[PATCH v2] x86/alternatives: Fix alt_max_short macro to really be a max()

Mathias Krause
2017-10-05T18:30:47Z

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 comments 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 <minipli@googlemail.com>
Reviewed-and-tested-by: Borislav Petkov <bp@suse.de>
---
v2:
- fix the comments even further, as requested by Boris
- add reviewed-and-tested-by for Boris

arch/x86/include/asm/alternative-asm.h | 4 +++-
arch/x86/include/asm/alternative.h | 6 +++---
2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/alternative-asm.h b/arch/x86/include/asm/alternative-asm.h
index e7636bac7372..6c98821fef5e 100644
--- a/arch/x86/include/asm/alternative-asm.h
+++ b/arch/x86/include/asm/alternative-asm.h
@@ -62,8 +62,10 @@
#define new_len2 145f-144f

/*
- * max without conditionals. Idea adapted from:
+ * gas compatible max based on the idea from:
* http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
+ *
+ * The additional "-" is needed because gas uses a "true" value of -1.
*/
#define alt_max_short(a, b) ((a) ^ (((a) ^ (b)) & -(-((a) < (b)))))

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index c096624137ae..ccbe24e697c4 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -103,12 +103,12 @@ static inline int alternatives_text_reserved(void *start, void *end)
alt_end_marker ":\n"

/*
- * max without conditionals. Idea adapted from:
+ * gas compatible max based on the idea 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