Re: [PATCH 0/9] Add new command to print the shadow stack backtrace

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

Hello Christina,

"Schimpe, Christina" <christina.schimpe@intel.com> writes:

>> -----Original Message-----
>> From: Schimpe, Christina
>> Sent: Monday, October 13, 2025 8:35 AM
>> To: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
>> Cc: gdb-patches@sourceware.org
>> Subject: RE: [PATCH 0/9] Add new command to print the shadow stack
>> backtrace
>> 
>> > -----Original Message-----
>> > From: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
>> > Sent: Monday, October 13, 2025 3:19 AM
>> > To: Schimpe, Christina <christina.schimpe@intel.com>
>> > Cc: gdb-patches@sourceware.org
>> > Subject: Re: [PATCH 0/9] Add new command to print the shadow stack
>> > backtrace
>> >
>> > Thiago Jung Bauermann <thiago.bauermann@linaro.org> writes:
>> >
>> > > I tested your patches and there's a bug unfortunately:
        ⋮
>> > The problem turned out to be simple. I just had to set the regnum for
>> > the GCSPR in the gdbarch:
>> >
>> > diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c index
>> > 95af82c26327..9e866fc319d4 100644
>> > --- a/gdb/aarch64-tdep.c
>> > +++ b/gdb/aarch64-tdep.c
>> > @@ -4780,6 +4780,10 @@ aarch64_gdbarch_init (struct gdbarch_info
>> info,
>> > struct gdbarch_list *arches)
>> >    /* Register a hook for converting a memory tag to a string.  */
>> >    set_gdbarch_memtag_to_string (gdbarch, aarch64_memtag_to_string);
>> >
>> > +  if (tdep->has_gcs ())
>> > +    /* AArch64's shadow stack pointer is the GCSPR.  */
>> > +    set_gdbarch_ssp_regnum (gdbarch, tdep->gcs_reg_base);
>> > +
>> >    /* ABI */
>> >    set_gdbarch_short_bit (gdbarch, 16);
>> >    set_gdbarch_int_bit (gdbarch, 32);
>> 
>> Ah, I forgot about that. Cool that it works now. 😊
>
> I thought about this again. In addition to setting the regnum for the shadow stack pointer,
> I would have expected that you also have to implement the gdbarch hook
> top_addr_empty_shadow_stack to make GCS work for shadow stack pointer unwinding
> and the shadow stack backtrace, or am I missing something?

Ah, I have a comment about this on my review of patch 1 that is sitting
in my drafts folder. I was waiting to send them all together, but it's
been taking me longer than I expected. I will just send what I have so
far.

But essentially, if there's no gdbarch_top_addr_empty_shadow_stack hook
this patch series changes the check of whether ssp is empty from a <=
comparison with the end of the range to a < comparison. Because of this,
AArch64 doesn't need to implement the hook to make the existing
aarch64-gcs*.exp tests work. It still needs it to make the "bt shadow"
command work though.

> Both, the aarch64 implememtation for top_addr_empty_shadow_stack and ssp regnum
> configuration, should then be part the first patch "Generalize handling ...".
> Does that make sense to you?

Yes, patch 1 should include the changes needed to make the existing
aarch64-gcs*.exp tests passing. As of v1, that's just the ssp regnum
configuration. Depending on what you decide to do for v2, that could
include the top_addr_empty_shadow_stack hook as well.

> Also I should probably add proper error messages when the configuration of ssp_regnum is missing,

Is that possible? How would GDB distinguish between missing ssp regnum
configuration versus the target not supporting shadow stacks?

> and improve the documentation for required gdbarch hooks to properly implement "bt shadow" in
> gdbarch_components.py.

-- 
Thiago