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Date:   Wed, 9 Aug 2023 13:35:32 +0800
From:   Pengfei Xu <pengfei.xu@intel.com>
To:     Andrew Morton <akpm@linux-foundation.org>
CC:     <dvyukov@google.com>, <nogikh@google.com>,
        <linux-kernel@vger.kernel.org>, Heng Su <heng.su@intel.com>,
        <syzkaller@googlegroups.com>, <keescook@google.com>,
        <mingo@elte.hu>, Mehta Sohil <sohil.mehta@intel.com>,
        <kirill.shutemov@linux.intel.com>, <peterz@infradead.org>
Subject: Re: [PATCH RESEND v1 1/1] x86/kernel: Increase kcov coverage under
 arch/x86/kernel folder
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Hi Andrew,

On 2023-08-07 at 12:58:03 -0700, Andrew Morton wrote:
> On Mon, 31 Jul 2023 11:04:18 +0800 Pengfei Xu <pengfei.xu@intel.com> wrote:
> 
> > Currently kcov instrument is disabled for object files under arch/x86/kernel
> > folder.
> > For object files under arch/x86/kernel, actually just disabling the kcov
> > instrument of files:"head32.o or head64.o and sev.o" could achieve
> > successful booting and provide kcov coverage for object files that do not
> > disable kcov instrument.
> > The additional kcov coverage collected from arch/x86/kernel folder helps
> > kernel fuzzing efforts to find bugs.
> > 
> > Link to related improvement discussion is below:
> > https://groups.google.com/g/syzkaller/c/Dsl-RYGCqs8/m/x-tfpTyFBAAJ
> > Related ticket is as follow:
> > https://bugzilla.kernel.org/show_bug.cgi?id=198443
> > 
> > ...
> >
> > --- a/arch/x86/kernel/Makefile
> > +++ b/arch/x86/kernel/Makefile
> > @@ -33,11 +33,10 @@ KCSAN_SANITIZE := n
> >  KMSAN_SANITIZE_head$(BITS).o				:= n
> >  KMSAN_SANITIZE_nmi.o					:= n
> >  
> > -# If instrumentation of this dir is enabled, boot hangs during first second.
> > -# Probably could be more selective here, but note that files related to irqs,
> > -# boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
> > -# non-deterministic coverage.
> > -KCOV_INSTRUMENT		:= n
> > +# If instrumentation of the following files is enabled, boot hangs during
> > +# first second.
> > +KCOV_INSTRUMENT_head$(BITS).o				:= n
> > +KCOV_INSTRUMENT_sev.o					:= n
> >  
> >  CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
> 
> This patch exposes (but probably doesn't cause) an objtool warning:
> 
> arch/x86/kernel/traps.o: warning: objtool: ibt_selftest+0x18: sibling call from callable instruction with modified stack frame

  Thanks for pointing out the problem!
  I found: v6.5-rc5 mainline kernel met the problem also:
"
vmlinux.o: warning: objtool: ibt_selftest+0x11: sibling call from callable instruction with modified stack frame
"

  Yes, it has something to do with the patch above, becasue after installed
above patch on top of v6.5-rc5 kernel, the offset changed from 0x11 to 0x1d:
"
vmlinux.o: warning: objtool: ibt_selftest+0x1d: sibling call from callable instruction with modified stack frame
"

  I'm still not quite sure the reason of warning from ibt_selftest function.
  Anyway, I will keep looking into this issue.

Best Regards,
Thanks!

> 
> gcc-12.2.0, x86_64 allmodconfig.
>