Re: next: i386-boot: clang-nightly: failed - intermittently - BUG: unable to handle page fault for address: 000024c0

[Nick Desaulniers]

On Wed, May 24, 2023 at 6:01 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, May 17, 2023 at 11:11:51AM -0700, Nick Desaulniers wrote:
> > On Wed, May 17, 2023 at 8:21 AM Naresh Kamboju
> > <naresh.kamboju@xxxxxxxxxx> wrote:
> > >
> > > Linux next-20230517 build with clang nightly for i386 boot fails intermittently.
> >
> > Keyword: intermittently. That will make tracking this down fun.
> >
> > Our CI also hit a boot failure on tip/master with the same splat:
> > https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/4998374271/jobs/8957285746
> > Though the CI pulled down a SHA
> > 0932447780e1f9a43bf68ef7fe3d9b41b46d58fc
> > which looks weird on
> > https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=0932447780e1f9a43bf68ef7fe3d9b41b46d58fc
> > >> Notice: this object is not reachable from any branch.
>
> Github isn't willing to show me content unless I log in or somesuch
> nonsense.

Ah, sorry about that.
https://paste.debian.net/1281050/ should be the log of ours.
https://storage.tuxsuite.com/public/clangbuiltlinux/continuous-integration2/builds/2QEtkwi60Mn3NLX4U0sDCAH0qqp/bzImage
is the corresponding build artifact.

There's ongoing discussion in #x86 on LinuxNet.  I suspect that a few
of Naresh's recent reports are all perhaps one single issue.

Arnd mentioned https://lore.kernel.org/all/CA+G9fYvVZ9WF-2zfrYeo3xnWNra0QGxLzei+b4yANZwEvr5CYw@xxxxxxxxxxxxxx/
which looks similar but is with GCC.

Either way, we're seeing this in mainline.

>
> > That this failed in -next and -tip in the same way makes me wonder if
> > something affecting this is coming in via -tip? Maybe the splat looks
> > familiar to x86 folks?
> >
> > I haven't been able to reproduce locally when my machine is relatively
> > load-less.  If I do a kernel build in the background, I was able to
> > get QEMU to hang, but without any splat. That was using tip/master @
> > f81d8f759e7f.
> >
> > Naresh, when you say "intermittent" do you have any data on the
> > relative frequency of this boot failure? (Also, please make sure to
> > use llvm@xxxxxxxxxxxxxxx in the future; we moved mailing lists years
> > ago).
> >
> > Looks like our CI report linked above has an additional splat though
> > via apply_alternatives and optimize_nops.
> >
> > >> [ 0.166742] Code: Unable to access opcode bytes at 0x36.
> >
> > Peter, that smells like perhaps either:
> > commit b6c881b248ef ("x86/alternative: Complicate optimize_nops() some more")
> > commit 6c480f222128 ("x86/alternative: Rewrite optimize_nops() some")
>
> So I did find me a 'funny' there, but nothing that explains boot fail.
>
> It would think that 'PAUSE' is a 2 byte NOP and replace it with NOP2;
> which is not quite the same thing. The below seems to cure that.
>
> Let me continue poking at things...
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 93aa95afd005..bb0a7b03e52f 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -159,9 +160,12 @@ void text_poke_early(void *addr, const void *opcode, size_t len);
>   */
>  static bool insn_is_nop(struct insn *insn)
>  {
> -       if (insn->opcode.bytes[0] == 0x90)
> +       /* Anything NOP, but not REP NOP. */
> +       if (insn->opcode.bytes[0] == 0x90 &&
> +           (!insn->prefixes.nbytes || insn->prefixes.bytes[0] != 0xF3))
>                 return true;
>
> +       /* NOPL */
>         if (insn->opcode.bytes[0] == 0x0F && insn->opcode.bytes[1] == 0x1F)
>                 return true;
>


-- 
Thanks,
~Nick Desaulniers