Re: [PATCH bpf-next v6 02/11] selftests/bpf: Fix test_kmods KDIR to honor O= and distro kernels

[sashiko-bot@kernel.org]

Sashiko AI review found 2 potential issue(s):
- [Medium] Relative paths passed via `O=` are incorrectly evaluated from the kernel source root instead of the invocation directory.
- [Medium] The `make clean` target unconditionally fails and aborts the build if the kernel source tree is unconfigured and distro headers are missing.
--

commit 533e8d39cc98f56f3dff4f8c5bff0dcd49fc6ef1
Author: Ricardo B. Marlière <rbm@suse.com>

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

This commit modifies test_kmods/Makefile to honor O= and KBUILD_OUTPUT, and
adds a fallback to /lib/modules/$(uname -r)/build for distro kernels. It also
makes missing KDIR silently skipped when building in permissive mode.

> diff --git a/tools/testing/selftests/bpf/test_kmods/Makefile b/tools/testing/selftests/bpf/test_kmods/Makefile
> index 63c4d3f6a12f6..10f202bd0d8c2 100644
> --- a/tools/testing/selftests/bpf/test_kmods/Makefile
> +++ b/tools/testing/selftests/bpf/test_kmods/Makefile
> @@ -1,5 +1,7 @@
>  TEST_KMOD_DIR := $(realpath $(dir $(abspath $(lastword $(MAKEFILE_LIST)))))
> -KDIR ?= $(abspath $(TEST_KMOD_DIR)/../../../../..)
> +SRCTREE_KDIR := $(abspath $(TEST_KMOD_DIR)/../../../../..)
> +KMOD_O := $(or $(O),$(KBUILD_OUTPUT))

Will this break out-of-tree builds if a user passes a relative path?

If a user builds selftests with a relative O= path (e.g. make O=out), KMOD_O
captures this relative string and passes it unaltered to the kernel build
system via make -C $(KDIR) O=$(KMOD_O).

Because make -C changes the working directory to $(KDIR) before evaluating
O=, the relative path will be evaluated relative to the kernel root rather
than the directory where make was originally invoked.

[ ... ]
> @@ -14,8 +16,24 @@ $(foreach m,$(MODULES),$(eval obj-m += $(m:.ko=.o)))
>  
>  CFLAGS_bpf_testmod.o = -I$(src)
>  
> +# When BPF_STRICT_BUILD != 0, a missing KDIR is fatal (the default).
> +# When permissive, skip silently.
> +PERMISSIVE := $(filter 0,$(BPF_STRICT_BUILD))
> +
>  all:
> -	$(Q)$(MAKE) -C $(KDIR) M=$(TEST_KMOD_DIR) modules
> +ifeq ($(PERMISSIVE),)
> +	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O),O=$(KMOD_O),KBUILD_OUTPUT=) \
> +		M=$(TEST_KMOD_DIR) modules

[ ... ]

>  clean:
> -	$(Q)$(MAKE) -C $(KDIR) M=$(TEST_KMOD_DIR) clean
> +ifeq ($(PERMISSIVE),)
> +	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O),O=$(KMOD_O),KBUILD_OUTPUT=) \
> +		M=$(TEST_KMOD_DIR) clean

Does this unconditionally abort make clean if the kernel source tree is
unconfigured and distro headers are missing?

If the system lacks installed kernel headers, /lib/modules/$(uname -r)/build
will not exist. In the default strict mode, GNU Make's -C flag will
immediately throw a fatal 'No such file or directory' error and abort the
make clean process.

> +else ifneq ("$(wildcard $(KDIR))", "")
> +	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O),O=$(KMOD_O),KBUILD_OUTPUT=) \
> +		M=$(TEST_KMOD_DIR) clean
> +endif

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260416-selftests-bpf_misconfig-v6-0-7efeab504af1@suse.com?part=2