[PATCH v2 2/3] cifs: don't update i_size in cifs_do_truncate() without a cached handle

[Frank Sorenson <sorenson@redhat.com>]

When cifs_do_truncate() is called from cifs_open() for an O_TRUNC open
and find_writable_file() returns NULL (no cached writable handle),
cifs_file_flush() returns 0 without sending a server set_file_size
request.  The outer "if (!rc)" block then falls through unconditionally
to cifs_setsize(), setting the local inode size to 0 even though the
server has not yet been told to truncate the file.

If the subsequent network open in cifs_open() fails (e.g.
STATUS_ACCESS_DENIED from cifs_nt_open()), the caller receives an error
but the local inode remains at size 0 while the server file is still at
its original size.

When cfile is NULL, the O_TRUNC flag in the subsequent cifs_open()
request tells the server to truncate; i_size is updated from the open
response.  Move the size update inside the "if (cfile)" arm so that it
only runs when the server has been explicitly told to truncate, and call
cifs_invalidate_cache() in the no-handle case to flush stale pages
before the open is sent.

Fixes: 110fee6b9bb5 ("smb: client: fix missing timestamp updates with O_TRUNC")
Cc: stable@vger.kernel.org
Cc: Paulo Alcantara <pc@manguebit.com>
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
---
 fs/smb/client/file.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/fs/smb/client/file.c b/fs/smb/client/file.c
index ac89c1ba56b1..92af1782bd73 100644
--- a/fs/smb/client/file.c
+++ b/fs/smb/client/file.c
@@ -1012,10 +1012,19 @@ static int cifs_do_truncate(const unsigned int xid, struct dentry *dentry)
 			server = tcon->ses->server;
 			rc = server->ops->set_file_size(xid, tcon,
 							cfile, 0, false);
-		}
-		if (!rc) {
-			netfs_resize_file(&cinode->netfs, 0, true);
-			cifs_setsize(inode, 0);
+			if (!rc) {
+				netfs_resize_file(&cinode->netfs, 0, true);
+				cifs_setsize(inode, 0);
+			}
+		} else {
+			/*
+			 * No cached handle; the server truncates as part of the
+			 * O_TRUNC open request that follows.  Invalidate stale
+			 * pages now so they are not written back after the
+			 * truncation; i_size is updated from the server's open
+			 * response.
+			 */
+			cifs_invalidate_cache(inode, 0);
 		}
 	}
 	if (cfile)
-- 
2.55.0