// SPDX-License-Identifier: GPL-2.0-or-later
//
// passwd [USER]
//
// Change a password. Setuid root, like su and for the same reason a program of
// its own: it has to write /etc/shadow, which no one else can.
//
// Anyone may change their own password after proving they know the old one;
// root may change anyone's without it. The new one is stored as a SHA-512
// crypt hash with a fresh random salt, and the file is replaced by rename, so
// a crash leaves the old shadow or the new one and never half of each.

#define _GNU_SOURCE
#include <crypt.h>
#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <pwd.h>
#include <shadow.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <termios.h>
#include <time.h>
#include <unistd.h>

static int
read_password(const char *prompt, char *buf, size_t n)
{
    int fd = open("/dev/tty", O_RDWR | O_NOCTTY);
    struct termios t, old;
    size_t i = 0;
    char c;

    if (fd < 0)
        return -1;
    if (tcgetattr(fd, &old) == 0) {
        t = old;
        t.c_lflag &= ~(ECHO | ECHONL);
        tcsetattr(fd, TCSAFLUSH, &t);
    }
    if (write(fd, prompt, strlen(prompt)) < 0) {}
    while (read(fd, &c, 1) == 1 && c != '\n' && c != '\r')
        if (i + 1 < n)
            buf[i++] = c;
    buf[i] = 0;
    tcsetattr(fd, TCSAFLUSH, &old);
    if (write(fd, "\n", 1) < 0) {}
    close(fd);
    return 0;
}

static void
make_salt(char *out)
{
    static const char a[] =
        "./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    unsigned char r[16];
    int fd = open("/dev/urandom", O_RDONLY);

    if (fd < 0 || read(fd, r, sizeof r) != (ssize_t)sizeof r) {
        fprintf(stderr, "passwd: no random numbers\n");
        exit(1);
    }
    close(fd);
    strcpy(out, "$6$");
    for (int i = 0; i < 16; i++)
        out[3 + i] = a[r[i] & 63];
    out[19] = '$';
    out[20] = 0;
}

int
main(int argc, char **argv)
{
    uid_t uid = getuid();
    struct passwd *me = getpwuid(uid), *pw;
    // getpwuid and getpwnam share one static buffer: keep the name we were
    // given, not a pointer into it.
    char *user = argc > 1 ? argv[1] : (me ? strdup(me->pw_name) : 0);
    char oldp[256], p1[256], p2[256], salt[24], *hash;
    FILE *in, *out;
    char line[1024], tmp[] = "/etc/shadow.new";
    long days = (long)(time(0) / 86400);
    int found = 0;

    if (!user || !(pw = getpwnam(user))) {
        fprintf(stderr, "passwd: unknown user\n");
        return 1;
    }
    {
        static struct passwd copy;
        copy = *pw;
        copy.pw_name = strdup(pw->pw_name);
        pw = &copy;
    }
    if (geteuid() != 0) {
        fprintf(stderr, "passwd: must be setuid root\n");
        return 1;
    }
    if (uid != 0 && pw->pw_uid != uid) {
        fprintf(stderr, "passwd: you may only change your own password\n");
        return 1;
    }

    if (uid != 0) {
        struct spwd *sp = getspnam(pw->pw_name);
        const char *cur = sp ? sp->sp_pwdp : "";
        if (cur[0]) {
            if (read_password("Current password: ", oldp, sizeof oldp) < 0)
                return 1;
            char *e = crypt(oldp, cur);
            memset(oldp, 0, sizeof oldp);
            if (!e || strcmp(e, cur) != 0) {
                sleep(2);
                fprintf(stderr, "passwd: incorrect password\n");
                return 1;
            }
        }
    }

    if (read_password("New password: ", p1, sizeof p1) < 0 ||
        read_password("Retype new password: ", p2, sizeof p2) < 0)
        return 1;
    if (strcmp(p1, p2) != 0) {
        fprintf(stderr, "passwd: the passwords do not match\n");
        return 1;
    }
    make_salt(salt);
    // An empty password stays empty: crypt of "" is a hash, and the user asked
    // for no password at all.
    hash = p1[0] ? crypt(p1, salt) : "";
    if (!hash) {
        fprintf(stderr, "passwd: cannot hash\n");
        return 1;
    }

    if (!(in = fopen("/etc/shadow", "r"))) {
        fprintf(stderr, "passwd: /etc/shadow: %s\n", strerror(errno));
        return 1;
    }
    umask(0077);
    if (!(out = fopen(tmp, "w"))) {
        fprintf(stderr, "passwd: %s: %s\n", tmp, strerror(errno));
        return 1;
    }
    while (fgets(line, sizeof line, in)) {
        size_t n = strlen(pw->pw_name);
        if (strncmp(line, pw->pw_name, n) == 0 && line[n] == ':') {
            // name:hash:lastchg:min:max:warn:inactive:expire:reserved
            char *f = strchr(line + n + 1, ':');
            fprintf(out, "%s:%s:%ld%s", pw->pw_name, hash, days,
                    f && (f = strchr(f + 1, ':')) ? f : ":0:::::\n");
            found = 1;
        } else {
            fputs(line, out);
        }
    }
    fclose(in);
    if (!found)
        fprintf(out, "%s:%s:%ld:0:::::\n", pw->pw_name, hash, days);
    memset(p1, 0, sizeof p1);
    memset(p2, 0, sizeof p2);

    struct group *g = getgrnam("shadow");
    fchmod(fileno(out), 0640);
    if (g)
        fchown(fileno(out), 0, g->gr_gid);
    if (fclose(out) != 0 || rename(tmp, "/etc/shadow") < 0) {
        fprintf(stderr, "passwd: could not write /etc/shadow: %s\n", strerror(errno));
        unlink(tmp);
        return 1;
    }
    printf("password updated\n");
    return 0;
}