linux-kernel - [PATCH 2/4] bitmap: relax find_nth_bit() limitation on return value

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Message-Id: <20240502233204.2255158-3-yury.norov@gmail.com>
Date: Thu,  2 May 2024 16:32:02 -0700
From: Yury Norov <yury.norov@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kuan-Wei Chiu <visitorckw@...il.com>
Subject: [PATCH 2/4] bitmap: relax find_nth_bit() limitation on return value

The function claims to return the bitmap size if Nth bit doesn't exist.
This rule is violated in inline case because the fns() that is used
there doesn't know anything about size of the bitmap.

So, relax this requirement to '>= size', and make the outline
implementation a bit cheaper.

All in-tree kernel users of find_nth_bit() are safe against that.

Reported-by: Rasmus Villemoes <linux@...musvillemoes.dk>
Closes: https://lore.kernel.org/all/Zi50cAgR8nZvgLa3@yury-ThinkPad/T/#m6da806a0525e74dcc91f35e5f20766ed4e853e8a
Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 include/linux/find.h | 2 +-
 lib/find_bit.c       | 2 +-
 lib/test_bitmap.c    | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/find.h b/include/linux/find.h
index c69598e383c1..02751e43d448 100644
--- a/include/linux/find.h
+++ b/include/linux/find.h
@@ -220,7 +220,7 @@ unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
  *	 idx = find_first_bit(addr, size);
  *
  * Returns the bit number of the N'th set bit.
- * If no such, returns @size.
+ * If no such, returns >= @size.
  */
 static inline
 unsigned long find_nth_bit(const unsigned long *addr, unsigned long size, unsigned long n)
diff --git a/lib/find_bit.c b/lib/find_bit.c
index 32f99e9a670e..0bddfc3ff248 100644
--- a/lib/find_bit.c
+++ b/lib/find_bit.c
@@ -87,7 +87,7 @@ out:										\
 	if (sz % BITS_PER_LONG)							\
 		tmp = (FETCH) & BITMAP_LAST_WORD_MASK(sz);			\
 found:										\
-	sz = min(idx * BITS_PER_LONG + fns(tmp, nr), sz);			\
+	sz = idx * BITS_PER_LONG + fns(tmp, nr);				\
 out:										\
 	sz;									\
 })
diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
index 6b2b33579f56..088838f829c9 100644
--- a/lib/test_bitmap.c
+++ b/lib/test_bitmap.c
@@ -244,7 +244,7 @@ static void __init test_find_nth_bit(void)
 	expect_eq_uint(60,  find_nth_bit(bmap, 64 * 3, 5));
 	expect_eq_uint(80,  find_nth_bit(bmap, 64 * 3, 6));
 	expect_eq_uint(123, find_nth_bit(bmap, 64 * 3, 7));
-	expect_eq_uint(64 * 3, find_nth_bit(bmap, 64 * 3, 8));
+	expect_eq_uint(0,   !!(find_nth_bit(bmap, 64 * 3, 8) < 64 * 3));
 
 	expect_eq_uint(10,  find_nth_bit(bmap, 64 * 3 - 1, 0));
 	expect_eq_uint(20,  find_nth_bit(bmap, 64 * 3 - 1, 1));
@@ -254,7 +254,7 @@ static void __init test_find_nth_bit(void)
 	expect_eq_uint(60,  find_nth_bit(bmap, 64 * 3 - 1, 5));
 	expect_eq_uint(80,  find_nth_bit(bmap, 64 * 3 - 1, 6));
 	expect_eq_uint(123, find_nth_bit(bmap, 64 * 3 - 1, 7));
-	expect_eq_uint(64 * 3 - 1, find_nth_bit(bmap, 64 * 3 - 1, 8));
+	expect_eq_uint(0,   !!(find_nth_bit(bmap, 64 * 3 - 1, 8) < 64 * 3 - 1));
 
 	for_each_set_bit(bit, exp1, EXP1_IN_BITS) {
 		b = find_nth_bit(exp1, EXP1_IN_BITS, cnt++);
-- 
2.40.1