From: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
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@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Jiri Hladky <hladky.jiri@xxxxxxxxxxxxxx>
Fixes: e01b70ef3eb3 ("x86: fix bug in arch/i386/lib/delay.c file, delay_loop function")
Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
---
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
