[PATCH v2] bitops: Avoid integer overflow in GENMASK(_ULL)

[PATCH v2] bitops: Avoid integer overflow in GENMASK(_ULL)

[Matthias Kaehlcke]

GENMASK(_ULL) performs a left-shift of ~0UL(L), which technically
results in an integer overflow. clang raises a warning if the overflow
occurs in a preprocessor expression. Clear the low-order bits through
a substraction instead of the left-shift to avoid the overflow.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
---
Changes in v2:
- use substraction instead of left-shift, get rid off odd right-shift
- also change GENMASK
- updated commit message

 include/linux/bitops.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index a83c822c35c2..8fbe259b197c 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -19,10 +19,11 @@
  * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
  */
 #define GENMASK(h, l) \
-	(((~0UL) << (l)) & (~0UL >> (BITS_PER_LONG - 1 - (h))))
+	(((~0UL) - (1UL << (l)) + 1) & (~0UL >> (BITS_PER_LONG - 1 - (h))))
 
 #define GENMASK_ULL(h, l) \
-	(((~0ULL) << (l)) & (~0ULL >> (BITS_PER_LONG_LONG - 1 - (h))))
+	(((~0ULL) - (1ULL << (l)) + 1) & \
+	 (~0ULL >> (BITS_PER_LONG_LONG - 1 - (h))))
 
 extern unsigned int __sw_hweight8(unsigned int w);
 extern unsigned int __sw_hweight16(unsigned int w);
-- 
2.14.0.rc1.383.gd1ce394fe2-goog

Re: [PATCH v2] bitops: Avoid integer overflow in GENMASK(_ULL)

[Andrew Morton]

On Thu,  3 Aug 2017 14:20:20 -0700 Matthias Kaehlcke <mka@chromium.org> wrote:

> GENMASK(_ULL) performs a left-shift of ~0UL(L), which technically
> results in an integer overflow. clang raises a warning if the overflow
> occurs in a preprocessor expression.

That's just irritaing.

> Clear the low-order bits through
> a substraction instead of the left-shift to avoid the overflow.

There's no change in .text size for a few files I tested so guess we
can live with it.