fix-SEEK_HOLE_DATA-for-blocksize-lt-pagesize

   1 ext4: fix SEEK_HOLE/SEEK_DATA for blocksize < pagesize
   2
   3 From: Jan Kara <jack@suse.cz>
   4
   5 ext4_find_unwritten_pgoff() does not properly handle a situation when
   6 starting index is in the middle of a page and blocksize < pagesize. The
   7 following command shows the bug on filesystem with 1k blocksize:
   8
   9   xfs_io -f -c "falloc 0 4k" \
  10             -c "pwrite 1k 1k" \
  11             -c "pwrite 3k 1k" \
  12             -c "seek -a -r 0" foo
  13
  14 In this example, neither lseek(fd, 1024, SEEK_HOLE) nor lseek(fd, 2048,
  15 SEEK_DATA) will return the correct result.
  16
  17 Fix the problem by neglecting buffers in a page before starting offset.
  18
  19 Reported-by: Andreas Gruenbacher <agruenba@redhat.com>
  20 Signed-off-by: Theodore Ts'o <tytso@mit.edu>
  21 Signed-off-by: Jan Kara <jack@suse.cz>
  22 CC: stable@vger.kernel.org # 3.8+
  23 ---
  24  fs/ext4/file.c | 3 +++
  25  1 file changed, 3 insertions(+)
  26
  27 This is an easy fix for the SEEK_HOLE/SEEK_DATA bug Andreas found. Longer
  28 term we want to move ext4 to use page_cache_seek_hole_data() however for
  29 stable backporting this is more suitable.
  30
  31 diff --git a/fs/ext4/file.c b/fs/ext4/file.c
  32 index b221d0b546b0..8ba34cbe8fe7 100644
  33 --- a/fs/ext4/file.c
  34 +++ b/fs/ext4/file.c
  35 @@ -530,6 +530,8 @@ static int ext4_find_unwritten_pgoff(struct inode *inode,
  36                                 lastoff = page_offset(page);
  37                                 bh = head = page_buffers(page);
  38                                 do {
  39 +                                       if (lastoff + bh->b_size <= startoff)
  40 +                                               goto next;
  41                                         if (buffer_uptodate(bh) ||
  42                                             buffer_unwritten(bh)) {
  43                                                 if (whence == SEEK_DATA)
  44 @@ -544,6 +546,7 @@ static int ext4_find_unwritten_pgoff(struct inode *inode,
  45                                                 unlock_page(page);
  46                                                 goto out;
  47                                         }
  48 +next:
  49                                         lastoff += bh->b_size;
  50                                         bh = bh->b_this_page;
  51                                 } while (bh != head);
  52 --
  53 2.12.3
  54
  55