The alt_max_short() macro in asm/alternative.h does not work as
intended, leading to nasty bugs. E.g. alt_max_short("1", "3")
evaluates to 3, but alt_max_short("3", "1") evaluates to 1 -- not
exactly the maximum of 1 and 3.
In fact, I had to learn it the hard way by crashing my kernel in not
so funny ways by attempting to make use of the ALTENATIVE_2 macro
with alternatives where the first one was larger than the second
one.
According to [1] and commit dbe4058a6a44 ("x86/alternatives: Fix
ALTERNATIVE_2 padding generation properly") the right handed side
should read "-(-(a < b))" not "-(-(a - b))". Fix that, to make the
macro work as intended.
While at it, fix up the comment regarding the additional "-", too.
It's not about gas' usage of s32 but brain dead logic of having a
"true" value of -1 for the < operator ... *sigh*
Btw., the one in asm/alternative-asm.h is correct. And, apparently,
all current users of ALTERNATIVE_2() pass same sized alternatives,
avoiding to hit the bug.
[1] http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
Fixes: dbe4058a6a44 ("x86/alternatives: Fix ALTERNATIVE_2 padding generation
properly")
Signed-off-by: Mathias Krause <mini...@googlemail.com>
Cc: Borislav Petkov <b...@suse.de>
---
arch/x86/include/asm/alternative.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/alternative.h
b/arch/x86/include/asm/alternative.h
index c096624137ae..7c553f48f163 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -106,9 +106,9 @@ static inline int alternatives_text_reserved(void *start,
void *end)
* max without conditionals. Idea adapted from:
* http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
*
- * The additional "-" is needed because gas works with s32s.
+ * The additional "-" is needed because gas uses a "true" value of -1.
*/
-#define alt_max_short(a, b) "((" a ") ^ (((" a ") ^ (" b ")) & -(-((" a ")
- (" b ")))))"
+#define alt_max_short(a, b) "((" a ") ^ (((" a ") ^ (" b ")) & -(-((" a ")
< (" b ")))))"
/*
* Pad the second replacement alternative with additional NOPs if it is
--
1.7.10.4
