smb3: allow previous versions to be mounted with snapshot= mount parm

mounting with the "snapshots=" mount parm allows a read-only
view of a previous version of a file system (see MS-SMB2
and "timewarp" tokens, section 2.2.13.2.6) based on the timestamp
passed in on the snapshots mount parm.

Add processing to optionally send this create context.

Example output:

/mnt1 is mounted with "snapshots=..." and will see an earlier
version of the directory, with three fewer files than /mnt2
the current version of the directory.

root@Ubuntu-17-Virtual-Machine:~/cifs-2.6# cat /proc/mounts | grep cifs
//172.22.149.186/public /mnt1 cifs
ro,relatime,vers=default,cache=strict,username=smfrench,uid=0,noforceuid,gid=0,noforcegid,addr=172.22.149.186,file_mode=0755,dir_mode=0755,soft,nounix,mapposix,rsize=1048576,wsize=1048576,echo_interval=60,snapshot=131748608570000000,actimeo=1

//172.22.149.186/public /mnt2 cifs
rw,relatime,vers=default,cache=strict,username=smfrench,uid=0,noforceuid,gid=0,noforcegid,addr=172.22.149.186,file_mode=0755,dir_mode=0755,soft,nounix,mapposix,rsize=1048576,wsize=1048576,echo_interval=60,actimeo=1

root@Ubuntu-17-Virtual-Machine:~/cifs-2.6# ls /mnt1
EmptyDir  newerdir
root@Ubuntu-17-Virtual-Machine:~/cifs-2.6# ls /mnt1/newerdir

root@Ubuntu-17-Virtual-Machine:~/cifs-2.6# ls /mnt2
EmptyDir  file  newerdir  newestdir  timestamp-trace.cap
root@Ubuntu-17-Virtual-Machine:~/cifs-2.6# ls /mnt2/newerdir
new-file-not-in-snapshot

Snapshots are extremely useful for comparing previous versions of files or directories,
and recovering from data corruptions or mistakes.
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>

fs/cifs/smb2pdu.c

@@ -1856,6 +1856,51 @@ add_durable_context(struct kvec *iov, unsigned int *num_iovec,
 	return 0;
 }
 
+/* See MS-SMB2 2.2.13.2.7 */
+static struct crt_twarp_ctxt *
+create_twarp_buf(__u64 timewarp)
+{
+	struct crt_twarp_ctxt *buf;
+
+	buf = kzalloc(sizeof(struct crt_twarp_ctxt), GFP_KERNEL);
+	if (!buf)
+		return NULL;
+
+	buf->ccontext.DataOffset = cpu_to_le16(offsetof
+					(struct crt_twarp_ctxt, Timestamp));
+	buf->ccontext.DataLength = cpu_to_le32(8);
+	buf->ccontext.NameOffset = cpu_to_le16(offsetof
+				(struct crt_twarp_ctxt, Name));
+	buf->ccontext.NameLength = cpu_to_le16(4);
+	/* SMB2_CREATE_TIMEWARP_TOKEN is "TWrp" */
+	buf->Name[0] = 'T';
+	buf->Name[1] = 'W';
+	buf->Name[2] = 'r';
+	buf->Name[3] = 'p';
+	buf->Timestamp = cpu_to_le64(timewarp);
+	return buf;
+}
+
+/* See MS-SMB2 2.2.13.2.7 */
+static int
+add_twarp_context(struct kvec *iov, unsigned int *num_iovec, __u64 timewarp)
+{
+	struct smb2_create_req *req = iov[0].iov_base;
+	unsigned int num = *num_iovec;
+
+	iov[num].iov_base = create_twarp_buf(timewarp);
+	if (iov[num].iov_base == NULL)
+		return -ENOMEM;
+	iov[num].iov_len = sizeof(struct crt_twarp_ctxt);
+	if (!req->CreateContextsOffset)
+		req->CreateContextsOffset = cpu_to_le32(
+				sizeof(struct smb2_create_req) +
+				iov[num - 1].iov_len);
+	le32_add_cpu(&req->CreateContextsLength, sizeof(struct crt_twarp_ctxt));
+	*num_iovec = num + 1;
+	return 0;
+}
+
 static int
 alloc_path_with_tree_prefix(__le16 **out_path, int *out_size, int *out_len,
 			    const char *treename, const __le16 *path)
@@ -2168,6 +2213,21 @@ SMB2_open_init(struct cifs_tcon *tcon, struct smb_rqst *rqst, __u8 *oplock,
 			return rc;
 	}
 
+	if (tcon->snapshot_time) {
+		cifs_dbg(FYI, "adding snapshot context\n");
+		if (n_iov > 2) {
+			struct create_context *ccontext =
+			    (struct create_context *)iov[n_iov-1].iov_base;
+			ccontext->Next =
+				cpu_to_le32(iov[n_iov-1].iov_len);
+		}
+
+		rc = add_twarp_context(iov, &n_iov, tcon->snapshot_time);
+		if (rc)
+			return rc;
+	}
+
+
 	rqst->rq_nvec = n_iov;
 	return 0;
 }

fs/cifs/smb2pdu.h

@@ -767,6 +767,14 @@ struct create_durable_handle_reconnect_v2 {
 	struct durable_reconnect_context_v2 dcontext;
 } __packed;
 
+/* See MS-SMB2 2.2.13.2.5 */
+struct crt_twarp_ctxt {
+	struct create_context ccontext;
+	__u8	Name[8];
+	__le64	Timestamp;
+
+} __packed;
+
 #define COPY_CHUNK_RES_KEY_SIZE	24
 struct resume_key_req {
 	char ResumeKey[COPY_CHUNK_RES_KEY_SIZE];