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

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

From: Jan Kara <j...@suse.cz>

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 <agrue...@redhat.com>
Signed-off-by: Theodore Ts'o <ty...@mit.edu>
Signed-off-by: Jan Kara <j...@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

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

--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -469,6 +469,8 @@ 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)
@@ -483,6 +485,7 @@ 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);


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