// SPDX-License-Identifier: GPL-2.0
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <sched.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/vfs.h>
#include <unistd.h>

#ifndef MS_NOSYMFOLLOW
# define MS_NOSYMFOLLOW 256     /* Do not follow symlinks */
#endif

#ifndef ST_NOSYMFOLLOW
# define ST_NOSYMFOLLOW 0x2000  /* Do not follow symlinks */
#endif

#define DATA "/tmp/data"
#define LINK "/tmp/symlink"
#define TMP  "/tmp"

static void die(char *fmt, ...)
{
        va_list ap;

        va_start(ap, fmt);
        vfprintf(stderr, fmt, ap);
        va_end(ap);
        exit(EXIT_FAILURE);
}

static void vmaybe_write_file(bool enoent_ok, char *filename, char *fmt,
                va_list ap)
{
        ssize_t written;
        char buf[4096];
        int buf_len;
        int fd;

        buf_len = vsnprintf(buf, sizeof(buf), fmt, ap);
        if (buf_len < 0)
                die("vsnprintf failed: %s\n", strerror(errno));

        if (buf_len >= sizeof(buf))
                die("vsnprintf output truncated\n");

        fd = open(filename, O_WRONLY);
        if (fd < 0) {
                if ((errno == ENOENT) && enoent_ok)
                        return;
                die("open of %s failed: %s\n", filename, strerror(errno));
        }

        written = write(fd, buf, buf_len);
        if (written != buf_len) {
                if (written >= 0) {
                        die("short write to %s\n", filename);
                } else {
                        die("write to %s failed: %s\n",
                                filename, strerror(errno));
                }
        }

        if (close(fd) != 0)
                die("close of %s failed: %s\n", filename, strerror(errno));
}

static void maybe_write_file(char *filename, char *fmt, ...)
{
        va_list ap;

        va_start(ap, fmt);
        vmaybe_write_file(true, filename, fmt, ap);
        va_end(ap);
}

static void write_file(char *filename, char *fmt, ...)
{
        va_list ap;

        va_start(ap, fmt);
        vmaybe_write_file(false, filename, fmt, ap);
        va_end(ap);
}

static void create_and_enter_ns(void)
{
        uid_t uid = getuid();
        gid_t gid = getgid();

        if (unshare(CLONE_NEWUSER) != 0)
                die("unshare(CLONE_NEWUSER) failed: %s\n", strerror(errno));

        maybe_write_file("/proc/self/setgroups", "deny");
        write_file("/proc/self/uid_map", "0 %d 1", uid);
        write_file("/proc/self/gid_map", "0 %d 1", gid);

        if (setgid(0) != 0)
                die("setgid(0) failed %s\n", strerror(errno));
        if (setuid(0) != 0)
                die("setuid(0) failed %s\n", strerror(errno));

        if (unshare(CLONE_NEWNS) != 0)
                die("unshare(CLONE_NEWNS) failed: %s\n", strerror(errno));
}

static void setup_symlink(void)
{
        int data, err;

        data = creat(DATA, O_RDWR);
        if (data < 0)
                die("creat failed: %s\n", strerror(errno));

        err = symlink(DATA, LINK);
        if (err < 0)
                die("symlink failed: %s\n", strerror(errno));

        if (close(data) != 0)
                die("close of %s failed: %s\n", DATA, strerror(errno));
}

static void test_link_traversal(bool nosymfollow)
{
        int link;

        link = open(LINK, 0, O_RDWR);
        if (nosymfollow) {
                if ((link != -1 || errno != ELOOP)) {
                        die("link traversal unexpected result: %d, %s\n",
                                        link, strerror(errno));
                }
        } else {
                if (link < 0)
                        die("link traversal failed: %s\n", strerror(errno));

                if (close(link) != 0)
                        die("close of link failed: %s\n", strerror(errno));
        }
}

static void test_readlink(void)
{
        char buf[4096];
        ssize_t ret;

        bzero(buf, sizeof(buf));

        ret = readlink(LINK, buf, sizeof(buf));
        if (ret < 0)
                die("readlink failed: %s\n", strerror(errno));
        if (strcmp(buf, DATA) != 0)
                die("readlink strcmp failed: '%s' '%s'\n", buf, DATA);
}

static void test_realpath(void)
{
        char *path = realpath(LINK, NULL);

        if (!path)
                die("realpath failed: %s\n", strerror(errno));
        if (strcmp(path, DATA) != 0)
                die("realpath strcmp failed\n");

        free(path);
}

static void test_statfs(bool nosymfollow)
{
        struct statfs buf;
        int ret;

        ret = statfs(TMP, &buf);
        if (ret)
                die("statfs failed: %s\n", strerror(errno));

        if (nosymfollow) {
                if ((buf.f_flags & ST_NOSYMFOLLOW) == 0)
                        die("ST_NOSYMFOLLOW not set on %s\n", TMP);
        } else {
                if ((buf.f_flags & ST_NOSYMFOLLOW) != 0)
                        die("ST_NOSYMFOLLOW set on %s\n", TMP);
        }
}

static void run_tests(bool nosymfollow)
{
        test_link_traversal(nosymfollow);
        test_readlink();
        test_realpath();
        test_statfs(nosymfollow);
}

int main(int argc, char **argv)
{
        create_and_enter_ns();

        if (mount("testing", TMP, "ramfs", 0, NULL) != 0)
                die("mount failed: %s\n", strerror(errno));

        setup_symlink();
        run_tests(false);

        if (mount("testing", TMP, "ramfs", MS_REMOUNT|MS_NOSYMFOLLOW, NULL) != 0)
                die("remount failed: %s\n", strerror(errno));

        run_tests(true);

        return EXIT_SUCCESS;
}