/*
**  Issue as discovered by richard lucassen in
**  <http://marc.info/?t=121760392200003&r=1&w=2>
**  After setuid, open returns "permission denied" when trying to
**  open /dev/stdder (which symlinks to /proc/self/fd/2).
*/

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int open_file(char *pathname, char *msg);

int
main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "usage: self_fd /dev/stderr\n");
        /* Or, you could try some other file :-) */
        exit(1);
    }

    /* Check we are who we need to be */
    if (getuid() != 0) {
        fprintf(stderr, "I insist that I be run as root\n");
        exit(1);
    }

    open_file(argv[1], "root");

    /* Set ourselves to an arbitrary user id, group first */
    if (setgid(500) == -1) {
        perror("setgid(500)");
        exit(1);
    }
    if (setuid(500) == -1) {
        perror("setuid(500)");
        exit(1);
    }

    open_file(argv[1], "uid 500");

    return 0;
}

int
open_file(char *pathname, char *msg)
{
    int    fd;
    char   m[80];
    struct stat s;

    fd = open(pathname, O_WRONLY);
    if (fd == -1) {
        snprintf(m, sizeof m, "open(%s) as %s", pathname, msg);
        perror(m);
        if (stat(pathname, &s) == -1) {
            snprintf(m, sizeof m, "stat(%s) as %s", pathname, msg);
            perror(m);
        } else {
            fprintf(stderr, "%s: uid: %u, gid: %u\n", pathname,
                    s.st_uid, s.st_gid);
        }
        return 1;
    } else {
        if (fstat(fd, &s) == -1) {
            snprintf(m, sizeof m, "fstat(%s) as %s", pathname, msg);
            perror(m);
        } else {
            fprintf(stderr, "%s: uid: %u, gid: %u\n", pathname,
                    s.st_uid, s.st_gid);
        }
        close(fd);
        return 0;
    }
}