Re: ld.lld: error: inline assembly requires more registers than available at line 523

[Leonardo Bras Soares Passos]

On Wed, May 4, 2022 at 2:55 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> On Mon, May 02, 2022 at 06:29:38AM +0800, kernel test robot wrote:
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   672c0c5173427e6b3e2a9bbb7be51ceeec78093a
> > commit: ba1f77c546966c12d86220d078e5838dcaeab348 x86/kvm: Fix compilation warning in non-x86_64 builds
> > date:   2 months ago
> > config: i386-randconfig-a012-20220502 (https://download.01.org/0day-ci/archive/20220502/202205020637.JVJo3blw-lkp@xxxxxxxxx/config)
> > compiler: clang version 15.0.0 (https://github.com/llvm/llvm-project 09325d36061e42b495d1f4c7e933e260eac260ed)
> > reproduce (this is a W=1 build):
> >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> >         chmod +x ~/bin/make.cross
> >         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ba1f77c546966c12d86220d078e5838dcaeab348
> >         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> >         git fetch --no-tags linus master
> >         git checkout ba1f77c546966c12d86220d078e5838dcaeab348
> >         # save the config file
> >         mkdir build_dir && cp config build_dir/.config
> >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=i386 SHELL=/bin/bash
> >
> > If you fix the issue, kindly add following tag as appropriate
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> >
> > All errors (new ones prefixed by >>):
> >
> > >> ld.lld: error: inline assembly requires more registers than available at line 523
> > >> ld.lld: error: inline assembly requires more registers than available at line 523
> >
>
> I don't think this bisection result is correct; this error is visible at
> v5.17-rc3, which this commit is based on. This is likely the same root
> cause as https://github.com/ClangBuiltLinux/linux/issues/1589, just with
> LTO. I'll reduce something down and comment there if it turns out to be
> different.
>
> Cheers,
> Nathan
>

Hello Nathan,
I went down on this issue, but I have no idea how to proceed. These
are my findings:
- This commit (ba1f77c546966c12d86220d078e5838dcaeab348) fixes the
build, thus allowing this error to happen later (in LTO stage)
- The commit that introduced this issue was 69d0db01e210, due to removing :
   ubsan-cflags-$(CONFIG_UBSAN_OBJECT_SIZE)       += -fsanitize=object-size
- Only by having this flag enabled in ubsan-cflags-y on
torvalds/master causes the build to succeed

At this point, given the commit message in 69d0db01e210 I was thinking
of finding & fixing the code that causes the ld.lld error to happen,
but I was lacking the skills to extract any filename from LTO step.

But it's also possible to create a patch just adding
'ubsan-cflags-$(CONFIG_SOMETHING)      += -fsanitize=object-size'
and fix this build.

What would you suggest?

Best regards,
Leo