[PATCH v4 4/5] samples: Add example of using pidfd getfd in conjunction with user trap

[Sargun Dhillon <sargun@sargun.me>]

This sample adds the usage of SECCOMP_RET_USER_NOTIF together with pidfd
GETFD ioctl. It shows trapping a syscall, and handling it by extracting
the FD into the parent process without stopping the child process.
Although, in this example, there's no explicit policy separation in
the two processes, it can be generalized into the example of a transparent
proxy.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
---
 samples/seccomp/.gitignore        |   1 +
 samples/seccomp/Makefile          |   9 +-
 samples/seccomp/user-trap-pidfd.c | 185 ++++++++++++++++++++++++++++++
 3 files changed, 194 insertions(+), 1 deletion(-)
 create mode 100644 samples/seccomp/user-trap-pidfd.c

diff --git a/samples/seccomp/.gitignore b/samples/seccomp/.gitignore
index d1e2e817d556..f37e3692a1dd 100644
--- a/samples/seccomp/.gitignore
+++ b/samples/seccomp/.gitignore
@@ -2,3 +2,4 @@ bpf-direct
 bpf-fancy
 dropper
 user-trap
+user-trap-pidfd
diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile
index 82b7347318d1..c3880869cadc 100644
--- a/samples/seccomp/Makefile
+++ b/samples/seccomp/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 ifndef CROSS_COMPILE
-hostprogs-y := bpf-fancy dropper bpf-direct user-trap
+hostprogs-y := bpf-fancy dropper bpf-direct user-trap user-trap-pidfd
 
 HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
 HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include
@@ -24,6 +24,11 @@ HOSTCFLAGS_user-trap.o += -I$(objtree)/usr/include
 HOSTCFLAGS_user-trap.o += -idirafter $(objtree)/include
 user-trap-objs := user-trap.o user-trap-helper.o
 
+HOSTCFLAGS_user-trap-pidfd.o += -I$(objtree)/usr/include
+HOSTCFLAGS_user-trap-pidfd.o += -idirafter $(objtree)/include
+user-trap-pidfd-objs := user-trap-pidfd.o user-trap-helper.o
+
+
 # Try to match the kernel target.
 ifndef CONFIG_64BIT
 
@@ -39,10 +44,12 @@ HOSTCFLAGS_dropper.o += $(MFLAG)
 HOSTCFLAGS_bpf-helper.o += $(MFLAG)
 HOSTCFLAGS_bpf-fancy.o += $(MFLAG)
 HOSTCFLAGS_user-trap.o += $(MFLAG)
+HOSTCFLAGS_user-trap-pidfd.o += $(MFLAG)
 HOSTLDLIBS_bpf-direct += $(MFLAG)
 HOSTLDLIBS_bpf-fancy += $(MFLAG)
 HOSTLDLIBS_dropper += $(MFLAG)
 HOSTLDLIBS_user-trap += $(MFLAG)
+HOSTLDLIBS_user-trap-pidfd += $(MFLAG)
 endif
 always := $(hostprogs-y)
 endif
diff --git a/samples/seccomp/user-trap-pidfd.c b/samples/seccomp/user-trap-pidfd.c
new file mode 100644
index 000000000000..bd5730347a81
--- /dev/null
+++ b/samples/seccomp/user-trap-pidfd.c
@@ -0,0 +1,185 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/seccomp.h>
+#include <linux/prctl.h>
+#include <linux/pidfd.h>
+#include <sys/socket.h>
+#include <sys/prctl.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/ioctl.h>
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <netinet/in.h>
+#include "user-trap-helper.h"
+
+#define CHILD_PORT_TRY_BIND		80
+#define CHILD_PORT_ACTUAL_BIND	4998
+
+static int tracee(void)
+{
+	struct sockaddr_in addr = {
+		.sin_family	= AF_INET,
+		.sin_port	= htons(CHILD_PORT_TRY_BIND),
+		.sin_addr	= {
+			.s_addr	= htonl(INADDR_ANY)
+		}
+	};
+	socklen_t addrlen = sizeof(addr);
+	int sock, ret = 1;
+
+	sock = socket(AF_INET, SOCK_STREAM, 0);
+	if (sock == -1) {
+		perror("socket");
+		goto out;
+	}
+
+
+	if (bind(sock, (struct sockaddr *) &addr, sizeof(addr))) {
+		perror("bind");
+		goto out;
+	}
+
+	printf("Child successfully performed bind operation\n");
+	if (getsockname(sock, (struct sockaddr *) &addr, &addrlen)) {
+		perror("getsockname");
+		goto out;
+	}
+
+
+	printf("Socket bound to port %d\n", ntohs(addr.sin_port));
+	assert(ntohs(addr.sin_port) == CHILD_PORT_ACTUAL_BIND);
+
+	ret = 0;
+out:
+	return ret;
+}
+
+static int handle_req(int listener, int pidfd)
+{
+	struct sockaddr_in addr = {
+		.sin_family	= AF_INET,
+		.sin_port	= htons(CHILD_PORT_ACTUAL_BIND),
+		.sin_addr	= {
+			.s_addr	= htonl(INADDR_LOOPBACK)
+		}
+	};
+	struct seccomp_notif_sizes sizes;
+	struct seccomp_notif_resp *resp;
+	struct seccomp_notif *req;
+	int fd, ret = 1;
+
+	if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) {
+		perror("seccomp(GET_NOTIF_SIZES)");
+		goto out;
+	}
+	req = malloc(sizes.seccomp_notif);
+	if (!req)
+		goto out;
+	memset(req, 0, sizeof(*req));
+
+	resp = malloc(sizes.seccomp_notif_resp);
+	if (!resp)
+		goto out_free_req;
+	memset(resp, 0, sizeof(*resp));
+
+	if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
+		perror("ioctl recv");
+		goto out;
+	}
+	printf("Child tried to call bind with fd: %lld\n", req->data.args[0]);
+	fd = ioctl(pidfd, PIDFD_IOCTL_GETFD, req->data.args[0]);
+	if (fd == -1) {
+		perror("ioctl pidfd");
+		goto out_free_resp;
+	}
+	if (bind(fd, (struct sockaddr *) &addr, sizeof(addr))) {
+		perror("bind");
+		goto out_free_resp;
+	}
+
+	resp->id = req->id;
+	resp->error = 0;
+	resp->val = 0;
+	if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0) {
+		perror("ioctl send");
+		goto out_free_resp;
+	}
+
+	ret = 0;
+out_free_resp:
+	free(resp);
+out_free_req:
+	free(req);
+out:
+	return ret;
+}
+
+static int pidfd_open(pid_t pid, unsigned int flags)
+{
+	errno = 0;
+	return syscall(__NR_pidfd_open, pid, flags);
+}
+
+int main(void)
+{
+	int pidfd, listener, sk_pair[2], ret = 1;
+	pid_t pid;
+
+	if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair) < 0) {
+		perror("socketpair");
+		goto out;
+	}
+
+	pid = fork();
+	if (pid < 0) {
+		perror("fork");
+		goto close_pair;
+	}
+
+	if (pid == 0) {
+		if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+			perror("prctl(NO_NEW_PRIVS)");
+			exit(1);
+		}
+		listener = user_trap_syscall(__NR_bind,
+					     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+		if (listener < 0) {
+			perror("seccomp");
+			exit(1);
+		}
+		if (send_fd(sk_pair[1], listener) < 0)
+			exit(1);
+		close(listener);
+		exit(tracee());
+	}
+
+	pidfd = pidfd_open(pid, 0);
+	if (pidfd < 0) {
+		perror("pidfd_open");
+		goto kill_child;
+	}
+
+	listener = recv_fd(sk_pair[0]);
+	if (listener < 0)
+		goto kill_child;
+
+	if (handle_req(listener, pidfd))
+		goto kill_child;
+
+	/* Wait for child to finish */
+	waitpid(pid, NULL, 0);
+
+	ret = 0;
+	goto close_pair;
+
+kill_child:
+	kill(pid, SIGKILL);
+close_pair:
+	close(sk_pair[0]);
+	close(sk_pair[1]);
+out:
+	return ret;
+}
-- 
2.20.1