Re: [PATCH] GDB: aarch64: Fix inferior function call if GCS is present but disabled

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

Hello Luis,

Thank you for the review!

Luis <luis.machado.foss@gmail.com> writes:

> On 08/07/2026 07:03, Thiago Jung Bauermann wrote:
>> On AArch64, even if the Guarded Control Stack feature is present on the
>> system the inferior may not have enabled it.
>> There's a bug in GDB's GCS support in that if the system supports GCS,
>> GDB will always push a GCS entry when doing an inferior function call
>> even if the inferior doesn't have it enabled.  This causes inferior
>> function calls to fail. E.g.:
>>    (gdb) p foo ()
>>    Can't write to Guarded Control Stack.
>> The error message is because the GCSPR doesn't point to a valid memory
>> address.
>> Fix by checking whether GCS is enabled in the inferior before trying to
>> push a GCS entry.
>> Regression tested on an aarch64-linux-gnu QEMU VM with GCS present.
>> ---
>> GCS support first appeared in GDB 17, so if this patch is approved for
>> trunk is it ok to also commit it to the branch?
>
> Sounds good to me.
>
>>   gdb/aarch64-tdep.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>> diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
>> index 886e7ce1e7b6..b4437dcccc8a 100644
>> --- a/gdb/aarch64-tdep.c
>> +++ b/gdb/aarch64-tdep.c
>> @@ -1934,7 +1934,11 @@ static void
>>   aarch64_shadow_stack_push (gdbarch *gdbarch, CORE_ADDR new_addr,
>>   			   regcache *regcache)
>>   {
>> -  aarch64_push_gcs_entry (regcache, new_addr);
>> +  bool gcs_is_enabled;
>> +
>> +  gdbarch_get_shadow_stack_pointer (gdbarch, regcache, gcs_is_enabled);
>> +  if (gcs_is_enabled)
>> +    aarch64_push_gcs_entry (regcache, new_addr);
>>   }
>>     /* Implement the "push_dummy_call" gdbarch method.  */
>
> LGTM, with Baris' suggestion.
>
> Approved-By: Luis Machado <luis.machado.foss@gmail.com>

Pushed as commit 4410648da673, with Baris' suggestion.

Also pushed as commit cbdcbf506376 to gdb-17-branch.

-- 
Thiago
(he/him)