[CIFS] Fix potential data corruption when writing out cached dirty pages

Fix RedHat bug 329431

The idea here is separate "conscious" from "unconscious" flushes.
Conscious flushes are those due to a fsync() or close(). Unconscious
ones are flushes that occur as a side effect of some other operation or
due to memory pressure.

Currently, when an error occurs during an unconscious flush (ENOSPC or
EIO), we toss out the page and don't preserve that error to report to
the user when a conscious flush occurs. If after the unconscious flush,
there are no more dirty pages for the inode, the conscious flush will
simply return success even though there were previous errors when writing
out pages. This can lead to data corruption.

The easiest way to reproduce this is to mount up a CIFS share that's
very close to being full or where the user is very close to quota. mv
a file to the share that's slightly larger than the quota allows. The
writes will all succeed (since they go to pagecache). The mv will do a
setattr to set the new file's attributes. This calls
filemap_write_and_wait,
which will return an error since all of the pages can't be written out.
Then later, when the flush and release ops occur, there are no more
dirty pages in pagecache for the file and those operations return 0. mv
then assumes that the file was written out correctly and deletes the
original.

CIFS already has a write_behind_rc variable where it stores the results
from earlier flushes, but that value is only reported in cifs_close.
Since the VFS ignores the return value from the release operation, this
isn't helpful. We should be reporting this error during the flush
operation.

This patch does the following:

1) changes cifs_fsync to use filemap_write_and_wait and cifs_flush and also
sync to check its return code. If it returns successful, they then check
the value of write_behind_rc to see if an earlier flush had reported any
errors. If so, they return that error and clear write_behind_rc.

2) sets write_behind_rc in a few other places where pages are written
out as a side effect of other operations and the code waits on them.

3) changes cifs_setattr to only call filemap_write_and_wait for
ATTR_SIZE changes.

4) makes cifs_writepages accurately distinguish between EIO and ENOSPC
errors when writing out pages.

Some simple testing indicates that the patch works as expected and that
it fixes the reproduceable known problem.

Acked-by: Dave Kleikamp <shaggy@austin.rr.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 12 insertions, 15 deletions

diff --git a/fs/cifs/README b/fs/cifs/README
index b806b11b5560..bf11329ac784 100644
--- a/fs/cifs/README
+++ b/fs/cifs/README
@@ -225,12 +225,9 @@ If no password is provided, mount.cifs will prompt for password entry
 
 Restrictions
 ============
-Servers must support the NTLM SMB dialect (which is the most recent, supported 
-by Samba and Windows NT version 4, 2000 and XP and many other SMB/CIFS servers) 
 Servers must support either "pure-TCP" (port 445 TCP/IP CIFS connections) or RFC 
-1001/1002 support for "Netbios-Over-TCP/IP." Neither of these is likely to be a 
-problem as most servers support this.  IPv6 support is planned for the future,
-and is almost complete.
+1001/1002 support for "Netbios-Over-TCP/IP." This is not likely to be a 
+problem as most servers support this.
 
 Valid filenames differ between Windows and Linux.  Windows typically restricts
 filenames which contain certain reserved characters (e.g.the character : 
@@ -458,6 +455,8 @@ A partial list of the supported mount options follows:
                 byte range locks).
  remount        remount the share (often used to change from ro to rw mounts
                 or vice versa)
+ cifsacl        Report mode bits (e.g. on stat) based on the Windows ACL for
+                the file. (EXPERIMENTAL)
  servern        Specify the server 's netbios name (RFC1001 name) to use
                 when attempting to setup a session to the server.  This is
                 This is needed for mounting to some older servers (such
@@ -584,8 +583,8 @@ Experimental            When set to 1 used to enable certain experimental
                         performance enhancement was disabled when
                         signing turned on in case buffer was modified
                         just before it was sent, also this flag will
-                        be used to use the new experimental sessionsetup
-                        code).
+                        be used to use the new experimental directory change 
+                        notification code).
 
 These experimental features and tracing can be enabled by changing flags in 
 /proc/fs/cifs (after the cifs module has been installed or built into the 
@@ -608,7 +607,8 @@ the start of smb requests and responses can be enabled via:
 Two other experimental features are under development. To test these
 requires enabling CONFIG_CIFS_EXPERIMENTAL
 
-        ipv6 enablement
+        cifsacl support needed to retrieve approximated mode bits based on
+                the contents on the CIFS ACL.
 
         DNOTIFY fcntl: needed for support of directory change 
                             notification and perhaps later for file leases)
@@ -625,10 +625,7 @@ that they represent all for that share, not just those for which the server
 returned success.
         
 Also note that "cat /proc/fs/cifs/DebugData" will display information about 
-the active sessions and the shares that are mounted.  Note: NTLMv2 enablement 
-will not work since its implementation is not quite complete yet. Do not alter
-the ExtendedSecurity configuration value unless you are doing specific testing.
-Enabling extended security works to Windows 2000 Workstations and XP but not to 
-Windows 2000 server or Samba since it does not usually send "raw NTLMSSP" 
-(instead it sends NTLMSSP encapsulated in SPNEGO/GSSAPI, which support is not 
-complete in the CIFS VFS yet).  
+the active sessions and the shares that are mounted.
+Enabling Kerberos (extended security) works when CONFIG_CIFS_EXPERIMENTAL is enabled
+but requires a user space helper (from the Samba project). NTLM and NTLMv2 and
+LANMAN support do not require this helpr.