RE: [PATCH v2 1/9] gdb: Generalize handling of the shadow stack pointer.

Schimpe, Christina
2026-04-15T07:36:08Z

> -----Original Message-----
> From: Tom Tromey <tom@tromey.com>
> Sent: Dienstag, 14. April 2026 19:34
> To: Schimpe, Christina <christina.schimpe@intel.com>
> Cc: Tom Tromey <tom@tromey.com>; gdb-patches@sourceware.org;
> thiago.bauermann@linaro.org
> Subject: Re: [PATCH v2 1/9] gdb: Generalize handling of the shadow stack
> pointer.
>
> >>>>> Schimpe, Christina <christina.schimpe@intel.com> writes:
>
> > Sorry for getting back delayed response. I am seeing the following
> > when running check-gdbarch.py:
>
> > ~~~
> > $ ./check-gdbarch.py
> > never set: shadow_stack_element_size_aligned ~~~
>
> > Given that I assign a pre-default I don't have to configure it for amd64
> specifically.
> > But starting with this patch, this message will always appear when running
> the script.
>
> > Should we change the script for this case?
>
> It depends.
>
> This means that no architecture sets this gdbarch parameter.
> So:
>
> 1. Maybe it's not needed at all and should be removed, or 2. Maybe it's used
> in some later patch and the warning should be
> ignored, or
> 3. Maybe it might be used in the future and we don't want to
> hard-code the value and so it should be marked 'unused'
> in gdbarch_components.py

It is currently not used in any subsequent patch. GCS, amd64, and x32 all use an 8‑byte offset.
In theory, Linux kernel support for 32‑bit shadow stacks could be added in the future, which
would require a 4‑byte offset. For other architectures, I don't know about the details.

So based on that I'd go with option 3, do you agree ?

I could add a comment such as:

+ # Currently unused but we wanted to keep this hook around,
+ # since x86 32-bit shadow stacks would require a 4-byte offset,
+ # for instance. But this is not supported yet by the Linux kernel
+ # as x86 shadow stack for user space is only supported for amd64
+ # Linux starting with Linux kernel v6.6.
+ unused=True,

Alternatively, I could re-use the already existing shorter comment if you prefer:

+ # Currently unused but we wanted to keep this hook around.
+ unused=True,

I'd prefer the first comment, but keeping it short would be fine with me, too.

Thanks,
Christina
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