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

[Peter Maydell]

On Fri, 10 Mar 2023 at 18:20, Alex Bennée <alex.bennee@linaro.org> wrote:
>
>
> (adding some more gdb types to CC)
>
> Fabiano Rosas <farosas@suse.de> writes:
>
> > Peter Maydell <peter.maydell@linaro.org> writes:
> >
> >> On Fri, 10 Mar 2023 at 10:31, Alex Bennée <alex.bennee@linaro.org> wrote:
> >>>
> >>> You need a very new gdb to be able to run with pauth support otherwise
> >>> your likely to hit asserts and aborts. Disable pauth for now until we
> >>> can properly probe support in gdb.
> >>>
> >>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> >>
> >> If it makes gdb fall over, then shouldn't we be disabling
> >> the pauth gdbstub stuff entirely ? Otherwise even if our
> >> tests are fine our users will not be...
> >>
> >
> > Have you seem my message on IRC about changing the feature name in the
> > XML? I think the issue is that we're putting the .xml in a "namespace"
> > where GDB expects to only find stuff which it has code to
> > support. Changing from "org.gnu.gdb.aarch64.pauth" to
> > "org.qemu.aarch64.pauth" made it stop crashing and I can read the
> > registers just fine.
>
> That would work, although I would prefer to probe support so we can use
> the official namespace.

I don't think there's a way to probe for this problem. I spoke
to Luis about this, and apparently it's a bug in how gdb handles
the pauth XML description (fixed in gdb commit 1ba3a3222039eb25).
A gdb without any pauth support at all will be fine; a gdb with
the bug will report that it has pauth support but will crash
if you feed it the whole set of XML that QEMU has; a gdb
with the bug fixed will also report pauth support but won't crash.
(The bug only manifests if the full XML includes registers that GDB
doesn't care about, like the system registers; if the stub sends
only registers GDB knows about then it won't crash.)

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.

thanks
-- PMM