#define _BSD_SOURCE
#include <errno.h>
#include <grp.h>
#include <pwd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char **argv) {
	if(argc < 3) {
		fprintf(stderr, "Usage: %s username[:group] command [arguments]\n", argv[0]);
		return 1;
	}

	char *username_group = strdup(argv[1]);
	if(username_group == NULL) {
		perror("strdup");
		return 1;
	}
	char *colon_position, *username, *group;
	if((colon_position = strchr(username_group, ':')) != NULL) {
		*colon_position = 0;
		username = username_group;
		group = colon_position + 1;
	} else {
		username = username_group;
		group = NULL;
	}

	errno = 0;
	struct passwd *passwd_entry = getpwnam(username);
	if(passwd_entry == NULL) {
		perror("getpwnam");
		return 1;
	}

	uid_t ruid = passwd_entry->pw_uid;

	uid_t rgid;

	if(group != NULL) {
		errno = 0;
		struct group *group_entry = getgrnam(group);
		if(group_entry == NULL) {
			perror("getgrnam");
			return 1;
		}
		rgid = group_entry->gr_gid;
	} else {
		rgid = passwd_entry->pw_gid;
	}

	free(username_group);

	if(setreuid(ruid, -1) != 0) {
		perror("setreuid");
		return 1;
	}
	if(setregid(rgid, -1) != 0) {
		perror("setregid");
		return 1;
	}

	char **daemon_argv = &argv[2];
	const char *daemon_command = daemon_argv[0];

	execvp(daemon_command, daemon_argv);

	perror("execvp");
	return 1;
}