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From: Masahiro Yamada <masahiroy@kernel.org>
Date: Sun, 12 May 2024 22:04:19 +0900
Message-ID: <CAK7LNARrG0u2pSzAkdbzrcf_i7ByhEnDwHH=4LtaE_H7OgL=0A@mail.gmail.com>
Subject: Re: Incorrect thinlto cache directory path in Makefile
To: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, 
	Linux Kernel Build System <linux-kbuild@vger.kernel.org>, Sami Tolvanen <samitolvanen@google.com>, 
	Kees Cook <keescook@chromium.org>, Xu Zhen <xuzhen@fastmail.com>, 
	Nathan Chancellor <nathan@kernel.org>
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+CC: Nathan


On Sat, May 11, 2024 at 8:18=E2=80=AFPM Bagas Sanjaya <bagasdotme@gmail.com=
> wrote:
>
> Hi,
>
> Xu Zhen <xuzhen@fastmail.com> reported on Bugzilla (https://bugzilla.kern=
el.org/show_bug.cgi?id=3D218825) thinlto build directory problem (especiall=
y on
> DKMS modules). He wrote:
>
> > In Makefile, the ld flag `--thinlto-cache-dir' is set to
> > `$(extmod_prefix).thinlto-cache'. But at that time, the variable extmod=
_prefix
> > had not been assigned yet. Therefore, the thinlto cache dir is always c=
reated
> > in the current directory.
> >
> > Even worse, the cache dir cannot be deleted when executing `make clean'=
 This
> > is because its path was written as `$(KBUILD_EXTMOD)/.thinlto-cache' in=
 the
> > clean: target.
> >
> > Some users have been troubled by it: https://github.com/dell/dkms/issue=
s/292
> >
> > This bug was introduced by commit dc5723b02e523b2c4a68667f7e28c65018f72=
02f


I agree this is a bug.


line 945:
  KBUILD_LDFLAGS  +=3D --thinlto-cache-dir=3D$(extmod_prefix).thinlto-cache


line 1034:
  KBUILD_LDFLAGS  +=3D $(call ld-option,--no-warn-rwx-segments)


line 1095:
  export extmod_prefix =3D $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)


$(call ld-option ) at line 1034 adds --thinlto-cache-dir=3D.thinlto-cache
because --thinlto-cache-dir=3D$(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


--=20
Best Regards
Masahiro Yamada