[PATCH 4.9 025/152] xfs: Honor FALLOC_FL_KEEP_SIZE when punching ends of files

[Greg Kroah-Hartman]

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

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

From: Calvin Owens <calvinowens@xxxxxx>

commit 3dd09d5a8589c640abb49cfcf92b4ed669eafad1 upstream.

When punching past EOF on XFS, fallocate(mode=PUNCH_HOLE|KEEP_SIZE) will
round the file size up to the nearest multiple of PAGE_SIZE:

  calvinow@vm-disks/generic-xfs-1 ~$ dd if=/dev/urandom of=test bs=2048 count=1
  calvinow@vm-disks/generic-xfs-1 ~$ stat test
    Size: 2048            Blocks: 8          IO Block: 4096   regular file
  calvinow@vm-disks/generic-xfs-1 ~$ fallocate -n -l 2048 -o 2048 -p test
  calvinow@vm-disks/generic-xfs-1 ~$ stat test
    Size: 4096            Blocks: 8          IO Block: 4096   regular file

Commit 3c2bdc912a1cc050 ("xfs: kill xfs_zero_remaining_bytes") replaced
xfs_zero_remaining_bytes() with calls to iomap helpers. The new helpers
don't enforce that [pos,offset) lies strictly on [0,i_size) when being
called from xfs_free_file_space(), so by "leaking" these ranges into
xfs_zero_range() we get this buggy behavior.

Fix this by reintroducing the checks xfs_zero_remaining_bytes() did
against i_size at the bottom of xfs_free_file_space().

Reported-by: Aaron Gao <gzh@xxxxxx>
Fixes: 3c2bdc912a1cc050 ("xfs: kill xfs_zero_remaining_bytes")
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Brian Foster <bfoster@xxxxxxxxxx>
Signed-off-by: Calvin Owens <calvinowens@xxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/xfs/xfs_bmap_util.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -1318,8 +1318,16 @@ xfs_free_file_space(
 	/*
 	 * Now that we've unmap all full blocks we'll have to zero out any
 	 * partial block at the beginning and/or end.  xfs_zero_range is
-	 * smart enough to skip any holes, including those we just created.
+	 * smart enough to skip any holes, including those we just created,
+	 * but we must take care not to zero beyond EOF and enlarge i_size.
 	 */
+
+	if (offset >= XFS_ISIZE(ip))
+		return 0;
+
+	if (offset + len > XFS_ISIZE(ip))
+		len = XFS_ISIZE(ip) - offset;
+
 	return xfs_zero_range(ip, offset, len, NULL);
 }