linux-kernel - [PATCH v2] bitops: Avoid integer overflow in GENMASK(

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Date:   Thu,  3 Aug 2017 14:20:20 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     zijun_hu <zijun_hu@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Doug Anderson <dianders@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: [PATCH v2] bitops: Avoid integer overflow in GENMASK(_ULL)

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@...omium.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