Patch "log2: make order_base_2() behave correctly on const input value zero" has been added to the 3.18-stable tree

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

log2: make order_base_2() behave correctly on const input value zero

to the 3.18-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:
log2-make-order_base_2-behave-correctly-on-const-input-value-zero.patch
and it can be found in the queue-3.18 subdirectory.

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

From: Ard Biesheuvel <[email protected]>
Date: Thu, 2 Feb 2017 18:05:26 +0000
Subject: log2: make order_base_2() behave correctly on const input value zero

From: Ard Biesheuvel <[email protected]>

commit 29905b52fad0854351f57bab867647e4982285bf upstream.

The function order_base_2() is defined (according to the comment block) as returning zero on input zero, but subsequently passes the input into roundup_pow_of_two(), which is explicitly undefined for input zero.

This has gone unnoticed until now, but optimization passes in GCC 7 may produce constant folded function instances where a constant value of zero is passed into order_base_2(), resulting in link errors against the deliberately undefined '____ilog2_NaN'.

So update order_base_2() to adhere to its own documented interface.

[ See

     http://marc.info/?l=linux-kernel&m=147672952517795&w=2

and follow-up discussion for more background. The gcc "optimization pass" is really just broken, but now the GCC trunk problem seems to have escaped out of just specially built daily images, so we need to
work around it in mainline. - Linus ]

---
 include/linux/log2.h |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

@@ -194,6 +194,17 @@ unsigned long __rounddown_pow_of_two(uns
  *  ... and so on.
  */
 
-#define order_base_2(n) ilog2(roundup_pow_of_two(n))
+static inline __attribute_const__
+int __order_base_2(unsigned long n)
+{
+	return n > 1 ? ilog2(n - 1) + 1 : 0;
+}
 
+#define order_base_2(n)				\
+(						\
+	__builtin_constant_p(n) ? (		\
+		((n) == 0 || (n) == 1) ? 0 :	\
+		ilog2((n) - 1) + 1) :		\
+	__order_base_2(n)			\
+)
 #endif /* _LINUX_LOG2_H */

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

queue-3.18/log2-make-order_base_2-behave-correctly-on-const-input-value-zero.patch