instance: handle setgid() before setuid()

When attempting to run a service with an unprivileged user and group
id procd, the following error might occur:

	procd: failed to set uid:1000, gid:1000

This is due to the fact that procd first performs the setuid(), then
the setgid() call.

Usually there no sufficient permissions after a setuid() anymore to
change the effective group id of the process.

Refactor the code to:

  * Swap the invocations (first gid, then uid)
  * Don't set user or group id if it is 0
  * Handle errors independently and make them more verbose

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/service/instance.c b/service/instance.c
index 1fb65fa..3367885 100644
--- a/service/instance.c
+++ b/service/instance.c
@@ -282,12 +282,15 @@ instance_run(struct service_instance *in, int _stdout, int _stderr)
 		closefd(_stderr);
 	}
 
-	if (in->uid || in->gid) {
-		if (setuid(in->uid) || setgid(in->gid)) {
-			ERROR("failed to set uid:%d, gid:%d\n", in->uid, in->gid);
-			exit(127);
-		}
+	if (in->gid && setgid(in->gid)) {
+		ERROR("failed to set group id %d: %d (%s)\n", in->gid, errno, strerror(errno));
+		exit(127);
+	}
+	if (in->uid && setuid(in->uid)) {
+		ERROR("failed to set user id %d: %d (%s)\n", in->uid, errno, strerror(errno));
+		exit(127);
 	}
+
 	execvp(argv[0], argv);
 	exit(127);
 }