[PATCH 2/7] linux/bitmap.h: handle constant zero-size bitmaps correctly

[Rasmus Villemoes]

The static inlines in bitmap.h do not handle a compile-time constant
nbits==0 correctly (they dereference the passed src or dst pointers,
despite only 0 words being valid to access). I had the 0-day buildbot
chew on a patch [1] that would cause build failures for such cases
without complaining, suggesting that we don't have any such users
currently, at least for the 70 .config/arch combinations that was
built. Should any turn up, make sure they use the out-of-line versions,
which do handle nbits==0 correctly.

This is of course not the most efficient, but it's much less churn than
teaching all the static inlines an "if (zero_const_nbits())", and since
we don't have any current instances, this doesn't affect existing code
at all.

[1] lkml.kernel.org/r/20180815085539.27485-1-linux@xxxxxxxxxxxxxxxxxx

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
 include/linux/bitmap.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 1ee46f492267..b91a6b5d3e78 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -196,8 +196,13 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
 #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
 #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
 
+/*
+ * The static inlines below do not handle constant nbits==0 correctly,
+ * so make such users (should any ever turn up) call the out-of-line
+ * versions.
+ */
 #define small_const_nbits(nbits) \
-	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG)
+	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
 
 static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
 {
-- 
2.16.4