[PATCH 06/11] sane_fdtable_size(): don't bother looking at descriptors we are not going to copy

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

when given a max_fds argument lower than that current size (that
can happen when called from close_range(..., CLOSE_RANGE_UNSHARE)),
we can ignore all descriptors >= max_fds.

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
 fs/file.c | 43 +++++++++++++++++++++++--------------------
 1 file changed, 23 insertions(+), 20 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index fbcd3da46109..894bd18241b5 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -272,20 +272,6 @@ static inline bool fd_is_open(unsigned int fd, const struct fdtable *fdt)
 	return test_bit(fd, fdt->open_fds);
 }
 
-static unsigned int count_open_files(struct fdtable *fdt)
-{
-	unsigned int size = fdt->max_fds;
-	unsigned int i;
-
-	/* Find the last open fd */
-	for (i = size / BITS_PER_LONG; i > 0; ) {
-		if (fdt->open_fds[--i])
-			break;
-	}
-	i = (i + 1) * BITS_PER_LONG;
-	return i;
-}
-
 /*
  * Note that a sane fdtable size always has to be a multiple of
  * BITS_PER_LONG, since we have bitmaps that are sized by this.
@@ -297,16 +283,33 @@ static unsigned int count_open_files(struct fdtable *fdt)
  *
  * Rather than make close_range() have to worry about this,
  * just make that BITS_PER_LONG alignment be part of a sane
- * fdtable size. Becuase that's really what it is.
+ * fdtable size. Because that's really what it is.
  */
 static unsigned int sane_fdtable_size(struct fdtable *fdt, unsigned int max_fds)
 {
-	unsigned int count;
+	const unsigned int min_words = BITS_TO_LONGS(NR_OPEN_DEFAULT);  // 1
+	unsigned long mask;
+	unsigned int words;
+
+	if (max_fds > fdt->max_fds)
+		max_fds = fdt->max_fds;
+
+	if (max_fds == NR_OPEN_DEFAULT)
+		return NR_OPEN_DEFAULT;
+
+	/*
+	 * What follows is a simplified find_last_bit().  There's no point
+	 * finding exact last bit, when we are going to round it up anyway.
+	 */
+	words = BITS_TO_LONGS(max_fds);
+	mask = BITMAP_LAST_WORD_MASK(max_fds);
+
+	while (words > min_words && !(fdt->open_fds[words - 1] & mask)) {
+		mask = ~0UL;
+		words--;
+	}
 
-	count = count_open_files(fdt);
-	if (max_fds < NR_OPEN_DEFAULT)
-		max_fds = NR_OPEN_DEFAULT;
-	return ALIGN(min(count, max_fds), BITS_PER_LONG);
+	return words * BITS_PER_LONG;
 }
 
 /*
-- 
2.39.2