Merge branch 'SCM_PIDFD-SCM_PEERPIDFD'

Alexander Mikhalitsyn says:

====================
Add SCM_PIDFD and SO_PEERPIDFD

1. Implement SCM_PIDFD, a new type of CMSG type analogical to SCM_CREDENTIALS,
but it contains pidfd instead of plain pid, which allows programmers not
to care about PID reuse problem.

2. Add SO_PEERPIDFD which allows to get pidfd of peer socket holder pidfd.
This thing is direct analog of SO_PEERCRED which allows to get plain PID.

3. Add SCM_PIDFD / SO_PEERPIDFD kselftest

Idea comes from UAPI kernel group:
https://uapi-group.org/kernel-features/

Big thanks to Christian Brauner and Lennart Poettering for productive
discussions about this and Luca Boccassi for testing and reviewing this.

=== Motivation behind this patchset

Eric Dumazet raised a question:
> It seems that we already can use pidfd_open() (since linux-5.3), and
> pass the resulting fd in af_unix SCM_RIGHTS message ?

Yes, it's possible, but it means that from the receiver side we need
to trust the sent pidfd (in SCM_RIGHTS),
or always use combination of SCM_RIGHTS+SCM_CREDENTIALS, then we can
extract pidfd from SCM_RIGHTS,
then acquire plain pid from pidfd and after compare it with the pid
from SCM_CREDENTIALS.

A few comments from other folks regarding this.

Christian Brauner wrote:

>Let me try and provide some of the missing background.

>There are a range of use-cases where we would like to authenticate a
>client through sockets without being susceptible to PID recycling
>attacks. Currently, we can't do this as the race isn't fully fixable.
>We can only apply mitigations.

>What this patchset will allows us to do is to get a pidfd without the
>client having to send us an fd explicitly via SCM_RIGHTS. As that's
>already possibly as you correctly point out.

>But for protocols like polkit this is quite important. Every message is
>standalone and we would need to force a complete protocol change where
>we would need to require that every client allocate and send a pidfd via
>SCM_RIGHTS. That would also mean patching through all polkit users.

>For something like systemd-journald where we provide logging facilities
>and want to add metadata to the log we would also immensely benefit from
>being able to get a receiver-side controlled pidfd.

>With the message type we envisioned we don't need to change the sender
>at all and can be safe against pid recycling.

>Link: https://gitlab.freedesktop.org/polkit/polkit/-/merge_requests/154
>Link: https://uapi-group.org/kernel-features

Lennart Poettering wrote:

>So yes, this is of course possible, but it would mean the pidfd would
>have to be transported as part of the user protocol, explicitly sent
>by the sender. (Moreover, the receiver after receiving the pidfd would
>then still have to somehow be able to prove that the pidfd it just
>received actually refers to the peer's process and not some random
>process. – this part is actually solvable in userspace, but ugly)

>The big thing is simply that we want that the pidfd is associated
>*implicity* with each AF_UNIX connection, not explicitly. A lot of
>userspace already relies on this, both in the authentication area
>(polkit) as well as in the logging area (systemd-journald). Right now
>using the PID field from SO_PEERCREDS/SCM_CREDENTIALS is racy though
>and very hard to get right. Making this available as pidfd too, would
>solve this raciness, without otherwise changing semantics of it all:
>receivers can still enable the creds stuff as they wish, and the data
>is then implicitly appended to the connections/datagrams the sender
>initiates.

>Or to turn this around: things like polkit are typically used to
>authenticate arbitrary dbus methods calls: some service implements a
>dbus method call, and when an unprivileged client then issues that
>call, it will take the client's info, go to polkit and ask it if this
>is ok. If we wanted to send the pidfd as part of the protocol we
>basically would have to extend every single method call to contain the
>client's pidfd along with it as an additional argument, which would be
>a massive undertaking: it would change the prototypes of basically
>*all* methods a service defines… And that's just ugly.

>Note that Alex' patch set doesn't expose anything that wasn't exposed
>before, or attach, propagate what wasn't before. All it does, is make
>the field already available anyway (the struct ucred .pid field)
>available also in a better way (as a pidfd), to solve a variety of
>races, with no effect on the protocol actually spoken within the
>AF_UNIX transport. It's a seamless improvement of the status quo.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

arch/alpha/include/uapi/asm/socket.h

@@ -137,6 +137,9 @@
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+#define SO_PEERPIDFD		77
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64

arch/mips/include/uapi/asm/socket.h

@@ -148,6 +148,9 @@
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+#define SO_PEERPIDFD		77
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64

arch/parisc/include/uapi/asm/socket.h

@@ -129,6 +129,9 @@
 
 #define SO_RCVMARK		0x4049
 
+#define SO_PASSPIDFD		0x404A
+#define SO_PEERPIDFD		0x404B
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64

arch/sparc/include/uapi/asm/socket.h

@@ -130,6 +130,9 @@
 
 #define SO_RCVMARK               0x0054
 
+#define SO_PASSPIDFD             0x0055
+#define SO_PEERPIDFD             0x0056
+
 #if !defined(__KERNEL__)
 
 

include/linux/net.h

@@ -43,6 +43,7 @@ struct net;
 #define SOCK_PASSSEC		4
 #define SOCK_SUPPORT_ZC		5
 #define SOCK_CUSTOM_SOCKOPT	6
+#define SOCK_PASSPIDFD		7
 
 #ifndef ARCH_HAS_SOCKET_TYPES
 /**

include/linux/socket.h

@@ -177,6 +177,7 @@ static inline size_t msg_data_left(struct msghdr *msg)
 #define	SCM_RIGHTS	0x01		/* rw: access rights (array of int) */
 #define SCM_CREDENTIALS 0x02		/* rw: struct ucred		*/
 #define SCM_SECURITY	0x03		/* rw: security label		*/
+#define SCM_PIDFD	0x04		/* ro: pidfd (int)		*/
 
 struct ucred {
 	__u32	pid;

include/net/scm.h

@@ -120,12 +120,44 @@ static inline bool scm_has_secdata(struct socket *sock)
 }
 #endif /* CONFIG_SECURITY_NETWORK */
 
+static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm)
+{
+	struct file *pidfd_file = NULL;
+	int pidfd;
+
+	/*
+	 * put_cmsg() doesn't return an error if CMSG is truncated,
+	 * that's why we need to opencode these checks here.
+	 */
+	if ((msg->msg_controllen <= sizeof(struct cmsghdr)) ||
+	    (msg->msg_controllen - sizeof(struct cmsghdr)) < sizeof(int)) {
+		msg->msg_flags |= MSG_CTRUNC;
+		return;
+	}
+
+	WARN_ON_ONCE(!scm->pid);
+	pidfd = pidfd_prepare(scm->pid, 0, &pidfd_file);
+
+	if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {
+		if (pidfd_file) {
+			put_unused_fd(pidfd);
+			fput(pidfd_file);
+		}
+
+		return;
+	}
+
+	if (pidfd_file)
+		fd_install(pidfd, pidfd_file);
+}
+
 static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
 				struct scm_cookie *scm, int flags)
 {
 	if (!msg->msg_control) {
-		if (test_bit(SOCK_PASSCRED, &sock->flags) || scm->fp ||
-		    scm_has_secdata(sock))
+		if (test_bit(SOCK_PASSCRED, &sock->flags) ||
+		    test_bit(SOCK_PASSPIDFD, &sock->flags) ||
+		    scm->fp || scm_has_secdata(sock))
 			msg->msg_flags |= MSG_CTRUNC;
 		scm_destroy(scm);
 		return;
@@ -141,6 +173,9 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
 		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(ucreds), &ucreds);
 	}
 
+	if (test_bit(SOCK_PASSPIDFD, &sock->flags))
+		scm_pidfd_recv(msg, scm);
+
 	scm_destroy_cred(scm);
 
 	scm_passec(sock, msg, scm);

include/uapi/asm-generic/socket.h

@@ -132,6 +132,9 @@
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+#define SO_PEERPIDFD		77
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))

net/core/sock.c

@@ -1246,6 +1246,13 @@ int sk_setsockopt(struct sock *sk, int level, int optname,
 			clear_bit(SOCK_PASSCRED, &sock->flags);
 		break;
 
+	case SO_PASSPIDFD:
+		if (valbool)
+			set_bit(SOCK_PASSPIDFD, &sock->flags);
+		else
+			clear_bit(SOCK_PASSPIDFD, &sock->flags);
+		break;
+
 	case SO_TIMESTAMP_OLD:
 	case SO_TIMESTAMP_NEW:
 	case SO_TIMESTAMPNS_OLD:
@@ -1732,6 +1739,10 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
 		v.val = !!test_bit(SOCK_PASSCRED, &sock->flags);
 		break;
 
+	case SO_PASSPIDFD:
+		v.val = !!test_bit(SOCK_PASSPIDFD, &sock->flags);
+		break;
+
 	case SO_PEERCRED:
 	{
 		struct ucred peercred;
@@ -1747,6 +1758,39 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
 		goto lenout;
 	}
 
+	case SO_PEERPIDFD:
+	{
+		struct pid *peer_pid;
+		struct file *pidfd_file = NULL;
+		int pidfd;
+
+		if (len > sizeof(pidfd))
+			len = sizeof(pidfd);
+
+		spin_lock(&sk->sk_peer_lock);
+		peer_pid = get_pid(sk->sk_peer_pid);
+		spin_unlock(&sk->sk_peer_lock);
+
+		if (!peer_pid)
+			return -ESRCH;
+
+		pidfd = pidfd_prepare(peer_pid, 0, &pidfd_file);
+		put_pid(peer_pid);
+		if (pidfd < 0)
+			return pidfd;
+
+		if (copy_to_sockptr(optval, &pidfd, len) ||
+		    copy_to_sockptr(optlen, &len, sizeof(int))) {
+			put_unused_fd(pidfd);
+			fput(pidfd_file);
+
+			return -EFAULT;
+		}
+
+		fd_install(pidfd, pidfd_file);
+		return 0;
+	}
+
 	case SO_PEERGROUPS:
 	{
 		const struct cred *cred;

net/mptcp/sockopt.c

@@ -355,6 +355,7 @@ static int mptcp_setsockopt_sol_socket(struct mptcp_sock *msk, int optname,
 	case SO_BROADCAST:
 	case SO_BSDCOMPAT:
 	case SO_PASSCRED:
+	case SO_PASSPIDFD:
 	case SO_PASSSEC:
 	case SO_RXQ_OVFL:
 	case SO_WIFI_STATUS:

net/unix/Kconfig

@@ -4,7 +4,7 @@
 #
 
 config UNIX
-	tristate "Unix domain sockets"
+	bool "Unix domain sockets"
 	help
 	  If you say Y here, you will include support for Unix domain sockets;
 	  sockets are the standard Unix mechanism for establishing and
@@ -14,10 +14,6 @@ config UNIX
 	  an embedded system or something similar, you therefore definitely
 	  want to say Y here.
 
-	  To compile this driver as a module, choose M here: the module will be
-	  called unix.  Note that several important services won't work
-	  correctly if you say M here and then neglect to load the module.
-
 	  Say Y unless you know what you are doing.
 
 config UNIX_SCM

net/unix/af_unix.c

@@ -921,11 +921,26 @@ static void unix_unhash(struct sock *sk)
 	 */
 }
 
+static bool unix_bpf_bypass_getsockopt(int level, int optname)
+{
+	if (level == SOL_SOCKET) {
+		switch (optname) {
+		case SO_PEERPIDFD:
+			return true;
+		default:
+			return false;
+		}
+	}
+
+	return false;
+}
+
 struct proto unix_dgram_proto = {
 	.name			= "UNIX",
 	.owner			= THIS_MODULE,
 	.obj_size		= sizeof(struct unix_sock),
 	.close			= unix_close,
+	.bpf_bypass_getsockopt	= unix_bpf_bypass_getsockopt,
 #ifdef CONFIG_BPF_SYSCALL
 	.psock_update_sk_prot	= unix_dgram_bpf_update_proto,
 #endif
@@ -937,6 +952,7 @@ struct proto unix_stream_proto = {
 	.obj_size		= sizeof(struct unix_sock),
 	.close			= unix_close,
 	.unhash			= unix_unhash,
+	.bpf_bypass_getsockopt	= unix_bpf_bypass_getsockopt,
 #ifdef CONFIG_BPF_SYSCALL
 	.psock_update_sk_prot	= unix_stream_bpf_update_proto,
 #endif
@@ -1361,7 +1377,8 @@ static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
 		if (err)
 			goto out;
 
-		if (test_bit(SOCK_PASSCRED, &sock->flags) &&
+		if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
+		     test_bit(SOCK_PASSPIDFD, &sock->flags)) &&
 		    !unix_sk(sk)->addr) {
 			err = unix_autobind(sk);
 			if (err)
@@ -1469,7 +1486,8 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
 	if (err)
 		goto out;
 
-	if (test_bit(SOCK_PASSCRED, &sock->flags) && !u->addr) {
+	if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
+	     test_bit(SOCK_PASSPIDFD, &sock->flags)) && !u->addr) {
 		err = unix_autobind(sk);
 		if (err)
 			goto out;
@@ -1670,6 +1688,8 @@ static void unix_sock_inherit_flags(const struct socket *old,
 {
 	if (test_bit(SOCK_PASSCRED, &old->flags))
 		set_bit(SOCK_PASSCRED, &new->flags);
+	if (test_bit(SOCK_PASSPIDFD, &old->flags))
+		set_bit(SOCK_PASSPIDFD, &new->flags);
 	if (test_bit(SOCK_PASSSEC, &old->flags))
 		set_bit(SOCK_PASSSEC, &new->flags);
 }
@@ -1819,8 +1839,10 @@ static bool unix_passcred_enabled(const struct socket *sock,
 				  const struct sock *other)
 {
 	return test_bit(SOCK_PASSCRED, &sock->flags) ||
+	       test_bit(SOCK_PASSPIDFD, &sock->flags) ||
 	       !other->sk_socket ||
-	       test_bit(SOCK_PASSCRED, &other->sk_socket->flags);
+	       test_bit(SOCK_PASSCRED, &other->sk_socket->flags) ||
+	       test_bit(SOCK_PASSPIDFD, &other->sk_socket->flags);
 }
 
 /*
@@ -1904,7 +1926,8 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 			goto out;
 	}
 
-	if (test_bit(SOCK_PASSCRED, &sock->flags) && !u->addr) {
+	if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
+	     test_bit(SOCK_PASSPIDFD, &sock->flags)) && !u->addr) {
 		err = unix_autobind(sk);
 		if (err)
 			goto out;
@@ -2718,7 +2741,8 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 			/* Never glue messages from different writers */
 			if (!unix_skb_scm_eq(skb, &scm))
 				break;
-		} else if (test_bit(SOCK_PASSCRED, &sock->flags)) {
+		} else if (test_bit(SOCK_PASSCRED, &sock->flags) ||
+			   test_bit(SOCK_PASSPIDFD, &sock->flags)) {
 			/* Copy credentials */
 			scm_set_cred(&scm, UNIXCB(skb).pid, UNIXCB(skb).uid, UNIXCB(skb).gid);
 			unix_set_secdata(&scm, skb);

tools/include/uapi/asm-generic/socket.h

@@ -121,6 +121,9 @@
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+#define SO_PEERPIDFD		77
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))

tools/testing/selftests/net/.gitignore

@@ -29,6 +29,7 @@ reuseport_bpf_numa
 reuseport_dualstack
 rxtimestamp
 sctp_hello
+scm_pidfd
 sk_bind_sendto_listen
 sk_connect_zero_addr
 socket

tools/testing/selftests/net/af_unix/Makefile

@@ -1,3 +1,4 @@
-TEST_GEN_PROGS := diag_uid test_unix_oob unix_connect
+CFLAGS += $(KHDR_INCLUDES)
+TEST_GEN_PROGS := diag_uid test_unix_oob unix_connect scm_pidfd
 
 include ../../lib.mk

tools/testing/selftests/net/af_unix/scm_pidfd.c

@@ -0,0 +1,430 @@
+// SPDX-License-Identifier: GPL-2.0 OR MIT
+#define _GNU_SOURCE
+#include <error.h>
+#include <limits.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/socket.h>
+#include <linux/socket.h>
+#include <unistd.h>
+#include <string.h>
+#include <errno.h>
+#include <sys/un.h>
+#include <sys/signal.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+#include "../../kselftest_harness.h"
+
+#define clean_errno() (errno == 0 ? "None" : strerror(errno))
+#define log_err(MSG, ...)                                                   \
+	fprintf(stderr, "(%s:%d: errno: %s) " MSG "\n", __FILE__, __LINE__, \
+		clean_errno(), ##__VA_ARGS__)
+
+#ifndef SCM_PIDFD
+#define SCM_PIDFD 0x04
+#endif
+
+static void child_die()
+{
+	exit(1);
+}
+
+static int safe_int(const char *numstr, int *converted)
+{
+	char *err = NULL;
+	long sli;
+
+	errno = 0;
+	sli = strtol(numstr, &err, 0);
+	if (errno == ERANGE && (sli == LONG_MAX || sli == LONG_MIN))
+		return -ERANGE;
+
+	if (errno != 0 && sli == 0)
+		return -EINVAL;
+
+	if (err == numstr || *err != '\0')
+		return -EINVAL;
+
+	if (sli > INT_MAX || sli < INT_MIN)
+		return -ERANGE;
+
+	*converted = (int)sli;
+	return 0;
+}
+
+static int char_left_gc(const char *buffer, size_t len)
+{
+	size_t i;
+
+	for (i = 0; i < len; i++) {
+		if (buffer[i] == ' ' || buffer[i] == '\t')
+			continue;
+
+		return i;
+	}
+
+	return 0;
+}
+
+static int char_right_gc(const char *buffer, size_t len)
+{
+	int i;
+
+	for (i = len - 1; i >= 0; i--) {
+		if (buffer[i] == ' ' || buffer[i] == '\t' ||
+		    buffer[i] == '\n' || buffer[i] == '\0')
+			continue;
+
+		return i + 1;
+	}
+
+	return 0;
+}
+
+static char *trim_whitespace_in_place(char *buffer)
+{
+	buffer += char_left_gc(buffer, strlen(buffer));
+	buffer[char_right_gc(buffer, strlen(buffer))] = '\0';
+	return buffer;
+}
+
+/* borrowed (with all helpers) from pidfd/pidfd_open_test.c */
+static pid_t get_pid_from_fdinfo_file(int pidfd, const char *key, size_t keylen)
+{
+	int ret;
+	char path[512];
+	FILE *f;
+	size_t n = 0;
+	pid_t result = -1;
+	char *line = NULL;
+
+	snprintf(path, sizeof(path), "/proc/self/fdinfo/%d", pidfd);
+
+	f = fopen(path, "re");
+	if (!f)
+		return -1;
+
+	while (getline(&line, &n, f) != -1) {
+		char *numstr;
+
+		if (strncmp(line, key, keylen))
+			continue;
+
+		numstr = trim_whitespace_in_place(line + 4);
+		ret = safe_int(numstr, &result);
+		if (ret < 0)
+			goto out;
+
+		break;
+	}
+
+out:
+	free(line);
+	fclose(f);
+	return result;
+}
+
+static int cmsg_check(int fd)
+{
+	struct msghdr msg = { 0 };
+	struct cmsghdr *cmsg;
+	struct iovec iov;
+	struct ucred *ucred = NULL;
+	int data = 0;
+	char control[CMSG_SPACE(sizeof(struct ucred)) +
+		     CMSG_SPACE(sizeof(int))] = { 0 };
+	int *pidfd = NULL;
+	pid_t parent_pid;
+	int err;
+
+	iov.iov_base = &data;
+	iov.iov_len = sizeof(data);
+
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = control;
+	msg.msg_controllen = sizeof(control);
+
+	err = recvmsg(fd, &msg, 0);
+	if (err < 0) {
+		log_err("recvmsg");
+		return 1;
+	}
+
+	if (msg.msg_flags & (MSG_TRUNC | MSG_CTRUNC)) {
+		log_err("recvmsg: truncated");
+		return 1;
+	}
+
+	for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
+	     cmsg = CMSG_NXTHDR(&msg, cmsg)) {
+		if (cmsg->cmsg_level == SOL_SOCKET &&
+		    cmsg->cmsg_type == SCM_PIDFD) {
+			if (cmsg->cmsg_len < sizeof(*pidfd)) {
+				log_err("CMSG parse: SCM_PIDFD wrong len");
+				return 1;
+			}
+
+			pidfd = (void *)CMSG_DATA(cmsg);
+		}
+
+		if (cmsg->cmsg_level == SOL_SOCKET &&
+		    cmsg->cmsg_type == SCM_CREDENTIALS) {
+			if (cmsg->cmsg_len < sizeof(*ucred)) {
+				log_err("CMSG parse: SCM_CREDENTIALS wrong len");
+				return 1;
+			}
+
+			ucred = (void *)CMSG_DATA(cmsg);
+		}
+	}
+
+	/* send(pfd, "x", sizeof(char), 0) */
+	if (data != 'x') {
+		log_err("recvmsg: data corruption");
+		return 1;
+	}
+
+	if (!pidfd) {
+		log_err("CMSG parse: SCM_PIDFD not found");
+		return 1;
+	}
+
+	if (!ucred) {
+		log_err("CMSG parse: SCM_CREDENTIALS not found");
+		return 1;
+	}
+
+	/* pidfd from SCM_PIDFD should point to the parent process PID */
+	parent_pid =
+		get_pid_from_fdinfo_file(*pidfd, "Pid:", sizeof("Pid:") - 1);
+	if (parent_pid != getppid()) {
+		log_err("wrong SCM_PIDFD %d != %d", parent_pid, getppid());
+		return 1;
+	}
+
+	return 0;
+}
+
+struct sock_addr {
+	char sock_name[32];
+	struct sockaddr_un listen_addr;
+	socklen_t addrlen;
+};
+
+FIXTURE(scm_pidfd)
+{
+	int server;
+	pid_t client_pid;
+	int startup_pipe[2];
+	struct sock_addr server_addr;
+	struct sock_addr *client_addr;
+};
+
+FIXTURE_VARIANT(scm_pidfd)
+{
+	int type;
+	bool abstract;
+};
+
+FIXTURE_VARIANT_ADD(scm_pidfd, stream_pathname)
+{
+	.type = SOCK_STREAM,
+	.abstract = 0,
+};
+
+FIXTURE_VARIANT_ADD(scm_pidfd, stream_abstract)
+{
+	.type = SOCK_STREAM,
+	.abstract = 1,
+};
+
+FIXTURE_VARIANT_ADD(scm_pidfd, dgram_pathname)
+{
+	.type = SOCK_DGRAM,
+	.abstract = 0,
+};
+
+FIXTURE_VARIANT_ADD(scm_pidfd, dgram_abstract)
+{
+	.type = SOCK_DGRAM,
+	.abstract = 1,
+};
+
+FIXTURE_SETUP(scm_pidfd)
+{
+	self->client_addr = mmap(NULL, sizeof(*self->client_addr), PROT_READ | PROT_WRITE,
+				 MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+	ASSERT_NE(MAP_FAILED, self->client_addr);
+}
+
+FIXTURE_TEARDOWN(scm_pidfd)
+{
+	close(self->server);
+
+	kill(self->client_pid, SIGKILL);
+	waitpid(self->client_pid, NULL, 0);
+
+	if (!variant->abstract) {
+		unlink(self->server_addr.sock_name);
+		unlink(self->client_addr->sock_name);
+	}
+}
+
+static void fill_sockaddr(struct sock_addr *addr, bool abstract)
+{
+	char *sun_path_buf = (char *)&addr->listen_addr.sun_path;
+
+	addr->listen_addr.sun_family = AF_UNIX;
+	addr->addrlen = offsetof(struct sockaddr_un, sun_path);
+	snprintf(addr->sock_name, sizeof(addr->sock_name), "scm_pidfd_%d", getpid());
+	addr->addrlen += strlen(addr->sock_name);
+	if (abstract) {
+		*sun_path_buf = '\0';
+		addr->addrlen++;
+		sun_path_buf++;
+	} else {
+		unlink(addr->sock_name);
+	}
+	memcpy(sun_path_buf, addr->sock_name, strlen(addr->sock_name));
+}
+
+static void client(FIXTURE_DATA(scm_pidfd) *self,
+		   const FIXTURE_VARIANT(scm_pidfd) *variant)
+{
+	int err;
+	int cfd;
+	socklen_t len;
+	struct ucred peer_cred;
+	int peer_pidfd;
+	pid_t peer_pid;
+	int on = 0;
+
+	cfd = socket(AF_UNIX, variant->type, 0);
+	if (cfd < 0) {
+		log_err("socket");
+		child_die();
+	}
+
+	if (variant->type == SOCK_DGRAM) {
+		fill_sockaddr(self->client_addr, variant->abstract);
+
+		if (bind(cfd, (struct sockaddr *)&self->client_addr->listen_addr, self->client_addr->addrlen)) {
+			log_err("bind");
+			child_die();
+		}
+	}
+
+	if (connect(cfd, (struct sockaddr *)&self->server_addr.listen_addr,
+		    self->server_addr.addrlen) != 0) {
+		log_err("connect");
+		child_die();
+	}
+
+	on = 1;
+	if (setsockopt(cfd, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on))) {
+		log_err("Failed to set SO_PASSCRED");
+		child_die();
+	}
+
+	if (setsockopt(cfd, SOL_SOCKET, SO_PASSPIDFD, &on, sizeof(on))) {
+		log_err("Failed to set SO_PASSPIDFD");
+		child_die();
+	}
+
+	close(self->startup_pipe[1]);
+
+	if (cmsg_check(cfd)) {
+		log_err("cmsg_check failed");
+		child_die();
+	}
+
+	/* skip further for SOCK_DGRAM as it's not applicable */
+	if (variant->type == SOCK_DGRAM)
+		return;
+
+	len = sizeof(peer_cred);
+	if (getsockopt(cfd, SOL_SOCKET, SO_PEERCRED, &peer_cred, &len)) {
+		log_err("Failed to get SO_PEERCRED");
+		child_die();
+	}
+
+	len = sizeof(peer_pidfd);
+	if (getsockopt(cfd, SOL_SOCKET, SO_PEERPIDFD, &peer_pidfd, &len)) {
+		log_err("Failed to get SO_PEERPIDFD");
+		child_die();
+	}
+
+	/* pid from SO_PEERCRED should point to the parent process PID */
+	if (peer_cred.pid != getppid()) {
+		log_err("peer_cred.pid != getppid(): %d != %d", peer_cred.pid, getppid());
+		child_die();
+	}
+
+	peer_pid = get_pid_from_fdinfo_file(peer_pidfd,
+					    "Pid:", sizeof("Pid:") - 1);
+	if (peer_pid != peer_cred.pid) {
+		log_err("peer_pid != peer_cred.pid: %d != %d", peer_pid, peer_cred.pid);
+		child_die();
+	}
+}
+
+TEST_F(scm_pidfd, test)
+{
+	int err;
+	int pfd;
+	int child_status = 0;
+
+	self->server = socket(AF_UNIX, variant->type, 0);
+	ASSERT_NE(-1, self->server);
+
+	fill_sockaddr(&self->server_addr, variant->abstract);
+
+	err = bind(self->server, (struct sockaddr *)&self->server_addr.listen_addr, self->server_addr.addrlen);
+	ASSERT_EQ(0, err);
+
+	if (variant->type == SOCK_STREAM) {
+		err = listen(self->server, 1);
+		ASSERT_EQ(0, err);
+	}
+
+	err = pipe(self->startup_pipe);
+	ASSERT_NE(-1, err);
+
+	self->client_pid = fork();
+	ASSERT_NE(-1, self->client_pid);
+	if (self->client_pid == 0) {
+		close(self->server);
+		close(self->startup_pipe[0]);
+		client(self, variant);
+		exit(0);
+	}
+	close(self->startup_pipe[1]);
+
+	if (variant->type == SOCK_STREAM) {
+		pfd = accept(self->server, NULL, NULL);
+		ASSERT_NE(-1, pfd);
+	} else {
+		pfd = self->server;
+	}
+
+	/* wait until the child arrives at checkpoint */
+	read(self->startup_pipe[0], &err, sizeof(int));
+	close(self->startup_pipe[0]);
+
+	if (variant->type == SOCK_DGRAM) {
+		err = sendto(pfd, "x", sizeof(char), 0, (struct sockaddr *)&self->client_addr->listen_addr, self->client_addr->addrlen);
+		ASSERT_NE(-1, err);
+	} else {
+		err = send(pfd, "x", sizeof(char), 0);
+		ASSERT_NE(-1, err);
+	}
+
+	close(pfd);
+	waitpid(self->client_pid, &child_status, 0);
+	ASSERT_EQ(0, WIFEXITED(child_status) ? WEXITSTATUS(child_status) : 1);
+}
+
+TEST_HARNESS_MAIN