aboutsummaryrefslogtreecommitdiffstats
path: root/arch/x86/include/asm
diff options
context:
space:
mode:
authorMathias Krause <minipli@googlemail.com>2017-10-05 20:30:12 +0200
committerThomas Gleixner <tglx@linutronix.de>2017-10-09 13:35:17 +0200
commit6b32c126d33d5cb379bca280ab8acedc1ca978ff (patch)
tree90b2a48d574c8ac2c055e69696fb989bc4f14900 /arch/x86/include/asm
parent924c6b900cfdf376b07bccfd80e62b21914f8a5a (diff)
x86/alternatives: Fix alt_max_short macro to really be a max()
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 Reviewed-and-tested-by: Borislav Petkov <bp@suse.de> Fixes: dbe4058a6a44 ("x86/alternatives: Fix ALTERNATIVE_2 padding generation properly") Signed-off-by: Mathias Krause <minipli@googlemail.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Borislav Petkov <bp@suse.de> Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/1507228213-13095-1-git-send-email-minipli@googlemail.com
Diffstat (limited to 'arch/x86/include/asm')
-rw-r--r--arch/x86/include/asm/alternative-asm.h4
-rw-r--r--arch/x86/include/asm/alternative.h6
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

Privacy Policy