From: Jeff Layton <jlayton@redhat.com>
Date: Wed, 3 Sep 2008 13:32:58 -0400
Subject: [fs] cifs: fix O_APPEND on directio mounts
Message-id: 1220463178-6396-1-git-send-email-jlayton@redhat.com
O-Subject: [RHEL5.3 PATCH] BZ#460063: cifs: fix O_APPEND on directio mounts
Bugzilla: 460063
RH-Acked-by: Peter Staubach <staubach@redhat.com>
RH-Acked-by: Rik van Riel <riel@redhat.com>

Rather late breaking patch to fix O_APPEND in CIFS when a share is
mounted with the "directio" mount option. This just recently went
upstream and to the stable kernel series and an exception was just filed
for it since it's a potential data corruption issue.

Applies cleanly on top of the cifs update patchset that I recently
posted, and to 5.2 kernels with a little bit of offset. Tested by myself
using a simple test that appends data to a file, and by the customer who
originally reported the problem.

Original patch description follows.

-------------[snip]-------------

Upstream commit 838726c4756813576078203eb7e1e219db0da870

The direct I/O write codepath for CIFS is done through
cifs_user_write(). That function does not currently call
generic_write_checks() so the file position isn't being properly set
when the file is opened with O_APPEND.  It's also not doing the other
"normal" checks that should be done for a write call.

The problem is currently that when you open a file with O_APPEND on a
mount with the directio mount option, the file position is set to the
beginning of the file. This makes any subsequent writes clobber the data
in the file starting at the beginning.

This seems to fix the problem in cursory testing. It is, however
important to note that NFS disallows the combination of
(O_DIRECT|O_APPEND). If my understanding is correct, the concern is
races with multiple clients appending to a file clobbering each others'
data. Since the write model for CIFS and NFS is pretty similar in this
regard, CIFS is probably subject to the same sort of races. What's
unclear to me is why this is a particular problem with O_DIRECT and not
with buffered writes...

Regardless, disallowing O_APPEND on an entire mount is probably not
reasonable, so we'll probably just have to deal with it and reevaluate
this flag combination when we get proper support for O_DIRECT. In the
meantime this patch at least fixes the existing problem.

Signed-off-by: Jeff Layton <jlayton@redhat.com>

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 50a19b2..086dbdc 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -940,6 +940,10 @@ ssize_t cifs_user_write(struct file *file, const char __user *write_data,
 		return -EBADF;
 	open_file = (struct cifsFileInfo *) file->private_data;
 
+	rc = generic_write_checks(file, poffset, &write_size, 0);
+	if (rc)
+		return rc;
+
 	xid = GetXid();
 
 #if LINUX_VERSION_CODE < KERNEL_VERSION(2, 6, 20)