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 17a0d0f5a1bf4f2fd15ccba6bcba03ae36bdb31f..ea1d00159ea65e0c02a3f7fb79662bcfd95a483b 100644 (file)
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -43,8 +43,8 @@ static void delay_loop(unsigned long loops)
                "       jnz 2b          \n"
                "3:     dec %0          \n"
 
-               : /* we don't need output */
-               :"a" (loops)
+               : "+a" (loops)
+               :
        );
 }