[tip: x86/misc] x86/delay: Fix the wrong asm constraint in delay_loop()

[tip-bot2 for Ammar Faizi]

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

Commit-ID:     b86eb74098a92afd789da02699b4b0dd3f73b889
Gitweb:        https://git.kernel.org/tip/b86eb74098a92afd789da02699b4b0dd3f73b889
Author:        Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
AuthorDate:    Tue, 29 Mar 2022 17:47:04 +07:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Tue, 05 Apr 2022 21:21:57 +02:00

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

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@xxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lore.kernel.org/r/20220329104705.65256-2-ammarfaizi2@xxxxxxxxxxx
---
 arch/x86/lib/delay.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index 65d15df..0e65d00 100644
--- 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)
+		:
 	);
 }