[tip: x86/cleanups] x86/uaccess: Improve __try_cmpxchg64_user_asm() for x86_32

[tip-bot2 for Uros Bizjak]

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID:     4630535c64725bcd722eb59fb8d92be8db943d3d
Gitweb:        https://git.kernel.org/tip/4630535c64725bcd722eb59fb8d92be8db943d3d
Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate:    Tue, 28 Jun 2022 18:16:12 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Mon, 15 Aug 2022 19:18:12 +02:00

x86/uaccess: Improve __try_cmpxchg64_user_asm() for x86_32

Improve __try_cmpxcgh64_user_asm() for !CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT
by relaxing the output register constraint from "c" to "q" constraint,
which allows the compiler to choose between %ecx or %ebx register.

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20220628161612.7993-1-ubizjak@xxxxxxxxx
---
 arch/x86/include/asm/uaccess.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 913e593..b0583c1 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -448,7 +448,7 @@ do {									\
 
 #ifdef CONFIG_X86_32
 /*
- * Unlike the normal CMPXCHG, hardcode ECX for both success/fail and error.
+ * Unlike the normal CMPXCHG, use output GPR for both success/fail and error.
  * There are only six GPRs available and four (EAX, EBX, ECX, and EDX) are
  * hardcoded by CMPXCHG8B, leaving only ESI and EDI.  If the compiler uses
  * both ESI and EDI for the memory operand, compilation will fail if the error
@@ -461,11 +461,12 @@ do {									\
 	__typeof__(*(_ptr)) __new = (_new);				\
 	asm volatile("\n"						\
 		     "1: " LOCK_PREFIX "cmpxchg8b %[ptr]\n"		\
-		     "mov $0, %%ecx\n\t"				\
-		     "setz %%cl\n"					\
+		     "mov $0, %[result]\n\t"				\
+		     "setz %b[result]\n"				\
 		     "2:\n"						\
-		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG, %%ecx) \
-		     : [result]"=c" (__result),				\
+		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG,	\
+					   %[result])			\
+		     : [result] "=q" (__result),			\
 		       "+A" (__old),					\
 		       [ptr] "+m" (*_ptr)				\
 		     : "b" ((u32)__new),				\