Re: [PATCH 09/11] tests/tcg: disable pauth for aarch64 gdb tests

[Peter Maydell]

On Wed, 15 Mar 2023 at 09:51, Luis Machado <luis.machado@arm.com> wrote:
> On 3/13/23 11:44, Luis Machado wrote:
> > On 3/13/23 11:22, Peter Maydell via Gdb wrote:
> >> Luis and I came up with two options:
> >>
> >> (1) leave QEMU outputting the pauth xml as-is, and tell people
> >> whose gdb 12 crashes that they should upgrade to a newer gdb
> >>
> >> (2) make QEMU output the pauth info under a different XML namespace,
> >> and tell people who need backtraces when pauth is enabled
> >> that they should upgrade to a newer gdb
> >>
> >> Neither of these feel great, but on balance I guess 2 is better?
> >>
> >> Luis: I think that rather than doing (2) with a QEMU namespace,
> >> we should define a gdb namespace for this. That makes it clear
> >> that this is still a gdb-upstream-sanctioned way of exposing
> >> the pauth registers.
> >
> > That should be fine as well, and would work to side-step the gdb 12 bug so 
> > it doesn't crash.
> >
> > We could name the feature "org.gnu.gdb.aarch64.pauth_v2" or somesuch, and 
> > slowly stop using the original
> > "org.gnu.gdb.aarch64.pauth" feature. I can document the requirements for a 
> > compliant pauth_v2.
> >
> > FYI, I've pushed a better documentation for the arm/aarch64 xml 
> > descriptions here:
> >
> > https://sourceware.org/git?p=binutils-gdb.git;a=commit;h=d7001b29e9f256dfc60acb481d9df8f91f2ee623
> > https://sourceware.org/git?p=binutils-gdb.git;a=commit;h=e0994165d1b8469dfc27b09b62ac74862d535812

> Just an update on this. I had a chat with Richard Henderson yesterday, and it 
> might actually be easier and more convenient to backport
> fixes to older gdb versions (at least gdb-12 and gdb-11, but gdb-10 and gdb-9 
> are also affected). This will ensure those won't crash when
> they connect to a qemu that advertises the pauth feature.
>
> It also means we won't need qemu-side changes. My understanding is that we're 
> close to the 8.0.0 release, and the code is already in place.

Having run into this problem in another couple of situations, one of
which involved gdb 10, I think I'm increasingly favouring option
2 here. The affected gdbs seem to be quite widely deployed, and
the bug results in crashes even for users who didn't really
care about pauth. So I'd rather we didn't release a QEMU 8.0
which crashes these affected deployed gdbs.

So:
 (a) if on the gdb side you can define (within the next week) a
suitable new XML name you want QEMU to expose, we can commit a
change to switch to that before we do the 8.0 release
 (b) if that's too tight a timescale, we can commit a patch which
just stops QEMU from exposing the pauth.xml, and we can come up
with a better solution after 8.0 releases

In fact, I think I'm going to submit a patch to do (b) for
now and we can follow up with a patch for (a) if we want.

thanks
-- PMM