The idea here is just to give a demonstration of how one could safely use
the SECCOMP_RET_USER_NOTIF feature to do mount policies. This particular
policy is (as noted in the comment) not very interesting, but it serves to
illustrate how one might apply a policy dodging the various TOCTOU issues.

Signed-off-by: Tycho Andersen <redacted>
CC: Kees Cook <redacted>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <redacted>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <christian@brauner.io>
CC: Tyler Hicks <redacted>
CC: Akihiro Suda <redacted>
---
v5: new in v5
v7: updates for v7 API changes
v8: * add some more comments about what's happening in main() (Kees)
    * move from ptrace API to SECCOMP_FILTER_FLAG_NEW_LISTENER
---
 samples/seccomp/.gitignore  |   1 +
 samples/seccomp/Makefile    |   7 +-
 samples/seccomp/user-trap.c | 345 ++++++++++++++++++++++++++++++++++++
 3 files changed, 352 insertions(+), 1 deletion(-)

diff --git a/samples/seccomp/.gitignore b/samples/seccomp/.gitignore
index 78fb78184291..d1e2e817d556 100644
--- a/samples/seccomp/.gitignore
+++ b/samples/seccomp/.gitignore

@@ -1,3 +1,4 @@
 bpf-direct
 bpf-fancy
 dropper
+user-trap

diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile
index cf34ff6b4065..4920903c8009 100644
--- a/samples/seccomp/Makefile
+++ b/samples/seccomp/Makefile

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 ifndef CROSS_COMPILE
-hostprogs-$(CONFIG_SAMPLE_SECCOMP) := bpf-fancy dropper bpf-direct
+hostprogs-$(CONFIG_SAMPLE_SECCOMP) := bpf-fancy dropper bpf-direct user-trap
 
 HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
 HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include

@@ -16,6 +16,10 @@ HOSTCFLAGS_bpf-direct.o += -I$(objtree)/usr/include
 HOSTCFLAGS_bpf-direct.o += -idirafter $(objtree)/include
 bpf-direct-objs := bpf-direct.o
 
+HOSTCFLAGS_user-trap.o += -I$(objtree)/usr/include
+HOSTCFLAGS_user-trap.o += -idirafter $(objtree)/include
+user-trap-objs := user-trap.o
+
 # Try to match the kernel target.
 ifndef CONFIG_64BIT
 

@@ -33,6 +37,7 @@ HOSTCFLAGS_bpf-fancy.o += $(MFLAG)
 HOSTLDLIBS_bpf-direct += $(MFLAG)
 HOSTLDLIBS_bpf-fancy += $(MFLAG)
 HOSTLDLIBS_dropper += $(MFLAG)
+HOSTLDLIBS_user-trap += $(MFLAG)
 endif
 always := $(hostprogs-m)
 endif

diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
new file mode 100644
index 000000000000..bba7ac803c6c
--- /dev/null
+++ b/samples/seccomp/user-trap.c

@@ -0,0 +1,345 @@
+#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 mknod? %d\n", req->data.nr);