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

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 ]

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/log2.h b/include/linux/log2.h
index fd7ff3d91e6a920ff084beca09d10b5b9abba981..ef3d4f67118ce0f60789e6e749a4773754e01e87 100644 (file)
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -203,6 +203,17 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
  *  ... 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 */