service: allow setting a dedicated group id

Sometimes is desirable to run a process with a specific group id
instead of the default one which is derived from passwd entry.

However, we still want to initialize supplementary group ids
(including the default one), thus we have to store the specific
one in a dedicated structure element.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

service/instance.c

@@ -50,6 +50,7 @@ enum {
 	INSTANCE_ATTR_WATCH,
 	INSTANCE_ATTR_ERROR,
 	INSTANCE_ATTR_USER,
+	INSTANCE_ATTR_GROUP,
 	INSTANCE_ATTR_STDOUT,
 	INSTANCE_ATTR_STDERR,
 	INSTANCE_ATTR_NO_NEW_PRIVS,
@@ -76,6 +77,7 @@ static const struct blobmsg_policy instance_attr[__INSTANCE_ATTR_MAX] = {
 	[INSTANCE_ATTR_WATCH] = { "watch", BLOBMSG_TYPE_ARRAY },
 	[INSTANCE_ATTR_ERROR] = { "error", BLOBMSG_TYPE_ARRAY },
 	[INSTANCE_ATTR_USER] = { "user", BLOBMSG_TYPE_STRING },
+	[INSTANCE_ATTR_GROUP] = { "group", BLOBMSG_TYPE_STRING },
 	[INSTANCE_ATTR_STDOUT] = { "stdout", BLOBMSG_TYPE_BOOL },
 	[INSTANCE_ATTR_STDERR] = { "stderr", BLOBMSG_TYPE_BOOL },
 	[INSTANCE_ATTR_NO_NEW_PRIVS] = { "no_new_privs", BLOBMSG_TYPE_BOOL },
@@ -364,12 +366,12 @@ instance_run(struct service_instance *in, int _stdout, int _stderr)
 		closefd(_stderr);
 	}
 
-	if (in->user && in->gid && initgroups(in->user, in->gid)) {
+	if (in->user && in->pw_gid && initgroups(in->user, in->pw_gid)) {
 		ERROR("failed to initgroups() for user %s: %m\n", in->user);
 		exit(127);
 	}
-	if (in->gid && setgid(in->gid)) {
-		ERROR("failed to set group id %d: %m\n", in->gid);
+	if (in->gr_gid && setgid(in->gr_gid)) {
+		ERROR("failed to set group id %d: %m\n", in->gr_gid);
 		exit(127);
 	}
 	if (in->uid && setuid(in->uid)) {
@@ -650,10 +652,13 @@ instance_config_changed(struct service_instance *in, struct service_instance *in
 	if (string_changed(in->user, in_new->user))
 		return true;
 
+	if (string_changed(in->group, in_new->group))
+		return true;
+
 	if (in->uid != in_new->uid)
 		return true;
 
-	if (in->gid != in_new->gid)
+	if (in->pw_gid != in_new->pw_gid)
 		return true;
 
 	if (string_changed(in->pidfile, in_new->pidfile))
@@ -909,7 +914,16 @@ instance_config_parse(struct service_instance *in)
 		if (p) {
 			in->user = strdup(user);
 			in->uid = p->pw_uid;
-			in->gid = p->pw_gid;
+			in->gr_gid = in->pw_gid = p->pw_gid;
+		}
+	}
+
+	if (tb[INSTANCE_ATTR_GROUP]) {
+		const char *group = blobmsg_get_string(tb[INSTANCE_ATTR_GROUP]);
+		struct group *p = getgrnam(group);
+		if (p) {
+			in->group = strdup(group);
+			in->gr_gid = p->gr_gid;
 		}
 	}
 
@@ -1039,6 +1053,7 @@ instance_free(struct service_instance *in)
 	instance_config_cleanup(in);
 	free(in->config);
 	free(in->user);
+	free(in->group);
 	free(in);
 }
 

service/instance.h

@@ -44,7 +44,9 @@ struct service_instance {
 
 	char *user;
 	uid_t uid;
-	gid_t gid;
+	gid_t pw_gid;
+	char *group;
+	gid_t gr_gid;
 
 	bool halt;
 	bool restart;