[PATCH 11/12] aarch64: redefine RETURN_ADDRESS to strip PAC

[Florian Weimer]

* Adhemerval Zanella via Libc-alpha:

> On 11/05/2020 09:38, Szabolcs Nagy wrote:
>> The 05/08/2020 14:44, Adhemerval Zanella via Libc-alpha wrote:
>>> On 30/04/2020 14:45, Szabolcs Nagy wrote:
>>>> +++ b/sysdeps/aarch64/sysdep.h
>>>> @@ -35,6 +35,16 @@
>>>>  
>>>>  #define PTR_SIZE	(1<<PTR_LOG_SIZE)
>>>>  
>>>> +/* Strip pointer authentication code from pointer p.  */
>>>> +#define XPAC(p) ({					\
>>>> +  register void *__ra asm ("x30") = (p);		\
>>>> +  asm ("hint 7 // xpaclri" : "+r"(__ra));		\
>>>> +  __ra;})
>>>> +
>>>> +/* This is needed when glibc is built with -mbranch-protection=pac-ret.  */
>>>> +#undef RETURN_ADDRESS
>>>> +#define RETURN_ADDRESS(n) XPAC(__builtin_return_address(n))
>>>> +
>>>
>>> Maybe use a inline function instead?
>> 
>> macro seems more reliable to me than always_inline
>> when poking at __builtin_return_address and x30,
>> but i'm not against always_inline if that's
>> considered better.
>
> I would prefer a static inline unless a macro is really required
> (either due some compiler limitation or bug).

I think __builtin_return_address is ill-defined: Does the frame count
that vanishes due to inlining?

So it's probably a case similar to alloca, where a macro has to be
used.