linux-kernel - [PATCH 1/2] lib/bitmap: fix off-by-one in bitmap_to

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Date:   Mon, 11 Jul 2022 20:09:29 +0200
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] lib/bitmap: fix off-by-one in bitmap_to_arr64()

GENMASK*() family takes the first and the last bits of the mask
*including* them. So, with the current code bitmap_to_arr64()
doesn't clear the tail properly:

nbits %  exp             mask                must be
1        GENMASK(1, 0)   0x3                 0x1
...
63       GENMASK(63, 0)  0xffffffffffffffff  0x7fffffffffffffff

This was found by making the function always available instead of
32-bit BE systems only (for reusing in some new functionality).
Turn the number of bits into the last bit set by subtracting 1.
@nbits is already checked to be positive beforehand.

Fixes: 0a97953fd221 ("lib: add bitmap_{from,to}_arr64")
Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>
---
 lib/bitmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/bitmap.c b/lib/bitmap.c
index b18e31ea6e66..e903e13c62e1 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -1564,7 +1564,7 @@ void bitmap_to_arr64(u64 *buf, const unsigned long *bitmap, unsigned int nbits)
 
 	/* Clear tail bits in the last element of array beyond nbits. */
 	if (nbits % 64)
-		buf[-1] &= GENMASK_ULL(nbits % 64, 0);
+		buf[-1] &= GENMASK_ULL((nbits - 1) % 64, 0);
 }
 EXPORT_SYMBOL(bitmap_to_arr64);
 #endif
-- 
2.36.1