Commit 0b2811ba authored by Yury Norov's avatar Yury Norov
Browse files

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: default avatarRasmus Villemoes <linux@rasmusvillemoes.dk>
Closes: https://lore.kernel.org/all/Zi50cAgR8nZvgLa3@yury-ThinkPad/T/#m6da806a0525e74dcc91f35e5f20766ed4e853e8a


Signed-off-by: default avatarYury Norov <yury.norov@gmail.com>
parent 77db1920

include/linux/find.h

+1 −1
Original line number Diff line number Diff line
@@ -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)

lib/find_bit.c

+1 −1
Original line number Diff line number Diff line
@@ -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;									\

})

lib/test_bitmap.c

+2 −2
Original line number Diff line number Diff line
@@ -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++);