x86: drop bogus "cc" clobber from __try_cmpxchg_user_asm()

[PATCH] x86: drop bogus "cc" clobber from __try_cmpxchg_user_asm()

Posted by Jan Beulich 3 years, 10 months ago

As noted (and fixed) a couple of times in the past, "=@cc<cond>" outputs
and clobbering of "cc" don't work well together. The compiler appears to
mean to reject such, but doesn't - in its upstream form - quite manage
to yet for "cc". Furthermore two similar macros don't clobber "cc", and
clobbering "cc" is pointless in asm()-s for x86 anyway - the compiler
always assumes status flags to be clobbered there.

Fixes: 989b5db215a2 ("x86/uaccess: Implement macros for CMPXCHG on user addresses")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
The change I continue to carry locally actually allows specifying that
"cc" is _not_ clobbered; it only happens to improve detection of bad
situations like the one here.

--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -439,7 +439,7 @@ do {									\
 		       [ptr] "+m" (*_ptr),				\
 		       [old] "+a" (__old)				\
 		     : [new] ltype (__new)				\
-		     : "memory", "cc");					\
+		     : "memory");					\
 	if (unlikely(__err))						\
 		goto label;						\
 	if (unlikely(!success))						\

Re: [PATCH] x86: drop bogus "cc" clobber from __try_cmpxchg_user_asm()

Posted by Paolo Bonzini 3 years, 10 months ago

On 6/7/22 17:00, Jan Beulich wrote:
> As noted (and fixed) a couple of times in the past, "=@cc<cond>" outputs
> and clobbering of "cc" don't work well together. The compiler appears to
> mean to reject such, but doesn't - in its upstream form - quite manage
> to yet for "cc". Furthermore two similar macros don't clobber "cc", and
> clobbering "cc" is pointless in asm()-s for x86 anyway - the compiler
> always assumes status flags to be clobbered there.
> 
> Fixes: 989b5db215a2 ("x86/uaccess: Implement macros for CMPXCHG on user addresses")
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> The change I continue to carry locally actually allows specifying that
> "cc" is _not_ clobbered; it only happens to improve detection of bad
> situations like the one here.
> 
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -439,7 +439,7 @@ do {									\
>   		       [ptr] "+m" (*_ptr),				\
>   		       [old] "+a" (__old)				\
>   		     : [new] ltype (__new)				\
> -		     : "memory", "cc");					\
> +		     : "memory");					\
>   	if (unlikely(__err))						\
>   		goto label;						\
>   	if (unlikely(!success))						\
> 

Queued, thanks.

Paolo