linux-kernel - Re: Incorrect thinlto cache directory path in Makefile

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Date: Sun, 12 May 2024 22:04:19 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Bagas Sanjaya <bagasdotme@...il.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	Linux Kernel Build System <linux-kbuild@...r.kernel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Xu Zhen <xuzhen@...tmail.com>, 
	Nathan Chancellor <nathan@...nel.org>
Subject: Re: Incorrect thinlto cache directory path in Makefile

+CC: Nathan


On Sat, May 11, 2024 at 8:18 PM Bagas Sanjaya <bagasdotme@...il.com> wrote:
>
> Hi,
>
> Xu Zhen <xuzhen@...tmail.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@...nel.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