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

static void
usage(const char *argv0)
{
    fprintf(stderr, "usage: %s --uid UID --gid GID -- cmd...\n", argv0);
}

static unsigned long
parse_ulong(const char *text)
{
    char *end = NULL;
    unsigned long value;

    errno = 0;
    value = strtoul(text, &end, 10);
    if (errno != 0 || end == text || *end != '\0')
        exit(2);

    return value;
}

int
main(int argc, char **argv)
{
    uid_t uid = (uid_t)-1;
    gid_t gid = (gid_t)-1;
    gid_t groups[1];
    int i;

    for (i = 1; i < argc; i++) {
        if (strcmp(argv[i], "--uid") == 0 && i + 1 < argc) {
            uid = (uid_t)parse_ulong(argv[++i]);
        } else if (strcmp(argv[i], "--gid") == 0 && i + 1 < argc) {
            gid = (gid_t)parse_ulong(argv[++i]);
        } else if (strcmp(argv[i], "--") == 0) {
            i++;
            break;
        } else {
            usage(argv[0]);
            return 2;
        }
    }

    if (uid == (uid_t)-1 || gid == (gid_t)-1 || i >= argc) {
        usage(argv[0]);
        return 2;
    }

    groups[0] = gid;
    if (setgroups(1, groups) != 0) {
        perror("setgroups");
        return 1;
    }
    if (setgid(gid) != 0) {
        perror("setgid");
        return 1;
    }
    if (setuid(uid) != 0) {
        perror("setuid");
        return 1;
    }

    execvp(argv[i], &argv[i]);
    perror("execvp");
    return 1;
}