[PATCH v4 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()`

Ammar Faizi
2022-03-01T09:46:50Z

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

The asm constraint does not reflect that the asm statement can modify
the value of @loops. But the asm statement in delay_loop() does change
the @loops.

If by any chance the compiler inlines this function, it may clobber
random stuff (e.g. local variable, important temporary value in reg,
etc.).

Fortunately, delay_loop() is only called indirectly (so it can't
inline), and then the register it clobbers is %rax (which is by the
nature of the calling convention, it's a caller saved register), so it
didn't yield any bug.

^ That shouldn't be an excuse for using the wrong constraint anyway.

This changes "a" (as an input) to "+a" (as an input and output).

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jiri Hladky <hladky.jiri@googlemail.com>
Fixes: e01b70ef3eb3 ("x86: fix bug in arch/i386/lib/delay.c file, delay_loop function")
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---

Fortunately, the constraint violation that's fixed by patch 1 doesn't
yield any bug due to the nature of System V ABI. Should we backport
this?

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 65d15df6212d..0e65d00e2339 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)
+ :
);
}

--
2.32.0