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bitmap: relax find_nth_bit() limitation on return value

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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@rasmusvillemoes.dk>
Closes: https://lore.kernel.org/all/Zi50cAgR8nZvgLa3@yury-ThinkPad/T/#m6da806a0525e74dcc91f35e5f20766ed4e853e8a
Signed-off-by: Yury Norov <yury.norov@gmail.com>
This commit is contained in:
Yury Norov 2024-05-02 10:12:56 -07:00
parent 77db1920a8
commit 0b2811ba11
3 changed files with 4 additions and 4 deletions
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2
include/linux/find.h
View File

@ -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)

2
lib/find_bit.c
View File

@ -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; \
})

4
lib/test_bitmap.c
View File

@ -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++);