x86/delay: Fix the wrong asm constraint in delay_loop()

[ Upstream commit b86eb74098a92afd789da02699b4b0dd3f73b889 ]

The asm constraint does not reflect the fact that the asm statement can
modify the value of the local variable loops. Which it does.

Specifying the wrong constraint may lead to undefined behavior, it may
clobber random stuff (e.g. local variable, important temporary value in
regs, etc.). This is especially dangerous when the compiler decides to
inline the function and since it doesn't know that the value gets
modified, it might decide to use it from a register directly without
reloading it.

Change the constraint to "+a" to denote that the first argument is an
input and an output argument.

  [ bp: Fix typo, massage commit message. ]

Fixes: e01b70ef3eb3 ("x86: fix bug in arch/i386/lib/delay.c file, delay_loop function")
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220329104705.65256-2-ammarfaizi2@gnuweeb.org
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index 65d15df6212d67e4b936f0b9e764efd2b7737308..0e65d00e2339ff95e022d03ee9012346d2ead8b3 100644 (file)
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -54,8 +54,8 @@ static void delay_loop(u64 __loops)
                "       jnz 2b          \n"
                "3:     dec %0          \n"
 
-               : /* we don't need output */
-               :"a" (loops)
+               : "+a" (loops)
+               :
        );
 }