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

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 30/04/2020 14:45, 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 that includes a
> pointer authentication code, so the definition is adjusted.
> 
> XPAC is added unconditionally for now, but it's only needed if
> glibc is compiled with -mbranch-protection=pac-ret. Inline asm
> is used instead of __builtin_aarch64_xpaclri since that's an
> undocumented builtin and not available in all supported gccs.
> ---
>  sysdeps/aarch64/sysdep.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/sysdeps/aarch64/sysdep.h b/sysdeps/aarch64/sysdep.h
> index 63a04a70cd..87f19b9bef 100644
> --- a/sysdeps/aarch64/sysdep.h
> +++ 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?

  #ifndef __ASSEMBLER__
  # include <sys/cdefs.h>
  /* Strip pointer authentication code from pointer p.  */
  static __always_inline void *
  return_address (unsigned int n)
  { 
    register void *ra asm ("x30") = __builtin_return_address (n);
    asm ("hint 7 // xpaclri" : "+r" (ra));
    return ra;
  }

  /* This is needed when glibc is built with -mbranch-protection=pac-ret.  */
  # undef RETURN_ADDRESS
  # define RETURN_ADDRESS(n) return_address (n)
  #endif


>  #ifdef	__ASSEMBLER__
>  
>  /* Syntactic details of assembler.  */