linux-kernel - [PATCH] x86: drop bogus "cc" clobber from __try_cmpxchg_user

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Date:   Tue, 7 Jun 2022 17:00:53 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH] x86: drop bogus "cc" clobber from __try_cmpxchg_user_asm()

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@...e.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))						\