linux-kernel - [PATCH 1/4] lib/bitmap: extend comment for bitmap_(from,to)

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Date:   Wed, 20 Apr 2022 15:25:27 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Yury Norov <yury.norov@...il.com>, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org
Subject: [PATCH 1/4] lib/bitmap: extend comment for bitmap_(from,to)_arr32()

On LE systems bitmaps are naturally ordered, therefore we can potentially
use bitmap_copy routines when converting from 32-bit arrays, even if host
system is 64-bit. But it may lead to out-of-bond access due to unsafe
typecast, and the bitmap_(from,to)_arr32 comment doesn't explain that
clearly.

Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 include/linux/bitmap.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index a89b626d0fbe..10d805c2893c 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -271,8 +271,12 @@ static inline void bitmap_copy_clear_tail(unsigned long *dst,
 }
 
 /*
- * On 32-bit systems bitmaps are represented as u32 arrays internally, and
- * therefore conversion is not needed when copying data from/to arrays of u32.
+ * On 32-bit systems bitmaps are represented as u32 arrays internally. On LE64
+ * machines the order of hi and lo parts of nubmers match the bitmap structure.
+ * In both cases conversion is not needed when copying data from/to arrays of
+ * u32. But in LE64 case, typecast in bitmap_copy_clear_tail() may lead to the
+ * out-of-bound access. To avoid that, both LE and BE variants of 64-bit
+ * architectures are not using bitmap_copy_clear_tail().
  */
 #if BITS_PER_LONG == 64
 void bitmap_from_arr32(unsigned long *bitmap, const u32 *buf,
-- 
2.32.0