su-exec.c

/*
 * Taken from https://github.com/ncopa/su-exec
 */

/* set user and group id and exec */

#include <sys/types.h>

#include <err.h>
#include <errno.h>
#include <grp.h>
#include <pwd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static char *argv0;

static void usage(int exitcode)
{
    printf("Usage: %s user-spec command [args]\n", argv0);
    exit(exitcode);
}

int main(int argc, char *argv[])
{
    char *user, *group, **cmdargv;
    char *end;

    uid_t uid = getuid();
    gid_t gid = getgid();

    argv0 = argv[0];
    if (argc < 3)
        usage(0);

    user = argv[1];
    group = strchr(user, ':');
    if (group)
        *group++ = '\0';

    cmdargv = &argv[2];

    struct passwd *pw = NULL;
    if (user[0] != '\0') {
        pw = getpwnam(user);
        uid_t nuid = strtol(user, &end, 10);
        if (*end == '\0')
            uid = nuid;
    }
    if (pw == NULL) {
        pw = getpwuid(uid);
    }
    if (pw != NULL) {
        uid = pw->pw_uid;
        gid = pw->pw_gid;
    }

    setenv("HOME", pw != NULL ? pw->pw_dir : "/", 1);

    if (group && group[0] != '\0') {
        /* group was specified, ignore grouplist for setgroups later */
        pw = NULL;

        struct group *gr = getgrnam(group);
        if (gr == NULL) {
            gid_t ngid = strtol(group, &end, 10);
            if (*end == '\0') {
                gr = getgrgid(ngid);
                if (gr == NULL)
                    gid = ngid;
            }
        }
        if (gr != NULL)
            gid = gr->gr_gid;
    }

    if (pw == NULL) {
        if (setgroups(1, &gid) < 0)
            err(1, "setgroups(%i)", gid);
    } else {
        int ngroups = 0;
        gid_t *glist = NULL;

        while (1) {
            int r = getgrouplist(pw->pw_name, gid, glist, &ngroups);

            if (r >= 0) {
                if (setgroups(ngroups, glist) < 0)
                    err(1, "setgroups");
                break;
            }

            glist = realloc(glist, ngroups * sizeof(gid_t));
            if (glist == NULL)
                err(1, "malloc");
        }
    }

    if (setgid(gid) < 0)
        err(1, "setgid(%i)", gid);

    if (setuid(uid) < 0)
        err(1, "setuid(%i)", uid);

    execvp(cmdargv[0], cmdargv);
    err(1, "%s", cmdargv[0]);

    return 1;
}