Re: [PATCH v3 12/13] aarch64: redefine RETURN_ADDRESS to strip PAC

[Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>]

On 15/05/2020 11:40, Szabolcs Nagy wrote:
> RETURN_ADDRESS is used at several places in glibc to mean a valid
> code address of the call site, but with pac-ret it has a pointer
> authentication code (PAC), so its definition is adjusted.
> 
> strip_pac is omitted if glibc is bulit without pac-ret, but it could

s/bulit/built

> be added unconditionally (that's just unnecessary operations).
> Inline asm is used instead of __builtin_aarch64_xpaclri since that
> is an undocumented builtin and not available in all supported gccs.
> 
> Note: such change indicates a problem in the pac-ret design: it
> can break code that uses __builtin_return_address and the breakage
> is only visible at runtime on a system with pac-ret enabled. It is
> not ideal that users need target specific inline asm to fix this up.
> For now we can recommend disabling pac-ret where this is a problem,
> but gcc might need improvements in this are to make pac-ret usable.
> 
> TODO: __builtin_return_address handling with pac-ret:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94891
LGTM, thanks.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

> ---
>  sysdeps/aarch64/sysdep.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/sysdeps/aarch64/sysdep.h b/sysdeps/aarch64/sysdep.h
> index c51572a690..7a70cf7a2b 100644
> --- a/sysdeps/aarch64/sysdep.h
> +++ b/sysdeps/aarch64/sysdep.h
> @@ -35,6 +35,23 @@
>  
>  #define PTR_SIZE	(1<<PTR_LOG_SIZE)
>  
> +#ifndef	__ASSEMBLER__
> +/* Strip pointer authentication code from pointer p.  */
> +static inline void *
> +strip_pac (void *p)
> +{
> +	register void *ra asm ("x30") = (p);
> +	asm ("hint 7 // xpaclri" : "+r"(ra));
> +	return ra;

Indentation seems off here (tab instead of double space).

> +}
> +
> +/* This is needed when glibc is built with -mbranch-protection=pac-ret.  */
> +# ifdef HAVE_AARCH64_PAC_RET
> +#  undef RETURN_ADDRESS
> +#  define RETURN_ADDRESS(n) strip_pac (__builtin_return_address (n))
> +# endif
> +#endif
> +
>  #ifdef	__ASSEMBLER__
>  
>  /* Syntactic details of assembler.  */
> 

Ok.