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

On 3/13/23 11:22, Peter Maydell via Gdb wrote:
> On Fri, 10 Mar 2023 at 18:20, Alex Benn=C3=A9e <[email protected]> w=
rote:
>>
>>
>> (adding some more gdb types to CC)
>>
>> Fabiano Rosas <[email protected]> writes:
>>
>>> Peter Maydell <[email protected]> writes:
>>>
>>>> On Fri, 10 Mar 2023 at 10:31, Alex Benn=C3=A9e <[email protected]=
> wrote:
>>>>>
>>>>> You need a very new gdb to be able to run with pauth support otherwis=
e
>>>>> your likely to hit asserts and aborts. Disable pauth for now until we
>>>>> can properly probe support in gdb.
>>>>>
>>>>> Signed-off-by: Alex Benn=C3=A9e <[email protected]>
>>>>
>>>> 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.

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 s=
lowly 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 description=
s here:

https://sourceware.org/git?p=3Dbinutils-gdb.git;a=3Dcommit;h=3Dd7001b29e9f2=
56dfc60acb481d9df8f91f2ee623
https://sourceware.org/git?p=3Dbinutils-gdb.git;a=3Dcommit;h=3De0994165d1b8=
469dfc27b09b62ac74862d535812

>
> thanks
> -- PMM

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