check_root() both checks to see if we are root (in the init namespace),
and if we are drops to an unprivileged user.  To make future cleanups
simpler, split the checking for root (now in check_root()) from the actual
dropping of privilege (now in drop_root()).

Note that this does slightly alter semantics.  Previously we would only
setuid() if we were originally root (in the init namespace).  Now we will
always setuid() and setgid(), though it won't actually change anything if
we weren't privileged to begin with.  This also means that we will now
always attempt to switch to the user specified with --runas, even if we
aren't (init namespace) root to begin with.  Obviously this will fail with
an error if we weren't privileged to start with.  --help and the man page
are updated accordingly.

Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
 conf.c  |  5 +++--
 passt.1 |  5 +++--
 util.c  | 29 ++++++++++++++++++++++++++---
 util.h  |  1 +
 4 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/conf.c b/conf.c
index 0fe5266..545f61d 100644
--- a/conf.c
+++ b/conf.c
@@ -747,8 +747,8 @@ static void usage(const char *name)
 	info(   "    default: run in background if started from a TTY");
 	info(   "  -e, --stderr		Log to stderr too");
 	info(   "    default: log to system logger only if started from a TTY");
-	info(   "  --runas UID|UID:GID 	Use given UID, GID if started as root");
-	info(   "    UID and GID can be numeric, or login and group names");
+	info(   "  --runas UID|UID:GID 	Run as given UID, GID, which can be");
+	info(   "    numeric, or login and group names");
 	info(   "    default: drop to user \"nobody\"");
 	info(   "  -h, --help		Display this help message and exit");
 
@@ -1500,6 +1500,7 @@ void conf(struct ctx *c, int argc, char **argv)
 	} while (name != -1);
 
 	check_root(&uid, &gid);
+	drop_root(uid, gid);
 
 	if (c->mode == MODE_PASTA) {
 		if (*netns && optind != argc) {
diff --git a/passt.1 b/passt.1
index 61f0e4c..88cc879 100644
--- a/passt.1
+++ b/passt.1
@@ -104,9 +104,10 @@ terminal, and to both system logger and standard error otherwise.
 
 .TP
 .BR \-\-runas " " \fIUID\fR|\fIUID:GID\fR|\fILOGIN\fR|\fILOGIN:GROUP\fR
-If started as root, change to given UID and corresponding group if UID is given,
+Attempt to change to given UID and corresponding group if UID is given,
 or to given UID and given GID if both are given. Alternatively, login name, or
-login name and group name can be passed.
+login name and group name can be passed. This requires privileges (either
+initial effective UID 0 or CAP_SETUID capability) to work.
 Default is to change to user \fInobody\fR if started as root.
 
 .TP
diff --git a/util.c b/util.c
index b2ccb3d..eb25c37 100644
--- a/util.c
+++ b/util.c
@@ -492,7 +492,13 @@ void check_root(uid_t *uid, gid_t *gid)
 	char buf[BUFSIZ];
 	int fd;
 
-	if (getuid() && geteuid())
+	if (!*uid)
+		*uid = geteuid();
+
+	if (!*gid)
+		*gid = getegid();
+
+	if (*uid)
 		return;
 
 	if ((fd = open("/proc/self/uid_map", O_RDONLY | O_CLOEXEC)) < 0)
@@ -524,11 +530,28 @@ void check_root(uid_t *uid, gid_t *gid)
 		*uid = *gid = 65534;
 #endif
 	}
+}
+
+/**
+ * drop_root() - Switch to given UID and GID
+ * @uid:	User ID to switch to
+ * @gid:	Group ID to switch to
+ */
+void drop_root(uid_t uid, gid_t gid)
+{
+	if (setgroups(0, NULL)) {
+		/* If we don't start with CAP_SETGID, this will EPERM */
+		if (errno != EPERM) {
+			err("Can't drop supplementary groups: %s",
+			    strerror(errno));
+			exit(EXIT_FAILURE);
+		}
+	}
 
-	if (!setgroups(0, NULL) && !setgid(*gid) && !setuid(*uid))
+	if (!setgid(gid) && !setuid(uid))
 		return;
 
-	fprintf(stderr, "Can't change user/group, exiting");
+	err("Can't change user/group, exiting");
 	exit(EXIT_FAILURE);
 }
 
diff --git a/util.h b/util.h
index 58312fb..e2f686b 100644
--- a/util.h
+++ b/util.h
@@ -235,6 +235,7 @@ void procfs_scan_listen(struct ctx *c, uint8_t proto, int ip_version, int ns,
 			uint8_t *map, uint8_t *exclude);
 void drop_caps(void);
 void check_root(uid_t *uid, gid_t *gid);
+void drop_root(uid_t uid, gid_t gid);
 int ns_enter(const struct ctx *c);
 void write_pidfile(int fd, pid_t pid);
 int __daemon(int pidfile_fd, int devnull_fd);
-- 
2.37.3