#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>
#include <stddef.h>
#include <sys/sysmacros.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/user.h>
#include <sys/ioctl.h>
#include <sys/ptrace.h>
#include <sys/mount.h>
#include <linux/limits.h>
#include <linux/filter.h>
#include <linux/seccomp.h>

#define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x)))

static int seccomp(unsigned int op, unsigned int flags, void *args)
{
        errno = 0;
        return syscall(__NR_seccomp, op, flags, args);
}

static int send_fd(int sock, int fd)
{
        struct msghdr msg = {};
        struct cmsghdr *cmsg;
        char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
        struct iovec io = {
                .iov_base = &c,
                .iov_len = 1,
        };

        msg.msg_iov = &io;
        msg.msg_iovlen = 1;
        msg.msg_control = buf;
        msg.msg_controllen = sizeof(buf);
        cmsg = CMSG_FIRSTHDR(&msg);
        cmsg->cmsg_level = SOL_SOCKET;
        cmsg->cmsg_type = SCM_RIGHTS;
        cmsg->cmsg_len = CMSG_LEN(sizeof(int));
        *((int *)CMSG_DATA(cmsg)) = fd;
        msg.msg_controllen = cmsg->cmsg_len;

        if (sendmsg(sock, &msg, 0) < 0) {
                perror("sendmsg");
                return -1;
        }

        return 0;
}

static int recv_fd(int sock)
{
        struct msghdr msg = {};
        struct cmsghdr *cmsg;
        char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
        struct iovec io = {
                .iov_base = &c,
                .iov_len = 1,
        };

        msg.msg_iov = &io;
        msg.msg_iovlen = 1;
        msg.msg_control = buf;
        msg.msg_controllen = sizeof(buf);

        if (recvmsg(sock, &msg, 0) < 0) {
                perror("recvmsg");
                return -1;
        }

        cmsg = CMSG_FIRSTHDR(&msg);

        return *((int *)CMSG_DATA(cmsg));
}

static int user_trap_syscall(int nr, unsigned int flags)
{
        struct sock_filter filter[] = {
                BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
                        offsetof(struct seccomp_data, nr)),
                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_USER_NOTIF),
                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
        };

        struct sock_fprog prog = {
                .len = (unsigned short)ARRAY_SIZE(filter),
                .filter = filter,
        };

        return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog);
}

static int handle_req(struct seccomp_notif *req,
                      struct seccomp_notif_resp *resp, int listener)
{
        char path[PATH_MAX], source[PATH_MAX], target[PATH_MAX];
        int ret = -1, mem;

        resp->id = req->id;
        resp->error = -EPERM;
        resp->val = 0;

        if (req->data.nr != __NR_mount) {
                fprintf(stderr, "huh? trapped something besides mount? %d\n", req->data.nr);
                return -1;
        }

        /* Only allow bind mounts. */
        if (!(req->data.args[3] & MS_BIND))
                return 0;

        /*
         * Ok, let's read the task's memory to see where they wanted their
         * mount to go.
         */
        snprintf(path, sizeof(path), "/proc/%d/mem", req->pid);
        mem = open(path, O_RDONLY);
        if (mem < 0) {
                perror("open mem");
                return -1;
        }

        /*
         * Now we avoid a TOCTOU: we referred to a pid by its pid, but since
         * the pid that made the syscall may have died, we need to confirm that
         * the pid is still valid after we open its /proc/pid/mem file. We can
         * ask the listener fd this as follows.
         *
         * Note that this check should occur *after* any task-specific
         * resources are opened, to make sure that the task has not died and
         * we're not wrongly reading someone else's state in order to make
         * decisions.
         */
        if (ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req->id) < 0) {
                fprintf(stderr, "task died before we could map its memory\n");
                goto out;
        }

        /*
         * Phew, we've got the right /proc/pid/mem. Now we can read it. Note
         * that to avoid another TOCTOU, we should read all of the pointer args
         * before we decide to allow the syscall.
         */
        if (lseek(mem, req->data.args[0], SEEK_SET) < 0) {
                perror("seek");
                goto out;
        }

        ret = read(mem, source, sizeof(source));
        if (ret < 0) {
                perror("read");
                goto out;
        }

        if (lseek(mem, req->data.args[1], SEEK_SET) < 0) {
                perror("seek");
                goto out;
        }

        ret = read(mem, target, sizeof(target));
        if (ret < 0) {
                perror("read");
                goto out;
        }

        /*
         * Our policy is to only allow bind mounts inside /tmp. This isn't very
         * interesting, because we could do unprivlieged bind mounts with user
         * namespaces already, but you get the idea.
         */
        if (!strncmp(source, "/tmp/", 5) && !strncmp(target, "/tmp/", 5)) {
                if (mount(source, target, NULL, req->data.args[3], NULL) < 0) {
                        ret = -1;
                        perror("actual mount");
                        goto out;
                }
                resp->error = 0;
        }

        /* Even if we didn't allow it because of policy, generating the
         * response was be a success, because we want to tell the worker EPERM.
         */
        ret = 0;

out:
        close(mem);
        return ret;
}

int main(void)
{
        int sk_pair[2], ret = 1, status, listener;
        pid_t worker = 0 , tracer = 0;

        if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair) < 0) {
                perror("socketpair");
                return 1;
        }

        worker = fork();
        if (worker < 0) {
                perror("fork");
                goto close_pair;
        }

        if (worker == 0) {
                listener = user_trap_syscall(__NR_mount,
                                             SECCOMP_FILTER_FLAG_NEW_LISTENER);
                if (listener < 0) {
                        perror("seccomp");
                        exit(1);
                }

                /*
                 * Drop privileges. We definitely can't mount as uid 1000.
                 */
                if (setuid(1000) < 0) {
                        perror("setuid");
                        exit(1);
                }

                /*
                 * Send the listener to the parent; also serves as
                 * synchronization.
                 */
                if (send_fd(sk_pair[1], listener) < 0)
                        exit(1);
                close(listener);

                if (mkdir("/tmp/foo", 0755) < 0) {
                        perror("mkdir");
                        exit(1);
                }

                /*
                 * Try a bad mount just for grins.
                 */
                if (mount("/dev/sda", "/tmp/foo", NULL, 0, NULL) != -1) {
                        fprintf(stderr, "huh? mounted /dev/sda?\n");
                        exit(1);
                }

                if (errno != EPERM) {
                        perror("bad error from mount");
                        exit(1);
                }

                /*
                 * Ok, we expect this one to succeed.
                 */
                if (mount("/tmp/foo", "/tmp/foo", NULL, MS_BIND, NULL) < 0) {
                        perror("mount");
                        exit(1);
                }

                exit(0);
        }

        /*
         * Get the listener from the child.
         */
        listener = recv_fd(sk_pair[0]);
        if (listener < 0)
                goto out_kill;

        /*
         * Fork a task to handle the requests. This isn't strictly necessary,
         * but it makes the particular writing of this sample easier, since we
         * can just wait ofr the tracee to exit and kill the tracer.
         */
        tracer = fork();
        if (tracer < 0) {
                perror("fork");
                goto out_kill;
        }

        if (tracer == 0) {
                struct seccomp_notif *req;
                struct seccomp_notif_resp *resp;
                struct seccomp_notif_sizes sizes;

                if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) {
                        perror("seccomp(GET_NOTIF_SIZES)");
                        goto out_close;
                }

                req = malloc(sizes.seccomp_notif);
                if (!req)
                        goto out_close;

                resp = malloc(sizes.seccomp_notif_resp);
                if (!resp)
                        goto out_req;
                memset(resp, 0, sizes.seccomp_notif_resp);

                while (1) {
                        memset(req, 0, sizes.seccomp_notif);
                        if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
                                perror("ioctl recv");
                                goto out_resp;
                        }

                        if (handle_req(req, resp, listener) < 0)
                                goto out_resp;

                        /*
                         * ENOENT here means that the task may have gotten a
                         * signal and restarted the syscall. It's up to the
                         * handler to decide what to do in this case, but for
                         * the sample code, we just ignore it. Probably
                         * something better should happen, like undoing the
                         * mount, or keeping track of the args to make sure we
                         * don't do it again.
                         */
                        if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0 &&
                            errno != ENOENT) {
                                perror("ioctl send");
                                goto out_resp;
                        }
                }
out_resp:
                free(resp);
out_req:
                free(req);
out_close:
                close(listener);
                exit(1);
        }

        close(listener);

        if (waitpid(worker, &status, 0) != worker) {
                perror("waitpid");
                goto out_kill;
        }

        if (umount2("/tmp/foo", MNT_DETACH) < 0 && errno != EINVAL) {
                perror("umount2");
                goto out_kill;
        }

        if (remove("/tmp/foo") < 0 && errno != ENOENT) {
                perror("remove");
                exit(1);
        }

        if (!WIFEXITED(status) || WEXITSTATUS(status)) {
                fprintf(stderr, "worker exited nonzero\n");
                goto out_kill;
        }

        ret = 0;

out_kill:
        if (tracer > 0)
                kill(tracer, SIGKILL);
        if (worker > 0)
                kill(worker, SIGKILL);

close_pair:
        close(sk_pair[0]);
        close(sk_pair[1]);
        return ret;
}