SUNRPC: Add rpcauth_list_flavors()

The gss_mech_list_pseudoflavors() function provides a list of
currently registered GSS pseudoflavors.  This list does not include
any non-GSS flavors that have been registered with the RPC client.
nfs4_find_root_sec() currently adds these extra flavors by hand.

Instead, nfs4_find_root_sec() should be looking at the set of flavors
that have been explicitly registered via rpcauth_register().  And,
other areas of code will soon need the same kind of list that
contains all flavors the kernel currently knows about (see below).

Rather than cloning the open-coded logic in nfs4_find_root_sec() to
those new places, introduce a generic RPC function that generates a
full list of registered auth flavors and pseudoflavors.

A new rpc_authops method is added that lists a flavor's
pseudoflavors, if it has any.  I encountered an interesting module
loader loop when I tried to get the RPC client to invoke
gss_mech_list_pseudoflavors() by name.

This patch is a pre-requisite for server trunking discovery, and a
pre-requisite for fixing up the in-kernel mount client to do better
automatic security flavor selection.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth_gss/auth_gss.c

@@ -1619,6 +1619,7 @@ static const struct rpc_authops authgss_ops = {
 	.crcreate	= gss_create_cred,
 	.pipes_create	= gss_pipes_dentries_create,
 	.pipes_destroy	= gss_pipes_dentries_destroy,
+	.list_pseudoflavors = gss_mech_list_pseudoflavors,
 };
 
 static const struct rpc_credops gss_credops = {

net/sunrpc/auth_gss/gss_mech_switch.c

@@ -239,14 +239,26 @@ gss_mech_get_by_pseudoflavor(u32 pseudoflavor)
 
 EXPORT_SYMBOL_GPL(gss_mech_get_by_pseudoflavor);
 
-int gss_mech_list_pseudoflavors(rpc_authflavor_t *array_ptr)
+/**
+ * gss_mech_list_pseudoflavors - Discover registered GSS pseudoflavors
+ * @array: array to fill in
+ * @size: size of "array"
+ *
+ * Returns the number of array items filled in, or a negative errno.
+ *
+ * The returned array is not sorted by any policy.  Callers should not
+ * rely on the order of the items in the returned array.
+ */
+int gss_mech_list_pseudoflavors(rpc_authflavor_t *array_ptr, int size)
 {
 	struct gss_api_mech *pos = NULL;
 	int j, i = 0;
 
 	spin_lock(&registered_mechs_lock);
 	list_for_each_entry(pos, &registered_mechs, gm_list) {
-		for (j=0; j < pos->gm_pf_num; j++) {
+		for (j = 0; j < pos->gm_pf_num; j++) {
+			if (i >= size)
+				return -ENOMEM;
 			array_ptr[i++] = pos->gm_pfs[j].pseudoflavor;
 		}
 	}
@@ -254,8 +266,6 @@ int gss_mech_list_pseudoflavors(rpc_authflavor_t *array_ptr)
 	return i;
 }
 
-EXPORT_SYMBOL_GPL(gss_mech_list_pseudoflavors);
-
 u32
 gss_svc_to_pseudoflavor(struct gss_api_mech *gm, u32 service)
 {