Patch "x86/alternatives: Fix alt_max_short macro to really be a max()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

x86/alternatives: Fix alt_max_short macro to really be a max()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-alternatives-fix-alt_max_short-macro-to-really-be-a-max.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mathias Krause <[email protected]>
Date: Thu, 5 Oct 2017 20:30:12 +0200
Subject: x86/alternatives: Fix alt_max_short macro to really be a max()

From: Mathias Krause <[email protected]>

commit 6b32c126d33d5cb379bca280ab8acedc1ca978ff upstream.

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

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

@@ -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)))))
 

@@ -102,12 +102,12 @@ static inline int alternatives_text_rese
 	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

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-alternatives-fix-alt_max_short-macro-to-really-be-a-max.patch