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Date:   Fri, 1 Oct 2021 22:10:24 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+488ddf8087564d6de6e2@...kaller.appspotmail.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        will@...nel.org, x86@...nel.org, live-patching@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [syzbot] upstream test error: KASAN: invalid-access Read in
 __entry_tramp_text_end

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:
> 
>   https://lore.kernel.org/lkml/20170207111011.GB28790@leverpostej/
>   https://lore.kernel.org/lkml/20170207160300.GB26173@leverpostej/
>   https://lore.kernel.org/lkml/20170208091250.GT6515@twins.programming.kicks-ass.net/
>
> ... 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).

-- 
Josh

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