Re: [PATCH] gen_compile_commands: honor the build directory for modules

[Nicolas Schier]

On Fri, Oct 16, 2026 at 01:25:33PM +0700, Van Vo wrote:
> Hi Nathan,
> 
> > How was this discovered?
> 
> This came from code inspection of standalone use of -d with an explicit
> modules.order argument, followed initially by a constructed reproducer.
> It was not a failure encountered through Kbuild's compile_commands.json
> target.
> 
> I have now reproduced it using real Kbuild-generated artifacts from
> 0fcb05b72344f40806cbe810de8872f1698bf053 on Linux/aarch64, with Python
> 3.11.13 and GCC 12.2.0. This was a partial lib/ build with CONFIG_TEST_LKM=m.
> 
> The following uses a disposable source tree at that base. The guards
> prevent overwriting an existing output directory or result file:
> 
>     (
>         set -eu
>         test ! -e out
>         test ! -e outside.json
>         test ! -e inside.json
>         make O=out -j1 allnoconfig
>         bash scripts/config --file out/.config --enable MODULES \
>             --enable RUNTIME_TESTING_MENU --module TEST_LKM
>         make O=out -j1 olddefconfig
>         make O=out -j1 V=1 lib/
> 
>         outside_exit=0
>         python3 -B scripts/clang-tools/gen_compile_commands.py -a ar \
>             -d out -o outside.json out/lib/modules.order ||
>             outside_exit=$?
>         printf 'outside_exit=%s\n' "$outside_exit"
> 
>         inside_exit=0
>         (cd out &&
>          python3 -B ../scripts/clang-tools/gen_compile_commands.py -a ar \
>              -o ../inside.json lib/modules.order) ||
>             inside_exit=$?
>         printf 'inside_exit=%s\n' "$inside_exit"
>         test "$outside_exit" -eq 1
>         test "$inside_exit" -eq 0
>         python3 -c \
>             'import json; assert len(json.load(open("inside.json"))) == 1'
>     )
> 
> Kbuild generated lib/test_module.o, lib/test_module.mod,
> lib/.test_module.o.cmd and lib/modules.order under out/. Both .mod and
> modules.order contain lib/test_module.o.
> 
> The old script's invocation from the source directory fails with:
> 
>     FileNotFoundError: [Errno 2] No such file or directory:
> 'lib/test_module.mod'
>     outside_exit=1
> 
> The same artifacts work when the old script is invoked from out/:
> 
>     inside_exit=0
> 
> The successful invocation produces one JSON entry for lib/test_module.c.
> The patched script succeeds from the source directory and produces an
> identical entry. The old script also fails with absolute paths for -d
> and modules.order; the patched script succeeds with those paths too.
> 
> Only the standalone generator invocation is outside the build directory;
> compilation itself runs under out/. This is not a full kernel build or
> a boot/runtime test.

Thanks for the detailed reproducer.

Quoting from Nathan:
| How was this discovered? This cannot happen within Kbuild because this
| script is always invoked from within the build folder, so was it a
| manual invocation or something else?

So, if I get youre reply correctly, the issue this was not discovered 
"in the wild" but can only be reproduced by manual use of 
gen_compile_commands.py.

-- 
Nicolas