smb: client: handle max length for SMB symlinks

We can't use PATH_MAX for SMB symlinks because

  (1) Windows Server will fail FSCTL_SET_REPARSE_POINT with
      STATUS_IO_REPARSE_DATA_INVALID when input buffer is larger than
      16K, as specified in MS-FSA 2.1.5.10.37.

  (2) The client won't be able to parse large SMB responses that
      includes SMB symlink path within SMB2_CREATE or SMB2_IOCTL
      responses.

Fix this by defining a maximum length value (4060) for SMB symlinks
that both client and server can handle.

Cc: David Howells <dhowells@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/reparse.c

@@ -35,6 +35,9 @@ int smb2_create_reparse_symlink(const unsigned int xid, struct inode *inode,
 	u16 len, plen;
 	int rc = 0;
 
+	if (strlen(symname) > REPARSE_SYM_PATH_MAX)
+		return -ENAMETOOLONG;
+
 	sym = kstrdup(symname, GFP_KERNEL);
 	if (!sym)
 		return -ENOMEM;
@@ -64,7 +67,7 @@ int smb2_create_reparse_symlink(const unsigned int xid, struct inode *inode,
 	if (rc < 0)
 		goto out;
 
-	plen = 2 * UniStrnlen((wchar_t *)path, PATH_MAX);
+	plen = 2 * UniStrnlen((wchar_t *)path, REPARSE_SYM_PATH_MAX);
 	len = sizeof(*buf) + plen * 2;
 	buf = kzalloc(len, GFP_KERNEL);
 	if (!buf) {

fs/smb/client/reparse.h

@@ -12,6 +12,8 @@
 #include "fs_context.h"
 #include "cifsglob.h"
 
+#define REPARSE_SYM_PATH_MAX 4060
+
 /*
  * Used only by cifs.ko to ignore reparse points from files when client or
  * server doesn't support FSCTL_GET_REPARSE_POINT.