linux-kernel - [PATCH 2/2] lib/bitmap: drop optimization of bitmap_{from,to}

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Message-Id: <20230227214524.914050-2-yury.norov@gmail.com>
Date:   Mon, 27 Feb 2023 13:45:24 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Alexander Lobakin <aleksander.lobakin@...el.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>,
        Guenter Roeck <linux@...ck-us.net>,
        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>, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>
Subject: [PATCH 2/2] lib/bitmap: drop optimization of bitmap_{from,to}_arr64

bitmap_{from,to}_arr64() optimization is overly optimistic on 32-bit LE
architectures when it's wired to bitmap_copy_clear_tail().

bitmap_copy_clear_tail() takes care of unused bits in the bitmap up to
the next word boundary. But on 32-bit machines when copying bits from
bitmap to array of 64-bit words, it's expected that the unused part of
a recipient array must be cleared up to 64-bit boundary, so the last 4
bytes may stay untouched when nbits % 64 <= 32.

While the copying part of the optimization works correct, that clear-tail
trick makes corresponding tests reasonably fail:

test_bitmap: bitmap_to_arr64(nbits == 1): tail is not safely cleared: 0xa5a5a5a500000001 (must be 0x0000000000000001)

Fix it by removing bitmap_{from,to}_arr64() optimization for 32-bit LE
arches.

Reported-by: Guenter Roeck <linux@...ck-us.net>
Link: https://lore.kernel.org/lkml/20230225184702.GA3587246@roeck-us.net/
Fixes: 0a97953fd221 ("lib: add bitmap_{from,to}_arr64")
Signed-off-by: Yury Norov <yury.norov@...il.com>
Tested-by: Guenter Roeck <linux@...ck-us.net>
---
 include/linux/bitmap.h | 8 +++-----
 lib/bitmap.c           | 2 +-
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 40e53a2ecc0d..7d4c90eb3df4 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -302,12 +302,10 @@ void bitmap_to_arr32(u32 *buf, const unsigned long *bitmap,
 #endif
 
 /*
- * On 64-bit systems bitmaps are represented as u64 arrays internally. On LE32
- * machines the order of hi and lo parts of numbers match the bitmap structure.
- * In both cases conversion is not needed when copying data from/to arrays of
- * u64.
+ * On 64-bit systems bitmaps are represented as u64 arrays internally. So,
+ * the conversion is not needed when copying data from/to arrays of u64.
  */
-#if (BITS_PER_LONG == 32) && defined(__BIG_ENDIAN)
+#if BITS_PER_LONG == 32
 void bitmap_from_arr64(unsigned long *bitmap, const u64 *buf, unsigned int nbits);
 void bitmap_to_arr64(u64 *buf, const unsigned long *bitmap, unsigned int nbits);
 #else
diff --git a/lib/bitmap.c b/lib/bitmap.c
index 1c81413c51f8..ddb31015e38a 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -1495,7 +1495,7 @@ void bitmap_to_arr32(u32 *buf, const unsigned long *bitmap, unsigned int nbits)
 EXPORT_SYMBOL(bitmap_to_arr32);
 #endif
 
-#if (BITS_PER_LONG == 32) && defined(__BIG_ENDIAN)
+#if BITS_PER_LONG == 32
 /**
  * bitmap_from_arr64 - copy the contents of u64 array of bits to bitmap
  *	@bitmap: array of unsigned longs, the destination bitmap
-- 
2.34.1