Fwd: Incorrect thinlto cache directory path in Makefile

Fwd: Incorrect thinlto cache directory path in Makefile

[Bagas Sanjaya]

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Hi,

Xu Zhen <xuzhen@fastmail.com> reported on Bugzilla (https://bugzilla.kernel.org/show_bug.cgi?id=218825) thinlto build directory problem (especially on
DKMS modules). He wrote:

> In Makefile, the ld flag `--thinlto-cache-dir' is set to
> `$(extmod_prefix).thinlto-cache'. But at that time, the variable extmod_prefix
> had not been assigned yet. Therefore, the thinlto cache dir is always created
> in the current directory.
> 
> Even worse, the cache dir cannot be deleted when executing `make clean'. This
> is because its path was written as `$(KBUILD_EXTMOD)/.thinlto-cache' in the
> clean: target.
> 
> Some users have been troubled by it: https://github.com/dell/dkms/issues/292
> 
> This bug was introduced by commit dc5723b02e523b2c4a68667f7e28c65018f7202f

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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Re: Incorrect thinlto cache directory path in Makefile

[Masahiro Yamada]

+CC: Nathan


On Sat, May 11, 2024 at 8:18 PM Bagas Sanjaya <bagasdotme@gmail.com> wrote:
>
> Hi,
>
> Xu Zhen <xuzhen@fastmail.com> reported on Bugzilla (https://bugzilla.kernel.org/show_bug.cgi?id=218825) thinlto build directory problem (especially on
> DKMS modules). He wrote:
>
> > In Makefile, the ld flag `--thinlto-cache-dir' is set to
> > `$(extmod_prefix).thinlto-cache'. But at that time, the variable extmod_prefix
> > had not been assigned yet. Therefore, the thinlto cache dir is always created
> > in the current directory.
> >
> > Even worse, the cache dir cannot be deleted when executing `make clean'. This
> > is because its path was written as `$(KBUILD_EXTMOD)/.thinlto-cache' in the
> > clean: target.
> >
> > Some users have been troubled by it: https://github.com/dell/dkms/issues/292
> >
> > This bug was introduced by commit dc5723b02e523b2c4a68667f7e28c65018f7202f


I agree this is a bug.




line 945:
  KBUILD_LDFLAGS  += --thinlto-cache-dir=$(extmod_prefix).thinlto-cache


line 1034:
  KBUILD_LDFLAGS  += $(call ld-option,--no-warn-rwx-segments)


line 1095:
  export extmod_prefix = $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)







$(call ld-option ) at line 1034 adds --thinlto-cache-dir=.thinlto-cache
because --thinlto-cache-dir=$(extmod_prefix).thinlto-cache was added
at line 945, but $(extmod_prefix) is empty until it is defined
at line 1095.



However, the offending line was already removed for another issue.

See the following in linux-next:


Author: Nathan Chancellor <nathan@kernel.org>
Date:   Wed May 1 15:55:25 2024 -0700

    kbuild: Remove support for Clang's ThinLTO caching




If --thinlto-cache-dir comes back,
I hope Nathan will take care of this issue.







> Thanks.
>
> --
> An old man doll... just what I always wanted! - Clara



-- 
Best Regards
Masahiro Yamada

Re: Incorrect thinlto cache directory path in Makefile

[Masahiro Yamada]

On Sun, May 12, 2024 at 10:04 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> +CC: Nathan
>
>
> On Sat, May 11, 2024 at 8:18 PM Bagas Sanjaya <bagasdotme@gmail.com> wrote:
> >
> > Hi,
> >
> > Xu Zhen <xuzhen@fastmail.com> reported on Bugzilla (https://bugzilla.kernel.org/show_bug.cgi?id=218825) thinlto build directory problem (especially on
> > DKMS modules). He wrote:
> >
> > > In Makefile, the ld flag `--thinlto-cache-dir' is set to
> > > `$(extmod_prefix).thinlto-cache'. But at that time, the variable extmod_prefix
> > > had not been assigned yet. Therefore, the thinlto cache dir is always created
> > > in the current directory.
> > >
> > > Even worse, the cache dir cannot be deleted when executing `make clean'. This
> > > is because its path was written as `$(KBUILD_EXTMOD)/.thinlto-cache' in the
> > > clean: target.
> > >
> > > Some users have been troubled by it: https://github.com/dell/dkms/issues/292
> > >
> > > This bug was introduced by commit dc5723b02e523b2c4a68667f7e28c65018f7202f
>
>
> I agree this is a bug.
>
>
>
>
> line 945:
>   KBUILD_LDFLAGS  += --thinlto-cache-dir=$(extmod_prefix).thinlto-cache
>
>
> line 1034:
>   KBUILD_LDFLAGS  += $(call ld-option,--no-warn-rwx-segments)


This one is probably OK because it is guarded by CONFIG_LD_IS_BFD,
while LTO requires LD_IS_LLD.






I see another ld-option at 1043.

  LDFLAGS_vmlinux += $(call ld-option,--pack-dyn-relocs=relr,-z
pack-relative-relocs)


Maybe, this is related to the issue
(although I did not attempt to reproduce it).




-- 
Best Regards
Masahiro Yamada