[PATCH v2] xfs: Honor FALLOC_FL_KEEP_SIZE when punching ends of files

[Calvin Owens]

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: <stable@xxxxxxxxxxxxxxx> # 4.8+
Signed-off-by: Calvin Owens <calvinowens@xxxxxx>
---
 fs/xfs/xfs_bmap_util.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index 8b75dce..0796ebc 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -1309,6 +1309,17 @@ xfs_free_file_space(
 	}
 
 	/*
+	 * Avoid doing I/O beyond eof - it's not necessary
+	 * since nothing can read beyond eof.  The space will
+	 * be zeroed when the file is extended anyway.
+	 */
+	if (offset >= XFS_ISIZE(ip))
+		return 0;
+
+	if ((offset + len) >= XFS_ISIZE(ip))
+		len = XFS_ISIZE(ip) - offset - 1;
+
+	/*
 	 * 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.
-- 
2.9.3