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

Josh Poimboeuf
2021-10-02T05:13:03Z

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