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

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 ]

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/log2.h b/include/linux/log2.h
index f38fae23bdac52977e563b33625da1fec8b76a7b..c373295f359fa582859962bbf3295dbd936111ad 100644 (file)
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -194,6 +194,17 @@ 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 */