SUNRPC: add svcauth_map_clnt_to_svc_cred_local

Add new funtion svcauth_map_clnt_to_svc_cred_local which maps a
generic cred to a svc_cred suitable for use in nfsd.

This is needed by the localio code to map nfs client creds to nfs
server credentials.

Following from net/sunrpc/auth_unix.c:unx_marshal() it is clear that
->fsuid and ->fsgid must be used (rather than ->uid and ->gid).  In
addition, these uid and gid must be translated with from_kuid_munged()
so local client uses correct uid and gid when acting as local server.

Jeff Layton noted:
  This is where the magic happens. Since we're working in
  kuid_t/kgid_t, we don't need to worry about further idmapping.

Suggested-by: NeilBrown <neilb@suse.de> # to approximate unx_marshal()
Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Co-developed-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

include/linux/sunrpc/svcauth.h

@@ -14,6 +14,7 @@
 #include <linux/sunrpc/msg_prot.h>
 #include <linux/sunrpc/cache.h>
 #include <linux/sunrpc/gss_api.h>
+#include <linux/sunrpc/clnt.h>
 #include <linux/hash.h>
 #include <linux/stringhash.h>
 #include <linux/cred.h>
@@ -157,6 +158,10 @@ extern enum svc_auth_status svc_set_client(struct svc_rqst *rqstp);
 extern int	svc_auth_register(rpc_authflavor_t flavor, struct auth_ops *aops);
 extern void	svc_auth_unregister(rpc_authflavor_t flavor);
 
+extern void	svcauth_map_clnt_to_svc_cred_local(struct rpc_clnt *clnt,
+						   const struct cred *,
+						   struct svc_cred *);
+
 extern struct auth_domain *unix_domain_find(char *name);
 extern void auth_domain_put(struct auth_domain *item);
 extern struct auth_domain *auth_domain_lookup(char *name, struct auth_domain *new);

net/sunrpc/svcauth.c

@@ -18,6 +18,7 @@
 #include <linux/sunrpc/svcauth.h>
 #include <linux/err.h>
 #include <linux/hash.h>
+#include <linux/user_namespace.h>
 
 #include <trace/events/sunrpc.h>
 
@@ -175,6 +176,33 @@ rpc_authflavor_t svc_auth_flavor(struct svc_rqst *rqstp)
 }
 EXPORT_SYMBOL_GPL(svc_auth_flavor);
 
+/**
+ * svcauth_map_clnt_to_svc_cred_local - maps a generic cred
+ * to a svc_cred suitable for use in nfsd.
+ * @clnt: rpc_clnt associated with nfs client
+ * @cred: generic cred associated with nfs client
+ * @svc: returned svc_cred that is suitable for use in nfsd
+ */
+void svcauth_map_clnt_to_svc_cred_local(struct rpc_clnt *clnt,
+					const struct cred *cred,
+					struct svc_cred *svc)
+{
+	struct user_namespace *userns = clnt->cl_cred ?
+		clnt->cl_cred->user_ns : &init_user_ns;
+
+	memset(svc, 0, sizeof(struct svc_cred));
+
+	svc->cr_uid = KUIDT_INIT(from_kuid_munged(userns, cred->fsuid));
+	svc->cr_gid = KGIDT_INIT(from_kgid_munged(userns, cred->fsgid));
+	svc->cr_flavor = clnt->cl_auth->au_flavor;
+	if (cred->group_info)
+		svc->cr_group_info = get_group_info(cred->group_info);
+	/* These aren't relevant for local (network is bypassed) */
+	svc->cr_principal = NULL;
+	svc->cr_gss_mech = NULL;
+}
+EXPORT_SYMBOL_GPL(svcauth_map_clnt_to_svc_cred_local);
+
 /**************************************************
  * 'auth_domains' are stored in a hash table indexed by name.
  * When the last reference to an 'auth_domain' is dropped,