures and defeats the permissive skip logic.
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commit ffa2987a74e06bf51c1a3ffcf2c85b1504b4d383
Author: Ricardo B. Marlière <rbm@suse.com>

selftests/bpf: Fix test_kmods KDIR to honor O= and distro kernels

This commit updates the test_kmods Makefile to honor O= and
KBUILD_OUTPUT directories, and falls back to distro kernel headers
when the source tree is unconfigured.

> diff --git a/tools/testing/selftests/bpf/test_kmods/Makefile b/tools/testing/selftests/bpf/test_kmods/Makefile
> index 63c4d3f6a12f..031c7454ce65 100644
> --- a/tools/testing/selftests/bpf/test_kmods/Makefile
> +++ b/tools/testing/selftests/bpf/test_kmods/Makefile
> @@ -1,5 +1,16 @@
[ ... ]
> +KMOD_O := $(or $(O),$(KBUILD_OUTPUT))
> +KMOD_O_VALID := $(if $(KMOD_O),$(if $(wildcard $(KMOD_O)/Module.symvers),$(KMOD_O)))
> +KDIR ?= $(if $(KMOD_O_VALID),$(SRCTREE_KDIR), \
> +	    $(if $(wildcard $(SRCTREE_KDIR)/Module.symvers),$(SRCTREE_KDIR), \
> +		/lib/modules/$(shell uname -r)/build))

Will falling back to the host kernel headers cause compilation errors when
building in-tree test modules like bpf_testmod?

Since bpf_testmod relies heavily on internal BPF structures, experimental
macros, and kfuncs that change frequently between kernel releases, compiling
it against /lib/modules/$(uname -r)/build on a system where the running
kernel does not exactly match the upstream source tree will likely fail.

Additionally, if the host headers are present, the permissive skip check
further down in the file will pass since the directory exists.

Because the compilation failure is not ignored by the parent bpf/Makefile,
won't this abort the entire BPF selftests build with confusing C compilation
errors instead of silently skipping the unconfigured kernel tree?

Should in-tree test modules avoid falling back to host distro headers to
prevent these version mismatches?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260429-selftests-bpf_misconfig-v9-0-c311f06b4791@suse.com?part=2