[PATCH] ext4: Fix SEEK_HOLE/SEEK_DATA for blocksize < pagesize

[Jan Kara <jack@suse.cz>]

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>
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/file.c | 3 +++
 1 file changed, 3 insertions(+)

This is an easy fix for the SEEK_HOLE/SEEK_DATA bug Andreas found. Longer
term we want to move ext4 to use page_cache_seek_hole_data() however for
stable backporting this is more suitable.

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index b221d0b546b0..8ba34cbe8fe7 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -530,6 +530,8 @@ 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)
@@ -544,6 +546,7 @@ 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);
-- 
2.12.3