Re: [syzbot] upstream test error: KASAN: invalid-access Read in __entry_tramp_text_end

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 28, 2021 at 06:36:37PM -0700, Josh Poimboeuf wrote:
> On Tue, Sep 28, 2021 at 11:35:43AM +0100, Mark Rutland wrote:
> > > In the other x86 thread Josh Poimboeuf suggested to use asm goto to a
> > > cold part of the function instead of .fixup:
> > > https://lore.kernel.org/lkml/20210927234543.6waods7rraxseind@treble/
> > > This sounds like a more reliable solution that will cause less
> > > maintenance burden. Would it work for arm64 as well?
> > 
> > Maybe we can use that when CC_HAS_ASM_GOTO_OUTPUT is avaiable, but in
> > general we can't rely on asm goto supporting output arguments (and IIRC
> > GCC doesn't support that at all), so we'd still have to support the
> > current fixup scheme.

gcc-11 has it

> Even without CC_HAS_ASM_GOTO_OUTPUT it should still be possible to hack
> something together if you split the original insn asm and the extable
> asm into separate statements, like:
> 
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 6b52182e178a..8f62469f2027 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -137,20 +139,21 @@ static inline unsigned long long native_read_msr_safe(unsigned int msr,
>  {
>  	DECLARE_ARGS(val, low, high);
>  
> +	*err = 0;
> +	asm volatile("417: rdmsr\n"
> +		     : EAX_EDX_RET(val, low, high)
> +		     : "c" (msr));
> +	asm_volatile_goto(_ASM_EXTABLE(417b, %l[Efault]) :::: Efault);

That's terrible :-) Could probably do with a comment, but might just
work..

> +
> +done:
>  	if (tracepoint_enabled(read_msr))
>  		do_trace_read_msr(msr, EAX_EDX_VAL(val, low, high), *err);
>  	return EAX_EDX_VAL(val, low, high);
> +
> +Efault:
> +	*err = -EIO;
> +	ZERO_ARGS(val, low, high);
> +	goto done;
>  }
>  
>  /* Can be uninlined because referenced by paravirt */
>