NFS: add XDR decoder for mountd version 3 auth-flavor lists

Introduce an xdr_stream-based XDR decoder that can unpack the auth-
flavor list returned in a MNT3 reply.

The nfs_mount() function's caller allocates an array, and passes the
size and a pointer to it.  The decoder decodes all the flavors it can
into the array, and returns the number of decoded flavors.

If the caller is not interested in the auth flavors, it can pass a
value of zero as the size of the pre-allocated array.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/internal.h

@@ -29,6 +29,12 @@ struct nfs_clone_mount {
 	rpc_authflavor_t authflavor;
 };
 
+/*
+ * Note: RFC 1813 doesn't limit the number of auth flavors that
+ * a server can return, so make something up.
+ */
+#define NFS_MAX_SECFLAVORS	(12)
+
 /*
  * In-kernel mount arguments
  */

fs/nfs/mount_clnt.c

@@ -33,6 +33,7 @@
 #define MNT_fhs_status_sz	(1)
 #define MNT_fhandle_sz		XDR_QUADLEN(NFS2_FHSIZE)
 #define MNT_fhandle3_sz		(1 + XDR_QUADLEN(NFS3_FHSIZE))
+#define MNT_authflav3_sz	(1 + NFS_MAX_SECFLAVORS)
 
 /*
  * XDR argument and result sizes
@@ -122,6 +123,8 @@ static struct {
 struct mountres {
 	int errno;
 	struct nfs_fh *fh;
+	unsigned int *auth_count;
+	rpc_authflavor_t *auth_flavors;
 };
 
 struct mnt_fhstatus {
@@ -334,6 +337,40 @@ static int decode_fhandle3(struct xdr_stream *xdr, struct mountres *res)
 	return 0;
 }
 
+static int decode_auth_flavors(struct xdr_stream *xdr, struct mountres *res)
+{
+	rpc_authflavor_t *flavors = res->auth_flavors;
+	unsigned int *count = res->auth_count;
+	u32 entries, i;
+	__be32 *p;
+
+	if (*count == 0)
+		return 0;
+
+	p = xdr_inline_decode(xdr, sizeof(entries));
+	if (unlikely(p == NULL))
+		return -EIO;
+	entries = ntohl(*p);
+	dprintk("NFS: received %u auth flavors\n", entries);
+	if (entries > NFS_MAX_SECFLAVORS)
+		entries = NFS_MAX_SECFLAVORS;
+
+	p = xdr_inline_decode(xdr, sizeof(u32) * entries);
+	if (unlikely(p == NULL))
+		return -EIO;
+
+	if (entries > *count)
+		entries = *count;
+
+	for (i = 0; i < entries; i++) {
+		flavors[i] = ntohl(*p++);
+		dprintk("NFS:\tflavor %u: %d\n", i, flavors[i]);
+	}
+	*count = i;
+
+	return 0;
+}
+
 static int xdr_decode_fhstatus3(struct rpc_rqst *req, __be32 *p,
 				struct mnt_fhstatus *res)
 {