ext4: fix SEEK_HOLE/SEEK_DATA for blocksize < pagesize

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@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
CC: stable@vger.kernel.org # 3.8+

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 58294c9a7e1d..0d7cf0cc9b87 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -537,6 +537,8 @@ static int ext4_find_unwritten_pgoff(struct inode *inode,
                                 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)
@@ -551,6 +553,7 @@ static int ext4_find_unwritten_pgoff(struct inode *inode,
                                                 unlock_page(page);
                                                 goto out;
                                         }
+next:
                                         lastoff += bh->b_size;
                                         bh = bh->b_this_page;
                                 } while (bh != head);