[PATCH] omfs: reject writes starting beyond EOF

[Hungyu Lin <dennylin0707@gmail.com>]

OMFS does not initialize gaps between EOF and a buffered write. When the
requested block lies beyond the allocated extents, omfs_get_block() grows
the last extent and maps the write to the newly allocated block without
accounting for the skipped logical blocks.

A one-byte pwrite() at offset 8192 into an empty file returns success, as
does fsync(), but after remounting the byte is at offset 0 and offset 8192
reads as zero.

Reject writes starting beyond i_size before block_write_begin() can
allocate or map buffers. Overwrites, appends and writes crossing EOF
remain allowed. Gap initialization can be implemented separately.

Fixes: 8f09e98768c1 ("omfs: add file routines")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 fs/omfs/file.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/omfs/file.c b/fs/omfs/file.c
index 79a413f1dc0..82c15756671 100644
--- a/fs/omfs/file.c
+++ b/fs/omfs/file.c
@@ -317,6 +317,10 @@ static int omfs_write_begin(const struct kiocb *iocb,
 {
 	int ret;
 
+	/* Initializing a gap between EOF and the write is not supported. */
+	if (pos > i_size_read(mapping->host))
+		return -EOPNOTSUPP;
+
 	ret = block_write_begin(mapping, pos, len, foliop, omfs_get_block);
 	if (unlikely(ret))
 		omfs_write_failed(mapping, pos + len);

base-commit: 05441d697f7f9a927264f4058cbbc6a03dfcac9d
-- 
2.50.1 (Apple Git-155)