[PATCH 4.4 08/58] ext4: fix SEEK_HOLE/SEEK_DATA for blocksize < pagesize

From: Greg Kroah-Hartman
Date: Wed Aug 09 2017 - 15:46:22 EST


4.4-stable review patch. If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack@xxxxxxx>

commit fcf5ea10992fbac3c7473a1db33d56a139333cd1 upstream.

ext4_find_unwritten_pgoff() does not properly handle a situation when
starting index is in the middle of a page and blocksize < pagesize. The
following command shows the bug on filesystem with 1k blocksize:

xfs_io -f -c "falloc 0 4k" \
-c "pwrite 1k 1k" \
-c "pwrite 3k 1k" \
-c "seek -a -r 0" foo

In this example, neither lseek(fd, 1024, SEEK_HOLE) nor lseek(fd, 2048,
SEEK_DATA) will return the correct result.

Fix the problem by neglecting buffers in a page before starting offset.

Reported-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
fs/ext4/file.c | 3 +++
1 file changed, 3 insertions(+)

--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -500,6 +500,8 @@ static int ext4_find_unwritten_pgoff(str
lastoff = page_offset(page);
bh = head = page_buffers(page);
do {
+ if (lastoff + bh->b_size <= startoff)
+ goto next;
if (buffer_uptodate(bh) ||
buffer_unwritten(bh)) {
if (whence == SEEK_DATA)
@@ -514,6 +516,7 @@ static int ext4_find_unwritten_pgoff(str
unlock_page(page);
goto out;
}
+next:
lastoff += bh->b_size;
bh = bh->b_this_page;
} while (bh != head);