[PATCH] lib/bitmap: rework bitmap_cut()
From: Yury Norov
Date: Fri Mar 06 2020 - 17:14:56 EST
bitmap_cut() refers src after memmove(). If dst and src overlap,
it may cause buggy behaviour because src may become inconsistent.
The function complexity is of O(nbits * cut_bits), which can be
improved to O(nbits).
We can also rely on bitmap_shift_right() to do most of the work.
I don't like interface of bitmap_cut(). The idea of copying of a
whole bitmap inside the function from src to dst doesn't look
useful in practice. The function is introduced a few weeks ago and
was most probably inspired by bitmap_shift_*. Looking at the code,
it's easy to see that bitmap_shift_* is usually passed with
dst == src. bitmap_cut() has a single user so far, and it also
calls it with dst == src.
This patch removes dst so the whole work is made in-place. It also
switches the function to bitmap_shift_right() internally in sake of
performance and simplicity.
I've added a very basic test too.
Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
---
include/linux/bitmap.h | 7 ++---
lib/bitmap.c | 51 +++++++++-------------------------
lib/test_bitmap.c | 32 +++++++++++++++++++++
net/netfilter/nft_set_pipapo.c | 3 +-
4 files changed, 49 insertions(+), 44 deletions(-)
diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 99058eb81042..ed60b7272437 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -59,7 +59,7 @@
* Iterate over all set regions
* bitmap_shift_right(dst, src, n, nbits) *dst = *src >> n
* bitmap_shift_left(dst, src, n, nbits) *dst = *src << n
- * bitmap_cut(dst, src, first, n, nbits) Cut n bits from first, copy rest
+ * bitmap_cut(bmap, first, n, nbits) Cut n bits from first, copy rest
* bitmap_replace(dst, old, new, mask, nbits) *dst = (*old & ~(*mask)) | (*new & *mask)
* bitmap_remap(dst, src, old, new, nbits) *dst = map(old, new)(src)
* bitmap_bitremap(oldbit, old, new, nbits) newbit = map(old, new)(oldbit)
@@ -140,9 +140,8 @@ extern void __bitmap_shift_right(unsigned long *dst, const unsigned long *src,
unsigned int shift, unsigned int nbits);
extern void __bitmap_shift_left(unsigned long *dst, const unsigned long *src,
unsigned int shift, unsigned int nbits);
-extern void bitmap_cut(unsigned long *dst, const unsigned long *src,
- unsigned int first, unsigned int cut,
- unsigned int nbits);
+extern void bitmap_cut(unsigned long *bmap, unsigned int first,
+ unsigned int cut, unsigned int nbits);
extern int __bitmap_and(unsigned long *dst, const unsigned long *bitmap1,
const unsigned long *bitmap2, unsigned int nbits);
extern void __bitmap_or(unsigned long *dst, const unsigned long *bitmap1,
diff --git a/lib/bitmap.c b/lib/bitmap.c
index 89260aa342d6..06e06e0c3096 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -170,67 +170,42 @@ EXPORT_SYMBOL(__bitmap_shift_left);
/**
* bitmap_cut() - remove bit region from bitmap and right shift remaining bits
- * @dst: destination bitmap, might overlap with src
- * @src: source bitmap
+ * @bmap: bitmap to cut
* @first: start bit of region to be removed
* @cut: number of bits to remove
* @nbits: bitmap size, in bits
*
- * Set the n-th bit of @dst iff the n-th bit of @src is set and
- * n is less than @first, or the m-th bit of @src is set for any
- * m such that @first <= n < nbits, and m = n + @cut.
- *
* In pictures, example for a big-endian 32-bit architecture:
*
- * @src:
+ * @bmap:
* 31 63
* | |
* 10000000 11000001 11110010 00010101 10000000 11000001 01110010 00010101
* | | | |
* 16 14 0 32
*
- * if @cut is 3, and @first is 14, bits 14-16 in @src are cut and @dst is:
+ * if @cut is 3, and @first is 14, bits 14-16 in @bmap are cut and @dst is:
*
* 31 63
* | |
* 10110000 00011000 00110010 00010101 00010000 00011000 00101110 01000010
* | | |
* 14 (bit 17 0 32
- * from @src)
- *
- * Note that @dst and @src might overlap partially or entirely.
- *
- * This is implemented in the obvious way, with a shift and carry
- * step for each moved bit. Optimisation is left as an exercise
- * for the compiler.
+ * from @bmap)
*/
-void bitmap_cut(unsigned long *dst, const unsigned long *src,
- unsigned int first, unsigned int cut, unsigned int nbits)
+void bitmap_cut(unsigned long *bmap, unsigned int first,
+ unsigned int cut, unsigned int nbits)
{
- unsigned int len = BITS_TO_LONGS(nbits);
- unsigned long keep = 0, carry;
- int i;
-
- memmove(dst, src, len * sizeof(*dst));
-
- if (first % BITS_PER_LONG) {
- keep = src[first / BITS_PER_LONG] &
- (~0UL >> (BITS_PER_LONG - first % BITS_PER_LONG));
- }
+ unsigned long tmp;
+ unsigned long *b = bmap + first / BITS_PER_LONG;
- while (cut--) {
- for (i = first / BITS_PER_LONG; i < len; i++) {
- if (i < len - 1)
- carry = dst[i + 1] & 1UL;
- else
- carry = 0;
+ if (first % BITS_PER_LONG)
+ tmp = b[0] & BITMAP_LAST_WORD_MASK(first);
- dst[i] = (dst[i] >> 1) | (carry << (BITS_PER_LONG - 1));
- }
- }
+ bitmap_shift_right(b, b, cut - first, nbits - first);
- dst[first / BITS_PER_LONG] &= ~0UL << (first % BITS_PER_LONG);
- dst[first / BITS_PER_LONG] |= keep;
+ if (first % BITS_PER_LONG)
+ b[0] = tmp | (b[0] & BITMAP_FIRST_WORD_MASK(first));
}
EXPORT_SYMBOL(bitmap_cut);
diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
index 6b13150667f5..4b2fef13003d 100644
--- a/lib/test_bitmap.c
+++ b/lib/test_bitmap.c
@@ -540,6 +540,37 @@ static void __init test_bitmap_arr32(void)
}
}
+struct test_bitmap_cut {
+ unsigned int first;
+ unsigned int last;
+ unsigned int nbits;
+ unsigned long in;
+ unsigned long out;
+};
+
+static struct test_bitmap_cut test_cut[] = {
+ { 0, 0, BITS_PER_LONG, 0xdeadbeefUL, 0xdeadbeefUL },
+ { 0, 8, BITS_PER_LONG, 0xdeadbeefUL, 0xdeadbeUL },
+ { 4, 8, BITS_PER_LONG, 0xdeadbeefUL, 0xdeadbefUL },
+ { 8, 24, BITS_PER_LONG, 0xdeadbeefUL, 0xdeefUL },
+ { 16, 32, BITS_PER_LONG, 0xdeadbeefUL, 0xbeefUL },
+};
+
+static void __init test_bitmap_cut(void)
+{
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(test_cut); i++) {
+ struct test_bitmap_cut *t = &test_cut[i];
+
+ bitmap_cut(&t->in, t->first, t->last, t->nbits);
+ if (!bitmap_equal(&t->in, &t->out, t->nbits)) {
+ pr_err("bitmap_cut #%d failed:\n%*pb.\nExpected:\n%*pb\n",
+ i, t->nbits, &t->in, t->nbits, &t->out);
+ }
+ }
+}
+
static void noinline __init test_mem_optimisations(void)
{
DECLARE_BITMAP(bmap1, 1024);
@@ -617,6 +648,7 @@ static void __init selftest(void)
test_copy();
test_replace();
test_bitmap_arr32();
+ test_bitmap_cut();
test_bitmap_parse();
test_bitmap_parse_user();
test_bitmap_parselist();
diff --git a/net/netfilter/nft_set_pipapo.c b/net/netfilter/nft_set_pipapo.c
index 26395c8188b1..68faaf9a4f66 100644
--- a/net/netfilter/nft_set_pipapo.c
+++ b/net/netfilter/nft_set_pipapo.c
@@ -1397,8 +1397,7 @@ static void pipapo_drop(struct nft_pipapo_match *m,
pos = f->lt + g * NFT_PIPAPO_BUCKETS * f->bsize;
for (b = 0; b < NFT_PIPAPO_BUCKETS; b++) {
- bitmap_cut(pos, pos, rulemap[i].to,
- rulemap[i].n,
+ bitmap_cut(pos, rulemap[i].to, rulemap[i].n,
f->bsize * BITS_PER_LONG);
pos += f->bsize;
--
2.20.1