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Date:   Fri, 1 Oct 2021 22:10:24 -0700
From:   Josh Poimboeuf <>
To:     Mark Rutland <>
Cc:     Peter Zijlstra <>,
        Dmitry Vyukov <>,
        syzbot <>,
        Linux ARM <>,,,,,,,,
        Thomas Gleixner <>
Subject: Re: [syzbot] upstream test error: KASAN: invalid-access Read in

On Fri, Oct 01, 2021 at 01:27:06PM +0100, Mark Rutland wrote:
> > So we may need to get rid of .fixup altogether.  Especially for arches
> > which support livepatch.
> > 
> > We can replace some of the custom .fixup handlers with generic handlers
> > like x86 does, which do the fixup work in exception context.  This
> > generally works better for more generic work like putting an error code
> > in a certain register and resuming execution at the subsequent
> > instruction.
> I reckon even ignoring the unwind problems this'd be a good thing since
> it'd save on redundant copies of the fixup logic that happen to be
> identical, and the common cases like uaccess all fall into this shape.
> As for how to do that, in the past Peter and I had come up with some
> assembler trickery to get the name of the error code register encoded
> into the extable info:
> ... but maybe that's already solved on x86 in a different way?

That's really cool :-) But it might be overkill for x86's needs.  For
the exceptions which rely on handlers rather than anonymous .fixup code,
the register assumptions are hard-coded in the assembler constraints.  I
think that works well enough.

> > However a lot of the .fixup code is rather custom and doesn't
> > necessarily work well with that model.
> Looking at arm64, even where we'd need custom handlers it does appear we
> could mostly do that out-of-line in the exception handler. The more
> exotic cases are largely in out-of-line asm functions, where we can move
> the fixups within the function, after the usual return.
> I reckon we can handle the fixups for load_unaligned_zeropad() in the
> exception handler.
> Is there anything specific that you think is painful in the exception
> handler?

Actually, after looking at all the x86 .fixup usage, I think we can make
this two-pronged approach work.  Either move the .fixup code to an
exception handler (with a hard-coded assembler constraint register) or
put it in the function (out-of-line where possible).  I'll try to work
up some patches (x86 only of course).


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