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

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Date: Sun, 12 May 2024 22:14:40 +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

On Sun, May 12, 2024 at 10:04 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> +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)


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