Re: [PATCH RFC] Makefile: Export absolute srctree path for out-of-tree builds

[Quentin Monnet <qmo@xxxxxxxx>]

2024-12-17 11:10 UTC+0800 ~ Li Zhijian <lizhijian@xxxxxxxxxxx>
> Fixes an issue where out-of-tree kselftest builds fail when building
> the BPF and bpftools components. The failure occurs because the top-level
> Makefile passes a relative srctree path ('..') to its sub-Makefiles, which
> leads to errors in locating necessary files.
> 
> For example, the following error is encountered:
> 
> ```
> $ make V=1 O=$build/ TARGETS=hid kselftest-all
> ...
> make -C ../tools/testing/selftests all
> make[4]: Entering directory '/path/to/linux/tools/testing/selftests/hid'
> make  -C /path/to/linux/tools/testing/selftests/../../../tools/lib/bpf OUTPUT=/path/to/linux/O/kselftest/hid/tools/build/libbpf/ \
>             EXTRA_CFLAGS='-g -O0'                                      \
>             DESTDIR=/path/to/linux/O/kselftest/hid/tools prefix= all install_headers
> make[5]: Entering directory '/path/to/linux/tools/lib/bpf'
> ...
> make[5]: Entering directory '/path/to/linux/tools/bpf/bpftool'
> Makefile:127: ../tools/build/Makefile.feature: No such file or directory
> make[5]: *** No rule to make target '../tools/build/Makefile.feature'.  Stop.
> ```


Another condition to reproduce the failure above is to have have 
$(srcroot) in the Makefile set to "." or "..", for example when your
$build is located right under the root of the repo [0].

[0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile?h=v6.13-rc3#n273


> To resolve this, the srctree is exported as an absolute path (abs_srctree)
> when performing an out-of-tree build. This ensures that all sub-Makefiles
> have the correct path to the source tree, preventing directory resolution
> errors.
> 
> Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
> ---
> Request for Additional Testing
> 
> We welcome all contributors and CI systems to test this change thoroughly.
> In theory, this change should not affect in-tree builds. However, to ensure
> stability and compatibility, we encourage testing across different
> configurations.
> 
> What has been tested?
> - out-of-tree kernel build
> - out-of-tree kselftest-all
> ---
>  Makefile | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/Makefile b/Makefile
> index e5b8a8832c0c..36e65806bb5e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -275,7 +275,8 @@ else ifeq ($(srcroot)/,$(dir $(CURDIR)))
>      srcroot := ..
>  endif
>  
> -export srctree := $(if $(KBUILD_EXTMOD),$(abs_srctree),$(srcroot))
> +srctree := $(if $(KBUILD_EXTMOD),$(abs_srctree),$(srcroot))
> +export srctree := $(if $(building_out_of_srctree),$(abs_srctree),$(srctree))
>  
>  ifdef building_out_of_srctree
>  export VPATH := $(srcroot)


The patch does fix the issue in the case you reported.

Maybe I'd write the change differently, though; rather than potentially
overwriting $(srctree) before exporting it, let's unroll with ifdef's to
make it clearer?

	ifdef building_out_of_srctree
	    srctree := $(abs_srctree)
	else ifdef KBUILD_EXTMOD
	    srctree := $(abs_srctree)
	else
	    srctree := $(srcroot)
	endif
	export srctree

(not tested)

Thanks,
Quentin