[PATCH net-next v2 0/3] Add SCM_PIDFD and SO_PEERPIDFD

[PATCH net-next v2 0/3] Add SCM_PIDFD and SO_PEERPIDFD

[Alexander Mikhalitsyn]

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.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>

Alexander Mikhalitsyn (3):
  scm: add SO_PASSPIDFD and SCM_PIDFD
  net: core: add getsockopt SO_PEERPIDFD
  selftests: net: add SCM_PIDFD / SO_PEERPIDFD test

 arch/alpha/include/uapi/asm/socket.h          |   3 +
 arch/mips/include/uapi/asm/socket.h           |   3 +
 arch/parisc/include/uapi/asm/socket.h         |   3 +
 arch/sparc/include/uapi/asm/socket.h          |   3 +
 include/linux/net.h                           |   1 +
 include/linux/socket.h                        |   1 +
 include/net/scm.h                             |  14 +-
 include/uapi/asm-generic/socket.h             |   3 +
 net/core/sock.c                               |  32 ++
 net/mptcp/sockopt.c                           |   1 +
 net/unix/af_unix.c                            |  18 +-
 tools/include/uapi/asm-generic/socket.h       |   3 +
 tools/testing/selftests/net/.gitignore        |   1 +
 tools/testing/selftests/net/af_unix/Makefile  |   3 +-
 .../testing/selftests/net/af_unix/scm_pidfd.c | 336 ++++++++++++++++++
 15 files changed, 417 insertions(+), 8 deletions(-)
 create mode 100644 tools/testing/selftests/net/af_unix/scm_pidfd.c

-- 
2.34.1

[PATCH net-next v2 1/3] scm: add SO_PASSPIDFD and SCM_PIDFD

[Alexander Mikhalitsyn]

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.

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.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
v2:
	According to review comments from Kuniyuki Iwashima and Christian Brauner:
	- use pidfd_create(..) retval as a result
	- whitespace change
---
 arch/alpha/include/uapi/asm/socket.h    |  2 ++
 arch/mips/include/uapi/asm/socket.h     |  2 ++
 arch/parisc/include/uapi/asm/socket.h   |  2 ++
 arch/sparc/include/uapi/asm/socket.h    |  2 ++
 include/linux/net.h                     |  1 +
 include/linux/socket.h                  |  1 +
 include/net/scm.h                       | 14 ++++++++++++--
 include/uapi/asm-generic/socket.h       |  2 ++
 net/core/sock.c                         | 11 +++++++++++
 net/mptcp/sockopt.c                     |  1 +
 net/unix/af_unix.c                      | 18 +++++++++++++-----
 tools/include/uapi/asm-generic/socket.h |  2 ++
 12 files changed, 51 insertions(+), 7 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
index 739891b94136..ff310613ae64 100644
--- a/arch/alpha/include/uapi/asm/socket.h
+++ b/arch/alpha/include/uapi/asm/socket.h
@@ -137,6 +137,8 @@
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64
diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
index 18f3d95ecfec..762dcb80e4ec 100644
--- a/arch/mips/include/uapi/asm/socket.h
+++ b/arch/mips/include/uapi/asm/socket.h
@@ -148,6 +148,8 @@
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64
diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
index f486d3dfb6bb..df16a3e16d64 100644
--- a/arch/parisc/include/uapi/asm/socket.h
+++ b/arch/parisc/include/uapi/asm/socket.h
@@ -129,6 +129,8 @@
 
 #define SO_RCVMARK		0x4049
 
+#define SO_PASSPIDFD		0x404A
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64
diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
index 2fda57a3ea86..6e2847804fea 100644
--- a/arch/sparc/include/uapi/asm/socket.h
+++ b/arch/sparc/include/uapi/asm/socket.h
@@ -130,6 +130,8 @@
 
 #define SO_RCVMARK               0x0054
 
+#define SO_PASSPIDFD             0x0055
+
 #if !defined(__KERNEL__)
 
 
diff --git a/include/linux/net.h b/include/linux/net.h
index b73ad8e3c212..c234dfbe7a30 100644
--- a/include/linux/net.h
+++ b/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
 /**
diff --git a/include/linux/socket.h b/include/linux/socket.h
index 13c3a237b9c9..6bf90f251910 100644
--- a/include/linux/socket.h
+++ b/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;
diff --git a/include/net/scm.h b/include/net/scm.h
index 585adc1346bd..0c717ae9c8db 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h
@@ -124,8 +124,9 @@ 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 +142,15 @@ 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)) {
+		int pidfd;
+
+		WARN_ON_ONCE(!scm->pid);
+		pidfd = pidfd_create(scm->pid, 0);
+
+		put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd);
+	}
+
 	scm_destroy_cred(scm);
 
 	scm_passec(sock, msg, scm);
diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
index 638230899e98..b76169fdb80b 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h
@@ -132,6 +132,8 @@
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
diff --git a/net/core/sock.c b/net/core/sock.c
index c25888795390..3f974246ba3e 100644
--- a/net/core/sock.c
+++ b/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:
@@ -1737,6 +1744,10 @@ 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;
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 8a9656248b0f..bd80e707d0b3 100644
--- a/net/mptcp/sockopt.c
+++ b/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:
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 0b0f18ecce44..b0ac768752fa 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1361,7 +1361,8 @@ 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 +1470,8 @@ 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 +1672,8 @@ 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 +1823,10 @@ 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);
 }
 
 /*
@@ -1922,7 +1928,8 @@ 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;
@@ -2824,7 +2831,8 @@ 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);
diff --git a/tools/include/uapi/asm-generic/socket.h b/tools/include/uapi/asm-generic/socket.h
index 8756df13be50..fbbc4bf53ee3 100644
--- a/tools/include/uapi/asm-generic/socket.h
+++ b/tools/include/uapi/asm-generic/socket.h
@@ -121,6 +121,8 @@
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
-- 
2.34.1

[PATCH net-next v2 2/3] net: core: add getsockopt SO_PEERPIDFD

[Alexander Mikhalitsyn]

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.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
v2:
	According to review comments from Kuniyuki Iwashima and Christian Brauner:
	- use pidfd_create(..) retval as a result
	- whitespace change
---
 arch/alpha/include/uapi/asm/socket.h    |  1 +
 arch/mips/include/uapi/asm/socket.h     |  1 +
 arch/parisc/include/uapi/asm/socket.h   |  1 +
 arch/sparc/include/uapi/asm/socket.h    |  1 +
 include/uapi/asm-generic/socket.h       |  1 +
 net/core/sock.c                         | 21 +++++++++++++++++++++
 tools/include/uapi/asm-generic/socket.h |  1 +
 7 files changed, 27 insertions(+)

diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
index ff310613ae64..e94f621903fe 100644
--- a/arch/alpha/include/uapi/asm/socket.h
+++ b/arch/alpha/include/uapi/asm/socket.h
@@ -138,6 +138,7 @@
 #define SO_RCVMARK		75
 
 #define SO_PASSPIDFD		76
+#define SO_PEERPIDFD		77
 
 #if !defined(__KERNEL__)
 
diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
index 762dcb80e4ec..60ebaed28a4c 100644
--- a/arch/mips/include/uapi/asm/socket.h
+++ b/arch/mips/include/uapi/asm/socket.h
@@ -149,6 +149,7 @@
 #define SO_RCVMARK		75
 
 #define SO_PASSPIDFD		76
+#define SO_PEERPIDFD		77
 
 #if !defined(__KERNEL__)
 
diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
index df16a3e16d64..be264c2b1a11 100644
--- a/arch/parisc/include/uapi/asm/socket.h
+++ b/arch/parisc/include/uapi/asm/socket.h
@@ -130,6 +130,7 @@
 #define SO_RCVMARK		0x4049
 
 #define SO_PASSPIDFD		0x404A
+#define SO_PEERPIDFD		0x404B
 
 #if !defined(__KERNEL__)
 
diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
index 6e2847804fea..682da3714686 100644
--- a/arch/sparc/include/uapi/asm/socket.h
+++ b/arch/sparc/include/uapi/asm/socket.h
@@ -131,6 +131,7 @@
 #define SO_RCVMARK               0x0054
 
 #define SO_PASSPIDFD             0x0055
+#define SO_PEERPIDFD             0x0056
 
 #if !defined(__KERNEL__)
 
diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
index b76169fdb80b..8ce8a39a1e5f 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h
@@ -133,6 +133,7 @@
 #define SO_RCVMARK		75
 
 #define SO_PASSPIDFD		76
+#define SO_PEERPIDFD		77
 
 #if !defined(__KERNEL__)
 
diff --git a/net/core/sock.c b/net/core/sock.c
index 3f974246ba3e..85c269ca9d8a 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1763,6 +1763,27 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
 		goto lenout;
 	}
 
+	case SO_PEERPIDFD:
+	{
+		struct pid *peer_pid;
+		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);
+
+		pidfd = pidfd_create(peer_pid, 0);
+
+		put_pid(peer_pid);
+
+		if (copy_to_sockptr(optval, &pidfd, len))
+			return -EFAULT;
+		goto lenout;
+	}
+
 	case SO_PEERGROUPS:
 	{
 		const struct cred *cred;
diff --git a/tools/include/uapi/asm-generic/socket.h b/tools/include/uapi/asm-generic/socket.h
index fbbc4bf53ee3..54d9c8bf7c55 100644
--- a/tools/include/uapi/asm-generic/socket.h
+++ b/tools/include/uapi/asm-generic/socket.h
@@ -122,6 +122,7 @@
 #define SO_RCVMARK		75
 
 #define SO_PASSPIDFD		76
+#define SO_PEERPIDFD		77
 
 #if !defined(__KERNEL__)
 
-- 
2.34.1

[PATCH net-next v2 3/3] selftests: net: add SCM_PIDFD / SO_PEERPIDFD test

[Alexander Mikhalitsyn]

Basic test to check consistency between:
- SCM_CREDENTIALS and SCM_PIDFD
- SO_PEERCRED and SO_PEERPIDFD

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 tools/testing/selftests/net/.gitignore        |   1 +
 tools/testing/selftests/net/af_unix/Makefile  |   3 +-
 .../testing/selftests/net/af_unix/scm_pidfd.c | 336 ++++++++++++++++++
 3 files changed, 339 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/net/af_unix/scm_pidfd.c

diff --git a/tools/testing/selftests/net/.gitignore b/tools/testing/selftests/net/.gitignore
index a6911cae368c..f2d23a1df596 100644
--- a/tools/testing/selftests/net/.gitignore
+++ b/tools/testing/selftests/net/.gitignore
@@ -25,6 +25,7 @@ reuseport_bpf_cpu
 reuseport_bpf_numa
 reuseport_dualstack
 rxtimestamp
+scm_pidfd
 sk_bind_sendto_listen
 sk_connect_zero_addr
 socket
diff --git a/tools/testing/selftests/net/af_unix/Makefile b/tools/testing/selftests/net/af_unix/Makefile
index 1e4b397cece6..221c387a7d7f 100644
--- a/tools/testing/selftests/net/af_unix/Makefile
+++ b/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
diff --git a/tools/testing/selftests/net/af_unix/scm_pidfd.c b/tools/testing/selftests/net/af_unix/scm_pidfd.c
new file mode 100644
index 000000000000..fa502510ee9e
--- /dev/null
+++ b/tools/testing/selftests/net/af_unix/scm_pidfd.c
@@ -0,0 +1,336 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <error.h>
+#include <limits.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>
+
+#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 pid_t client_pid;
+static char sock_name[32];
+
+static void die(int status)
+{
+	unlink(sock_name);
+	kill(client_pid, SIGTERM);
+	exit(status);
+}
+
+static void child_die()
+{
+	kill(getppid(), SIGTERM);
+	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;
+}
+
+void client(struct sockaddr_un *listen_addr)
+{
+	int cfd;
+	socklen_t len;
+	struct ucred peer_cred;
+	int peer_pidfd;
+	pid_t peer_pid;
+	int on = 0;
+
+	cfd = socket(AF_UNIX, SOCK_STREAM, 0);
+	if (cfd < 0) {
+		log_err("socket");
+		child_die();
+	}
+
+	if (connect(cfd, (struct sockaddr *)listen_addr,
+		    sizeof(*listen_addr)) != 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();
+	}
+
+	if (cmsg_check(cfd)) {
+		log_err("cmsg_check failed");
+		child_die();
+	}
+
+	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("Failed to get SO_PEERPIDFD");
+		child_die();
+	}
+
+	peer_pid = get_pid_from_fdinfo_file(peer_pidfd,
+					    "Pid:", sizeof("Pid:") - 1);
+	if (peer_pid != peer_cred.pid) {
+		log_err("Failed to get SO_PEERPIDFD");
+		child_die();
+	}
+}
+
+int main(int argc, char **argv)
+{
+	int lfd, pfd;
+	int child_status = 0;
+	struct sockaddr_un listen_addr;
+
+	lfd = socket(AF_UNIX, SOCK_STREAM, 0);
+	if (lfd < 0) {
+		perror("socket");
+		exit(1);
+	}
+
+	memset(&listen_addr, 0, sizeof(listen_addr));
+	listen_addr.sun_family = AF_UNIX;
+	sprintf(sock_name, "scm_pidfd_%d", getpid());
+	unlink(sock_name);
+	strcpy(listen_addr.sun_path, sock_name);
+
+	if ((bind(lfd, (struct sockaddr *)&listen_addr, sizeof(listen_addr))) !=
+	    0) {
+		perror("socket bind failed");
+		exit(1);
+	}
+
+	if (listen(lfd, 1) < 0) {
+		perror("listen");
+		exit(1);
+	}
+
+	client_pid = fork();
+	if (client_pid < 0) {
+		perror("fork");
+		exit(1);
+	}
+
+	if (client_pid == 0) {
+		client(&listen_addr);
+		exit(0);
+	}
+
+	pfd = accept(lfd, NULL, NULL);
+	if (pfd < 0) {
+		perror("accept");
+		die(1);
+	}
+
+	if (send(pfd, "x", sizeof(char), 0) < 0) {
+		perror("send");
+		die(1);
+	}
+
+	waitpid(client_pid, &child_status, 0);
+	die(WIFEXITED(child_status) ? WEXITSTATUS(child_status) : 1);
+	die(0);
+}
\ No newline at end of file
-- 
2.34.1

Re: [PATCH net-next v2 1/3] scm: add SO_PASSPIDFD and SCM_PIDFD

[Kuniyuki Iwashima]

From:   Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Date:   Tue, 21 Mar 2023 19:33:40 +0100
> 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.
> 
> 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.
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> Cc: Lennart Poettering <mzxreary@0pointer.de>
> Cc: linux-kernel@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Cc: linux-arch@vger.kernel.org
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Thanks,
Kuniyuki

Re: [PATCH net-next v2 2/3] net: core: add getsockopt SO_PEERPIDFD

[Kuniyuki Iwashima]

From:   Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Date:   Tue, 21 Mar 2023 19:33:41 +0100
> 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.
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> Cc: Lennart Poettering <mzxreary@0pointer.de>
> Cc: linux-kernel@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Cc: linux-arch@vger.kernel.org
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Thanks,
Kuniyuki

Re: [PATCH net-next v2 3/3] selftests: net: add SCM_PIDFD / SO_PEERPIDFD test

[Kuniyuki Iwashima]

From:   Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Date:   Tue, 21 Mar 2023 19:33:42 +0100
> Basic test to check consistency between:
> - SCM_CREDENTIALS and SCM_PIDFD
> - SO_PEERCRED and SO_PEERPIDFD
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Cc: linux-arch@vger.kernel.org
> Cc: linux-kselftest@vger.kernel.org
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> ---
>  tools/testing/selftests/net/.gitignore        |   1 +
>  tools/testing/selftests/net/af_unix/Makefile  |   3 +-
>  .../testing/selftests/net/af_unix/scm_pidfd.c | 336 ++++++++++++++++++
>  3 files changed, 339 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/net/af_unix/scm_pidfd.c
> 
> diff --git a/tools/testing/selftests/net/.gitignore b/tools/testing/selftests/net/.gitignore
> index a6911cae368c..f2d23a1df596 100644
> --- a/tools/testing/selftests/net/.gitignore
> +++ b/tools/testing/selftests/net/.gitignore
> @@ -25,6 +25,7 @@ reuseport_bpf_cpu
>  reuseport_bpf_numa
>  reuseport_dualstack
>  rxtimestamp
> +scm_pidfd
>  sk_bind_sendto_listen
>  sk_connect_zero_addr
>  socket
> diff --git a/tools/testing/selftests/net/af_unix/Makefile b/tools/testing/selftests/net/af_unix/Makefile
> index 1e4b397cece6..221c387a7d7f 100644
> --- a/tools/testing/selftests/net/af_unix/Makefile
> +++ b/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
> diff --git a/tools/testing/selftests/net/af_unix/scm_pidfd.c b/tools/testing/selftests/net/af_unix/scm_pidfd.c
> new file mode 100644
> index 000000000000..fa502510ee9e
> --- /dev/null
> +++ b/tools/testing/selftests/net/af_unix/scm_pidfd.c
> @@ -0,0 +1,336 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#define _GNU_SOURCE
> +#include <error.h>
> +#include <limits.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"

Could you rewrite with kselftest ?
https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html

Also, it would be better to have all combinations as FIXTURE_VARIANT()
covered by unix_passcred_enabled() like

  (self, peer) = (0, 0), (SO_PASSPIDFD, 0), (0, SO_PASSPIDFD),
                 (SO_PASSPIDFD, SO_PASSPIDFD), ...
                 (SO_PASSPIDFD, SO_PEERCRED), ...

Thanks,
Kuniyuki


> +
> +#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 pid_t client_pid;
> +static char sock_name[32];
> +
> +static void die(int status)
> +{
> +	unlink(sock_name);
> +	kill(client_pid, SIGTERM);
> +	exit(status);
> +}
> +
> +static void child_die()
> +{
> +	kill(getppid(), SIGTERM);
> +	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;
> +}
> +
> +void client(struct sockaddr_un *listen_addr)
> +{
> +	int cfd;
> +	socklen_t len;
> +	struct ucred peer_cred;
> +	int peer_pidfd;
> +	pid_t peer_pid;
> +	int on = 0;
> +
> +	cfd = socket(AF_UNIX, SOCK_STREAM, 0);
> +	if (cfd < 0) {
> +		log_err("socket");
> +		child_die();
> +	}
> +
> +	if (connect(cfd, (struct sockaddr *)listen_addr,
> +		    sizeof(*listen_addr)) != 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();
> +	}
> +
> +	if (cmsg_check(cfd)) {
> +		log_err("cmsg_check failed");
> +		child_die();
> +	}
> +
> +	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("Failed to get SO_PEERPIDFD");
> +		child_die();
> +	}
> +
> +	peer_pid = get_pid_from_fdinfo_file(peer_pidfd,
> +					    "Pid:", sizeof("Pid:") - 1);
> +	if (peer_pid != peer_cred.pid) {
> +		log_err("Failed to get SO_PEERPIDFD");
> +		child_die();
> +	}
> +}
> +
> +int main(int argc, char **argv)
> +{
> +	int lfd, pfd;
> +	int child_status = 0;
> +	struct sockaddr_un listen_addr;
> +
> +	lfd = socket(AF_UNIX, SOCK_STREAM, 0);
> +	if (lfd < 0) {
> +		perror("socket");
> +		exit(1);
> +	}
> +
> +	memset(&listen_addr, 0, sizeof(listen_addr));
> +	listen_addr.sun_family = AF_UNIX;
> +	sprintf(sock_name, "scm_pidfd_%d", getpid());
> +	unlink(sock_name);
> +	strcpy(listen_addr.sun_path, sock_name);
> +
> +	if ((bind(lfd, (struct sockaddr *)&listen_addr, sizeof(listen_addr))) !=
> +	    0) {
> +		perror("socket bind failed");
> +		exit(1);
> +	}
> +
> +	if (listen(lfd, 1) < 0) {
> +		perror("listen");
> +		exit(1);
> +	}
> +
> +	client_pid = fork();
> +	if (client_pid < 0) {
> +		perror("fork");
> +		exit(1);
> +	}
> +
> +	if (client_pid == 0) {
> +		client(&listen_addr);
> +		exit(0);
> +	}
> +
> +	pfd = accept(lfd, NULL, NULL);
> +	if (pfd < 0) {
> +		perror("accept");
> +		die(1);
> +	}
> +
> +	if (send(pfd, "x", sizeof(char), 0) < 0) {
> +		perror("send");
> +		die(1);
> +	}
> +
> +	waitpid(client_pid, &child_status, 0);
> +	die(WIFEXITED(child_status) ? WEXITSTATUS(child_status) : 1);
> +	die(0);
> +}
> \ No newline at end of file
> -- 
> 2.34.1

Re: [PATCH net-next v2 1/3] scm: add SO_PASSPIDFD and SCM_PIDFD

[kernel test robot]

Hi Alexander,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on net-next/main]

url:    https://github.com/intel-lab-lkp/linux/commits/Alexander-Mikhalitsyn/scm-add-SO_PASSPIDFD-and-SCM_PIDFD/20230322-024808
patch link:    https://lore.kernel.org/r/20230321183342.617114-2-aleksandr.mikhalitsyn%40canonical.com
patch subject: [PATCH net-next v2 1/3] scm: add SO_PASSPIDFD and SCM_PIDFD
config: arm-allmodconfig (https://download.01.org/0day-ci/archive/20230322/202303222101.avwNiFWQ-lkp@intel.com/config)
compiler: arm-linux-gnueabi-gcc (GCC) 12.1.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/intel-lab-lkp/linux/commit/491b69039f4479e1e0fb3af635c96989cdd23734
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Alexander-Mikhalitsyn/scm-add-SO_PASSPIDFD-and-SCM_PIDFD/20230322-024808
        git checkout 491b69039f4479e1e0fb3af635c96989cdd23734
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross W=1 O=build_dir ARCH=arm olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross W=1 O=build_dir ARCH=arm SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
| Link: https://lore.kernel.org/oe-kbuild-all/202303222101.avwNiFWQ-lkp@intel.com/

All errors (new ones prefixed by >>, old ones prefixed by <<):

>> ERROR: modpost: "pidfd_create" [net/unix/unix.ko] undefined!

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests

Re: [PATCH net-next v2 0/3] Add SCM_PIDFD and SO_PEERPIDFD

[Christian Brauner]

On Tue, Mar 21, 2023 at 07:33:39PM +0100, Alexander Mikhalitsyn wrote:
> 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.
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> Cc: Lennart Poettering <mzxreary@0pointer.de>
> 
> Alexander Mikhalitsyn (3):
>   scm: add SO_PASSPIDFD and SCM_PIDFD
>   net: core: add getsockopt SO_PEERPIDFD
>   selftests: net: add SCM_PIDFD / SO_PEERPIDFD test
> 
>  arch/alpha/include/uapi/asm/socket.h          |   3 +
>  arch/mips/include/uapi/asm/socket.h           |   3 +
>  arch/parisc/include/uapi/asm/socket.h         |   3 +
>  arch/sparc/include/uapi/asm/socket.h          |   3 +
>  include/linux/net.h                           |   1 +
>  include/linux/socket.h                        |   1 +
>  include/net/scm.h                             |  14 +-
>  include/uapi/asm-generic/socket.h             |   3 +
>  net/core/sock.c                               |  32 ++
>  net/mptcp/sockopt.c                           |   1 +
>  net/unix/af_unix.c                            |  18 +-
>  tools/include/uapi/asm-generic/socket.h       |   3 +
>  tools/testing/selftests/net/.gitignore        |   1 +
>  tools/testing/selftests/net/af_unix/Makefile  |   3 +-
>  .../testing/selftests/net/af_unix/scm_pidfd.c | 336 ++++++++++++++++++
>  15 files changed, 417 insertions(+), 8 deletions(-)
>  create mode 100644 tools/testing/selftests/net/af_unix/scm_pidfd.c

What's the commit for this work? Because this seems to fail to apply
cleanly on anything from v6.3-rc1 until v6.3-rc3.

Re: [PATCH net-next v2 0/3] Add SCM_PIDFD and SO_PEERPIDFD

[Aleksandr Mikhalitsyn]

On Wed, Mar 22, 2023 at 3:13 PM Christian Brauner <brauner@kernel.org> wrote:
>
> On Tue, Mar 21, 2023 at 07:33:39PM +0100, Alexander Mikhalitsyn wrote:
> > 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.
> >
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Paolo Abeni <pabeni@redhat.com>
> > Cc: Leon Romanovsky <leon@kernel.org>
> > Cc: David Ahern <dsahern@kernel.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> > Cc: Lennart Poettering <mzxreary@0pointer.de>
> >
> > Alexander Mikhalitsyn (3):
> >   scm: add SO_PASSPIDFD and SCM_PIDFD
> >   net: core: add getsockopt SO_PEERPIDFD
> >   selftests: net: add SCM_PIDFD / SO_PEERPIDFD test
> >
> >  arch/alpha/include/uapi/asm/socket.h          |   3 +
> >  arch/mips/include/uapi/asm/socket.h           |   3 +
> >  arch/parisc/include/uapi/asm/socket.h         |   3 +
> >  arch/sparc/include/uapi/asm/socket.h          |   3 +
> >  include/linux/net.h                           |   1 +
> >  include/linux/socket.h                        |   1 +
> >  include/net/scm.h                             |  14 +-
> >  include/uapi/asm-generic/socket.h             |   3 +
> >  net/core/sock.c                               |  32 ++
> >  net/mptcp/sockopt.c                           |   1 +
> >  net/unix/af_unix.c                            |  18 +-
> >  tools/include/uapi/asm-generic/socket.h       |   3 +
> >  tools/testing/selftests/net/.gitignore        |   1 +
> >  tools/testing/selftests/net/af_unix/Makefile  |   3 +-
> >  .../testing/selftests/net/af_unix/scm_pidfd.c | 336 ++++++++++++++++++
> >  15 files changed, 417 insertions(+), 8 deletions(-)
> >  create mode 100644 tools/testing/selftests/net/af_unix/scm_pidfd.c
>
> What's the commit for this work? Because this seems to fail to apply
> cleanly on anything from v6.3-rc1 until v6.3-rc3.

It's based on net-next https://git.kernel.org/netdev/net-next/c/a02d83f9947d

Kind regards,
Alex

>

Re: [PATCH net-next v2 2/3] net: core: add getsockopt SO_PEERPIDFD

[Christian Brauner]

On Tue, Mar 21, 2023 at 07:33:41PM +0100, Alexander Mikhalitsyn wrote:
> 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.
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> Cc: Lennart Poettering <mzxreary@0pointer.de>
> Cc: linux-kernel@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Cc: linux-arch@vger.kernel.org
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> ---
> v2:
> 	According to review comments from Kuniyuki Iwashima and Christian Brauner:
> 	- use pidfd_create(..) retval as a result
> 	- whitespace change
> ---
>  arch/alpha/include/uapi/asm/socket.h    |  1 +
>  arch/mips/include/uapi/asm/socket.h     |  1 +
>  arch/parisc/include/uapi/asm/socket.h   |  1 +
>  arch/sparc/include/uapi/asm/socket.h    |  1 +
>  include/uapi/asm-generic/socket.h       |  1 +
>  net/core/sock.c                         | 21 +++++++++++++++++++++
>  tools/include/uapi/asm-generic/socket.h |  1 +
>  7 files changed, 27 insertions(+)
> 
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index ff310613ae64..e94f621903fe 100644
> --- a/arch/alpha/include/uapi/asm/socket.h
> +++ b/arch/alpha/include/uapi/asm/socket.h
> @@ -138,6 +138,7 @@
>  #define SO_RCVMARK		75
>  
>  #define SO_PASSPIDFD		76
> +#define SO_PEERPIDFD		77
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> index 762dcb80e4ec..60ebaed28a4c 100644
> --- a/arch/mips/include/uapi/asm/socket.h
> +++ b/arch/mips/include/uapi/asm/socket.h
> @@ -149,6 +149,7 @@
>  #define SO_RCVMARK		75
>  
>  #define SO_PASSPIDFD		76
> +#define SO_PEERPIDFD		77
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> index df16a3e16d64..be264c2b1a11 100644
> --- a/arch/parisc/include/uapi/asm/socket.h
> +++ b/arch/parisc/include/uapi/asm/socket.h
> @@ -130,6 +130,7 @@
>  #define SO_RCVMARK		0x4049
>  
>  #define SO_PASSPIDFD		0x404A
> +#define SO_PEERPIDFD		0x404B
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> index 6e2847804fea..682da3714686 100644
> --- a/arch/sparc/include/uapi/asm/socket.h
> +++ b/arch/sparc/include/uapi/asm/socket.h
> @@ -131,6 +131,7 @@
>  #define SO_RCVMARK               0x0054
>  
>  #define SO_PASSPIDFD             0x0055
> +#define SO_PEERPIDFD             0x0056
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> index b76169fdb80b..8ce8a39a1e5f 100644
> --- a/include/uapi/asm-generic/socket.h
> +++ b/include/uapi/asm-generic/socket.h
> @@ -133,6 +133,7 @@
>  #define SO_RCVMARK		75
>  
>  #define SO_PASSPIDFD		76
> +#define SO_PEERPIDFD		77
>  
>  #if !defined(__KERNEL__)
>  
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 3f974246ba3e..85c269ca9d8a 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1763,6 +1763,27 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
>  		goto lenout;
>  	}
>  
> +	case SO_PEERPIDFD:
> +	{
> +		struct pid *peer_pid;
> +		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);
> +
> +		pidfd = pidfd_create(peer_pid, 0);
> +
> +		put_pid(peer_pid);
> +
> +		if (copy_to_sockptr(optval, &pidfd, len))
> +			return -EFAULT;

This leaks the pidfd. We could do:

	if (copy_to_sockptr(optval, &pidfd, len)) {
		close_fd(pidfd);
		return -EFAULT;
	}

but it's a nasty anti-pattern to install the fd in the caller's fdtable
and then close it again. So let's avoid it if we can. Since you can only
set one socket option per setsockopt() sycall we should be able to
reserve an fd and pidfd_file, do the stuff that might fail, and then
call fd_install. So that would roughly be:

	peer_pid = get_pid(sk->sk_peer_pid);
	pidfd_file = pidfd_file_create(peer_pid, 0, &pidfd);
	f (copy_to_sockptr(optval, &pidfd, len))
	       return -EFAULT;
	goto lenout:
	
	.
	.
	.

lenout:
	if (copy_to_sockptr(optlen, &len, sizeof(int)))
		return -EFAULT;

	// Made it safely, install pidfd now.
	fd_install(pidfd, pidfd_file)

(See below for the associated api I'm going to publish independent of
this as kernel/fork.c and fanotify both could use it.)

But now, let's look at net/socket.c there's another wrinkle. So let's say you
have successfully installed the pidfd then it seems you can still fail later:

        if (level == SOL_SOCKET)
                err = sock_getsockopt(sock, level, optname, optval, optlen);
        else if (unlikely(!sock->ops->getsockopt))
                err = -EOPNOTSUPP;
        else
                err = sock->ops->getsockopt(sock, level, optname, optval,
                                            optlen);

        if (!in_compat_syscall())
                err = BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock->sk, level, optname,
                                                     optval, optlen, max_optlen,
                                                     err);

out_put:
	fput_light(sock->file, fput_needed);
	return err;

If the bpf hook returns an error we've placed an fd into the caller's sockopt
buffer without their knowledge.

From 4fee16f0920308bee2531fd3b08484f607eb5830 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Wed, 22 Mar 2023 15:59:02 +0100
Subject: [PATCH 1/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] pid: add
 pidfd_file_create()

Reserve and fd and pidfile, do stuff that might fail, install fd when
point of no return.

[HERE BE DRAGONS - DRAFT - __UNTESTED__] pid: add pidfd_file_create()

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/pid.h |  1 +
 kernel/pid.c        | 45 +++++++++++++++++++++++++++++++++------------
 2 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 343abf22092e..c486dbc4d7b6 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -80,6 +80,7 @@ extern struct pid *pidfd_pid(const struct file *file);
 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
 struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
 int pidfd_create(struct pid *pid, unsigned int flags);
+struct file *pidfd_file_create(struct pid *pid, unsigned int flags, int *pidfd);
 
 static inline struct pid *get_pid(struct pid *pid)
 {
diff --git a/kernel/pid.c b/kernel/pid.c
index 3fbc5e46b721..8d0924f1dbf6 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -576,6 +576,32 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
 	return task;
 }
 
+struct file *pidfd_file_create(struct pid *pid, unsigned int flags, int *pidfd)
+{
+	int fd;
+	struct file *pidfile;
+
+	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
+		return ERR_PTR(-EINVAL);
+
+	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
+		return ERR_PTR(-EINVAL);
+
+	fd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
+	if (fd < 0)
+		return ERR_PTR(fd);
+
+	pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
+				     flags | O_RDWR | O_CLOEXEC);
+	if (IS_ERR(pidfile)) {
+		put_unused_fd(fd);
+		return pidfile;
+	}
+	get_pid(pid); /* held by pidfile now */
+	*pidfd = fd;
+	return pidfile;
+}
+
 /**
  * pidfd_create() - Create a new pid file descriptor.
  *
@@ -594,20 +620,15 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
  */
 int pidfd_create(struct pid *pid, unsigned int flags)
 {
-	int fd;
+	int pidfd;
+	struct file *pidfile;
 
-	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
-		return -EINVAL;
+	pidfile = pidfd_file_create(pid, flags, &pidfd);
+	if (IS_ERR(pidfile))
+		return PTR_ERR(pidfile);
 
-	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
-		return -EINVAL;
-
-	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
-			      flags | O_RDWR | O_CLOEXEC);
-	if (fd < 0)
-		put_pid(pid);
-
-	return fd;
+	fd_install(pidfd, pidfile);
+	return pidfd;
 }
 
 /**
-- 
2.34.1

From c336f1c6cc39faa5aef4fbedd3c4f8eca51d8436 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Wed, 22 Mar 2023 15:59:54 +0100
Subject: [PATCH 2/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] fork: use
 pidfd_file_create()

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 kernel/fork.c | 11 +----------
 1 file changed, 1 insertion(+), 10 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index f68954d05e89..c8dc78ee0a74 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2296,20 +2296,11 @@ static __latent_entropy struct task_struct *copy_process(
 	 * if the fd table isn't shared).
 	 */
 	if (clone_flags & CLONE_PIDFD) {
-		retval = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
-		if (retval < 0)
-			goto bad_fork_free_pid;
-
-		pidfd = retval;
-
-		pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
-					      O_RDWR | O_CLOEXEC);
+		pidfile = pidfd_file_create(pid, O_RDWR | O_CLOEXEC, &pidfd);
 		if (IS_ERR(pidfile)) {
-			put_unused_fd(pidfd);
 			retval = PTR_ERR(pidfile);
 			goto bad_fork_free_pid;
 		}
-		get_pid(pid);	/* held by pidfile now */
 
 		retval = put_user(pidfd, args->pidfd);
 		if (retval)
-- 
2.34.1

From 0897f68fe06a8777d8ec600fdc719143f76095b1 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Wed, 22 Mar 2023 16:02:50 +0100
Subject: [PATCH 3/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] fanotify: use
 pidfd_file_create()

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/notify/fanotify/fanotify_user.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 8f430bfad487..4a8db6b5f690 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -665,6 +665,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 	unsigned int pidfd_mode = info_mode & FAN_REPORT_PIDFD;
 	struct file *f = NULL;
 	int ret, pidfd = FAN_NOPIDFD, fd = FAN_NOFD;
+	struct file *pidfd_file = NULL;
 
 	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
 
@@ -718,9 +719,11 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 		    !pid_has_task(event->pid, PIDTYPE_TGID)) {
 			pidfd = FAN_NOPIDFD;
 		} else {
-			pidfd = pidfd_create(event->pid, 0);
-			if (pidfd < 0)
+			pidfd_file = pidfd_file_create(event->pid, 0, &pidfd);
+			if (IS_ERR(pidfd_file)) {
 				pidfd = FAN_EPIDFD;
+				pidfd_file = NULL;
+			}
 		}
 	}
 
@@ -750,6 +753,8 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 
 	if (f)
 		fd_install(fd, f);
+	if (pidfd_file)
+		fd_install(pidfd, pidfd_file);
 
 	return metadata.event_len;
 
@@ -759,8 +764,10 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 		fput(f);
 	}
 
-	if (pidfd >= 0)
-		close_fd(pidfd);
+	if (pidfd >= 0) {
+		put_unused_fd(pidfd);
+		fput(pidfd_file);
+	}
 
 	return ret;
 }
-- 
2.34.1

Re: [PATCH net-next v2 1/3] scm: add SO_PASSPIDFD and SCM_PIDFD

[Christian Brauner]

On Tue, Mar 21, 2023 at 07:33:40PM +0100, Alexander Mikhalitsyn wrote:
> 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.
> 
> 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.
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> Cc: Lennart Poettering <mzxreary@0pointer.de>
> Cc: linux-kernel@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Cc: linux-arch@vger.kernel.org
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> ---
> v2:
> 	According to review comments from Kuniyuki Iwashima and Christian Brauner:
> 	- use pidfd_create(..) retval as a result
> 	- whitespace change
> ---
>  arch/alpha/include/uapi/asm/socket.h    |  2 ++
>  arch/mips/include/uapi/asm/socket.h     |  2 ++
>  arch/parisc/include/uapi/asm/socket.h   |  2 ++
>  arch/sparc/include/uapi/asm/socket.h    |  2 ++
>  include/linux/net.h                     |  1 +
>  include/linux/socket.h                  |  1 +
>  include/net/scm.h                       | 14 ++++++++++++--
>  include/uapi/asm-generic/socket.h       |  2 ++
>  net/core/sock.c                         | 11 +++++++++++
>  net/mptcp/sockopt.c                     |  1 +
>  net/unix/af_unix.c                      | 18 +++++++++++++-----
>  tools/include/uapi/asm-generic/socket.h |  2 ++
>  12 files changed, 51 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index 739891b94136..ff310613ae64 100644
> --- a/arch/alpha/include/uapi/asm/socket.h
> +++ b/arch/alpha/include/uapi/asm/socket.h
> @@ -137,6 +137,8 @@
>  
>  #define SO_RCVMARK		75
>  
> +#define SO_PASSPIDFD		76
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> index 18f3d95ecfec..762dcb80e4ec 100644
> --- a/arch/mips/include/uapi/asm/socket.h
> +++ b/arch/mips/include/uapi/asm/socket.h
> @@ -148,6 +148,8 @@
>  
>  #define SO_RCVMARK		75
>  
> +#define SO_PASSPIDFD		76
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> index f486d3dfb6bb..df16a3e16d64 100644
> --- a/arch/parisc/include/uapi/asm/socket.h
> +++ b/arch/parisc/include/uapi/asm/socket.h
> @@ -129,6 +129,8 @@
>  
>  #define SO_RCVMARK		0x4049
>  
> +#define SO_PASSPIDFD		0x404A
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> index 2fda57a3ea86..6e2847804fea 100644
> --- a/arch/sparc/include/uapi/asm/socket.h
> +++ b/arch/sparc/include/uapi/asm/socket.h
> @@ -130,6 +130,8 @@
>  
>  #define SO_RCVMARK               0x0054
>  
> +#define SO_PASSPIDFD             0x0055
> +
>  #if !defined(__KERNEL__)
>  
>  
> diff --git a/include/linux/net.h b/include/linux/net.h
> index b73ad8e3c212..c234dfbe7a30 100644
> --- a/include/linux/net.h
> +++ b/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
>  /**
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index 13c3a237b9c9..6bf90f251910 100644
> --- a/include/linux/socket.h
> +++ b/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;
> diff --git a/include/net/scm.h b/include/net/scm.h
> index 585adc1346bd..0c717ae9c8db 100644
> --- a/include/net/scm.h
> +++ b/include/net/scm.h
> @@ -124,8 +124,9 @@ 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 +142,15 @@ 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)) {
> +		int pidfd;
> +
> +		WARN_ON_ONCE(!scm->pid);
> +		pidfd = pidfd_create(scm->pid, 0);
> +
> +		put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd);

So here we need to also make sure that we can't end up in a situation
where the receiver gets an error message and discards the message but
we've snuck an fd into their fdtable. So callers of scm_recv() should be
in a path where the message can't fail anymore and we're about to return
to userspace.

Re: [PATCH net-next v2 2/3] net: core: add getsockopt SO_PEERPIDFD

[Aleksandr Mikhalitsyn]

On Wed, Mar 22, 2023 at 4:35 PM Christian Brauner <brauner@kernel.org> wrote:
>
> On Tue, Mar 21, 2023 at 07:33:41PM +0100, Alexander Mikhalitsyn wrote:
> > 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.
> >
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Paolo Abeni <pabeni@redhat.com>
> > Cc: Leon Romanovsky <leon@kernel.org>
> > Cc: David Ahern <dsahern@kernel.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> > Cc: Lennart Poettering <mzxreary@0pointer.de>
> > Cc: linux-kernel@vger.kernel.org
> > Cc: netdev@vger.kernel.org
> > Cc: linux-arch@vger.kernel.org
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> > ---
> > v2:
> >       According to review comments from Kuniyuki Iwashima and Christian Brauner:
> >       - use pidfd_create(..) retval as a result
> >       - whitespace change
> > ---
> >  arch/alpha/include/uapi/asm/socket.h    |  1 +
> >  arch/mips/include/uapi/asm/socket.h     |  1 +
> >  arch/parisc/include/uapi/asm/socket.h   |  1 +
> >  arch/sparc/include/uapi/asm/socket.h    |  1 +
> >  include/uapi/asm-generic/socket.h       |  1 +
> >  net/core/sock.c                         | 21 +++++++++++++++++++++
> >  tools/include/uapi/asm-generic/socket.h |  1 +
> >  7 files changed, 27 insertions(+)
> >
> > diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> > index ff310613ae64..e94f621903fe 100644
> > --- a/arch/alpha/include/uapi/asm/socket.h
> > +++ b/arch/alpha/include/uapi/asm/socket.h
> > @@ -138,6 +138,7 @@
> >  #define SO_RCVMARK           75
> >
> >  #define SO_PASSPIDFD         76
> > +#define SO_PEERPIDFD         77
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> > index 762dcb80e4ec..60ebaed28a4c 100644
> > --- a/arch/mips/include/uapi/asm/socket.h
> > +++ b/arch/mips/include/uapi/asm/socket.h
> > @@ -149,6 +149,7 @@
> >  #define SO_RCVMARK           75
> >
> >  #define SO_PASSPIDFD         76
> > +#define SO_PEERPIDFD         77
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> > index df16a3e16d64..be264c2b1a11 100644
> > --- a/arch/parisc/include/uapi/asm/socket.h
> > +++ b/arch/parisc/include/uapi/asm/socket.h
> > @@ -130,6 +130,7 @@
> >  #define SO_RCVMARK           0x4049
> >
> >  #define SO_PASSPIDFD         0x404A
> > +#define SO_PEERPIDFD         0x404B
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> > index 6e2847804fea..682da3714686 100644
> > --- a/arch/sparc/include/uapi/asm/socket.h
> > +++ b/arch/sparc/include/uapi/asm/socket.h
> > @@ -131,6 +131,7 @@
> >  #define SO_RCVMARK               0x0054
> >
> >  #define SO_PASSPIDFD             0x0055
> > +#define SO_PEERPIDFD             0x0056
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> > index b76169fdb80b..8ce8a39a1e5f 100644
> > --- a/include/uapi/asm-generic/socket.h
> > +++ b/include/uapi/asm-generic/socket.h
> > @@ -133,6 +133,7 @@
> >  #define SO_RCVMARK           75
> >
> >  #define SO_PASSPIDFD         76
> > +#define SO_PEERPIDFD         77
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/net/core/sock.c b/net/core/sock.c
> > index 3f974246ba3e..85c269ca9d8a 100644
> > --- a/net/core/sock.c
> > +++ b/net/core/sock.c
> > @@ -1763,6 +1763,27 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
> >               goto lenout;
> >       }
> >
> > +     case SO_PEERPIDFD:
> > +     {
> > +             struct pid *peer_pid;
> > +             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);
> > +
> > +             pidfd = pidfd_create(peer_pid, 0);
> > +
> > +             put_pid(peer_pid);
> > +
> > +             if (copy_to_sockptr(optval, &pidfd, len))
> > +                     return -EFAULT;
>
> This leaks the pidfd. We could do:
>
>         if (copy_to_sockptr(optval, &pidfd, len)) {
>                 close_fd(pidfd);
>                 return -EFAULT;
>         }

Ah, my bad. Thanks for pointing this out!

>
> but it's a nasty anti-pattern to install the fd in the caller's fdtable
> and then close it again. So let's avoid it if we can. Since you can only
> set one socket option per setsockopt() sycall we should be able to
> reserve an fd and pidfd_file, do the stuff that might fail, and then
> call fd_install. So that would roughly be:
>
>         peer_pid = get_pid(sk->sk_peer_pid);
>         pidfd_file = pidfd_file_create(peer_pid, 0, &pidfd);
>         f (copy_to_sockptr(optval, &pidfd, len))
>                return -EFAULT;
>         goto lenout:
>
>         .
>         .
>         .
>
> lenout:
>         if (copy_to_sockptr(optlen, &len, sizeof(int)))
>                 return -EFAULT;
>
>         // Made it safely, install pidfd now.
>         fd_install(pidfd, pidfd_file)
>
> (See below for the associated api I'm going to publish independent of
> this as kernel/fork.c and fanotify both could use it.)
>
> But now, let's look at net/socket.c there's another wrinkle. So let's say you
> have successfully installed the pidfd then it seems you can still fail later:
>
>         if (level == SOL_SOCKET)
>                 err = sock_getsockopt(sock, level, optname, optval, optlen);
>         else if (unlikely(!sock->ops->getsockopt))
>                 err = -EOPNOTSUPP;
>         else
>                 err = sock->ops->getsockopt(sock, level, optname, optval,
>                                             optlen);
>
>         if (!in_compat_syscall())
>                 err = BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock->sk, level, optname,
>                                                      optval, optlen, max_optlen,
>                                                      err);
>
> out_put:
>         fput_light(sock->file, fput_needed);
>         return err;
>
> If the bpf hook returns an error we've placed an fd into the caller's sockopt
> buffer without their knowledge.

yes, so we need to postpone fd_install to the end of __sys_getsockopt.
I'll think about that.

>
> From 4fee16f0920308bee2531fd3b08484f607eb5830 Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@kernel.org>
> Date: Wed, 22 Mar 2023 15:59:02 +0100
> Subject: [PATCH 1/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] pid: add
>  pidfd_file_create()
>
> Reserve and fd and pidfile, do stuff that might fail, install fd when
> point of no return.
>
> [HERE BE DRAGONS - DRAFT - __UNTESTED__] pid: add pidfd_file_create()
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  include/linux/pid.h |  1 +
>  kernel/pid.c        | 45 +++++++++++++++++++++++++++++++++------------
>  2 files changed, 34 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 343abf22092e..c486dbc4d7b6 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -80,6 +80,7 @@ extern struct pid *pidfd_pid(const struct file *file);
>  struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
>  struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
>  int pidfd_create(struct pid *pid, unsigned int flags);
> +struct file *pidfd_file_create(struct pid *pid, unsigned int flags, int *pidfd);
>
>  static inline struct pid *get_pid(struct pid *pid)
>  {
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 3fbc5e46b721..8d0924f1dbf6 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -576,6 +576,32 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
>         return task;
>  }
>
> +struct file *pidfd_file_create(struct pid *pid, unsigned int flags, int *pidfd)
> +{
> +       int fd;
> +       struct file *pidfile;
> +
> +       if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
> +               return ERR_PTR(-EINVAL);
> +
> +       if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
> +               return ERR_PTR(-EINVAL);
> +
> +       fd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
> +       if (fd < 0)
> +               return ERR_PTR(fd);
> +
> +       pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
> +                                    flags | O_RDWR | O_CLOEXEC);
> +       if (IS_ERR(pidfile)) {
> +               put_unused_fd(fd);
> +               return pidfile;
> +       }
> +       get_pid(pid); /* held by pidfile now */
> +       *pidfd = fd;
> +       return pidfile;
> +}
> +
>  /**
>   * pidfd_create() - Create a new pid file descriptor.
>   *
> @@ -594,20 +620,15 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
>   */
>  int pidfd_create(struct pid *pid, unsigned int flags)
>  {
> -       int fd;
> +       int pidfd;
> +       struct file *pidfile;
>
> -       if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
> -               return -EINVAL;
> +       pidfile = pidfd_file_create(pid, flags, &pidfd);
> +       if (IS_ERR(pidfile))
> +               return PTR_ERR(pidfile);
>
> -       if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
> -               return -EINVAL;
> -
> -       fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
> -                             flags | O_RDWR | O_CLOEXEC);
> -       if (fd < 0)
> -               put_pid(pid);
> -
> -       return fd;
> +       fd_install(pidfd, pidfile);
> +       return pidfd;
>  }
>
>  /**
> --
> 2.34.1
>
> From c336f1c6cc39faa5aef4fbedd3c4f8eca51d8436 Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@kernel.org>
> Date: Wed, 22 Mar 2023 15:59:54 +0100
> Subject: [PATCH 2/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] fork: use
>  pidfd_file_create()
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  kernel/fork.c | 11 +----------
>  1 file changed, 1 insertion(+), 10 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index f68954d05e89..c8dc78ee0a74 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2296,20 +2296,11 @@ static __latent_entropy struct task_struct *copy_process(
>          * if the fd table isn't shared).
>          */
>         if (clone_flags & CLONE_PIDFD) {
> -               retval = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
> -               if (retval < 0)
> -                       goto bad_fork_free_pid;
> -
> -               pidfd = retval;
> -
> -               pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
> -                                             O_RDWR | O_CLOEXEC);
> +               pidfile = pidfd_file_create(pid, O_RDWR | O_CLOEXEC, &pidfd);
>                 if (IS_ERR(pidfile)) {
> -                       put_unused_fd(pidfd);
>                         retval = PTR_ERR(pidfile);
>                         goto bad_fork_free_pid;
>                 }
> -               get_pid(pid);   /* held by pidfile now */
>
>                 retval = put_user(pidfd, args->pidfd);
>                 if (retval)
> --
> 2.34.1
>
> From 0897f68fe06a8777d8ec600fdc719143f76095b1 Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@kernel.org>
> Date: Wed, 22 Mar 2023 16:02:50 +0100
> Subject: [PATCH 3/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] fanotify: use
>  pidfd_file_create()
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/notify/fanotify/fanotify_user.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 8f430bfad487..4a8db6b5f690 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -665,6 +665,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>         unsigned int pidfd_mode = info_mode & FAN_REPORT_PIDFD;
>         struct file *f = NULL;
>         int ret, pidfd = FAN_NOPIDFD, fd = FAN_NOFD;
> +       struct file *pidfd_file = NULL;
>
>         pr_debug("%s: group=%p event=%p\n", __func__, group, event);
>
> @@ -718,9 +719,11 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>                     !pid_has_task(event->pid, PIDTYPE_TGID)) {
>                         pidfd = FAN_NOPIDFD;
>                 } else {
> -                       pidfd = pidfd_create(event->pid, 0);
> -                       if (pidfd < 0)
> +                       pidfd_file = pidfd_file_create(event->pid, 0, &pidfd);
> +                       if (IS_ERR(pidfd_file)) {
>                                 pidfd = FAN_EPIDFD;
> +                               pidfd_file = NULL;
> +                       }
>                 }
>         }
>
> @@ -750,6 +753,8 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>
>         if (f)
>                 fd_install(fd, f);
> +       if (pidfd_file)
> +               fd_install(pidfd, pidfd_file);
>
>         return metadata.event_len;
>
> @@ -759,8 +764,10 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>                 fput(f);
>         }
>
> -       if (pidfd >= 0)
> -               close_fd(pidfd);
> +       if (pidfd >= 0) {
> +               put_unused_fd(pidfd);
> +               fput(pidfd_file);
> +       }
>
>         return ret;
>  }
> --
> 2.34.1
>

[PATCH 0/3] pidfd: add pidfd_prepare()

[Christian Brauner]

This adds the pidfd_prepare() helper which allows the caller to reserve
a pidfd number and allocates a new pidfd file that stashes the provided
struct pid.

This will allow us to remove places that either open code this
functionality e.g., during copy_process() or that currently call
pidfd_create() but then have to call close_fd() because there are still
failure points after pidfd_create() has been called.

Other functionality wants to make use of pidfd's as well and they need a
pidfd_prepare() internal api as well.

I've tested the fanotify and fork changes via LTP which provides
coverage for all the affected codepaths.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Christian Brauner (3):
      pid: add pidfd_prepare()
      fork: use pidfd_prepare()
      fanotify: use pidfd_prepare()

 fs/notify/fanotify/fanotify_user.c | 13 ++++---
 include/linux/pid.h                |  1 +
 kernel/fork.c                      | 12 +------
 kernel/pid.c                       | 69 +++++++++++++++++++++++++++++++-------
 4 files changed, 68 insertions(+), 27 deletions(-)
---
base-commit: 197b6b60ae7bc51dd0814953c562833143b292aa
change-id: 20230327-pidfd-file-api-8b28d68cf0a9

[PATCH 1/3] pid: add pidfd_prepare()

[Christian Brauner]

Add a new helper that allows to reserve a pidfd and allocates a new
pidfd file that stashes the provided struct pid. This will allow us to
remove places that either open code this function or that call
pidfd_create() but then have to call close_fd() because there are still
failure points after pidfd_create() has been called.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/pid.h |  1 +
 kernel/pid.c        | 69 +++++++++++++++++++++++++++++++++++++++++++----------
 2 files changed, 58 insertions(+), 12 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 343abf22092e..b75de288a8c2 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -80,6 +80,7 @@ extern struct pid *pidfd_pid(const struct file *file);
 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
 struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
 int pidfd_create(struct pid *pid, unsigned int flags);
+int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret);
 
 static inline struct pid *get_pid(struct pid *pid)
 {
diff --git a/kernel/pid.c b/kernel/pid.c
index 3fbc5e46b721..95e7e01574c8 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -576,6 +576,56 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
 	return task;
 }
 
+/**
+ * pidfd_prepare - allocate a new pidfd_file and reserve a pidfd
+ * @pid:   the struct pid for which to create a pidfd
+ * @flags: flags of the new @pidfd
+ * @pidfd: the pidfd to return
+ *
+ * Allocate a new file that stashes @pid and reserve a new pidfd number in the
+ * caller's file descriptor table. The pidfd is reserved but not installed yet.
+ *
+ * If this function returns successfully the caller is responsible to either
+ * call fd_install() passing the returned pidfd and pidfd file as arguments in
+ * order to install the pidfd into its file descriptor table or they must use
+ * put_unused_fd() and fput() on the returned pidfd and pidfd file
+ * respectively.
+ *
+ * This function is useful when a pidfd must already be reserved but there
+ * might still be points of failure afterwards and the caller wants to ensure
+ * that no pidfd is leaked into its file descriptor table.
+ *
+ * Return: On success, a reserved pidfd is returned from the function and a new
+ *         pidfd file is returned in the last argument to the function. On
+ *         error, a negative error code is returned from the function and the
+ *         last argument remains unchanged.
+ */
+int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
+{
+	int pidfd;
+	struct file *pidfd_file;
+
+	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
+		return -EINVAL;
+
+	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
+		return -EINVAL;
+
+	pidfd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
+	if (pidfd < 0)
+		return pidfd;
+
+	pidfd_file = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
+					flags | O_RDWR | O_CLOEXEC);
+	if (IS_ERR(pidfd_file)) {
+		put_unused_fd(pidfd);
+		return PTR_ERR(pidfd_file);
+	}
+	get_pid(pid); /* held by pidfd_file now */
+	*ret = pidfd_file;
+	return pidfd;
+}
+
 /**
  * pidfd_create() - Create a new pid file descriptor.
  *
@@ -594,20 +644,15 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
  */
 int pidfd_create(struct pid *pid, unsigned int flags)
 {
-	int fd;
+	int pidfd;
+	struct file *pidfd_file;
 
-	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
-		return -EINVAL;
-
-	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
-		return -EINVAL;
-
-	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
-			      flags | O_RDWR | O_CLOEXEC);
-	if (fd < 0)
-		put_pid(pid);
+	pidfd = pidfd_prepare(pid, flags, &pidfd_file);
+	if (pidfd < 0)
+		return pidfd;
 
-	return fd;
+	fd_install(pidfd, pidfd_file);
+	return pidfd;
 }
 
 /**

-- 
2.34.1

[PATCH 2/3] fork: use pidfd_prepare()

[Christian Brauner]

Stop open-coding get_unused_fd_flags() and anon_inode_getfile(). That's
brittle just for keeping the flags between both calls in sync. Use the
dedicated helper.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 kernel/fork.c | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index c0257cbee093..1f5eb854ba3e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2291,21 +2291,11 @@ static __latent_entropy struct task_struct *copy_process(
 	 * if the fd table isn't shared).
 	 */
 	if (clone_flags & CLONE_PIDFD) {
-		retval = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
+		retval = pidfd_prepare(pid, O_RDWR | O_CLOEXEC, &pidfile);
 		if (retval < 0)
 			goto bad_fork_free_pid;
-
 		pidfd = retval;
 
-		pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
-					      O_RDWR | O_CLOEXEC);
-		if (IS_ERR(pidfile)) {
-			put_unused_fd(pidfd);
-			retval = PTR_ERR(pidfile);
-			goto bad_fork_free_pid;
-		}
-		get_pid(pid);	/* held by pidfile now */
-
 		retval = put_user(pidfd, args->pidfd);
 		if (retval)
 			goto bad_fork_put_pidfd;

-- 
2.34.1

[PATCH 3/3] fanotify: use pidfd_prepare()

[Christian Brauner]

We generally try to avoid installing a file descriptor into the caller's
file descriptor table just to close it again via close_fd() in case an
error occurs. Instead we reserve a file descriptor but don't install it
into the caller's file descriptor table yet. If we fail for other,
unrelated reasons we can just close the reserved file descriptor and if
we make it past all meaningful error paths we just install it. Fanotify
gets this right already for one fd type but not for pidfds.

Use the new pidfd_prepare() helper to reserve a pidfd and a pidfd file
and switch to the more common fd allocation and installation pattern.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/notify/fanotify/fanotify_user.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 8f430bfad487..22fb1cf7e1fc 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -663,7 +663,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 	struct fanotify_info *info = fanotify_event_info(event);
 	unsigned int info_mode = FAN_GROUP_FLAG(group, FANOTIFY_INFO_MODES);
 	unsigned int pidfd_mode = info_mode & FAN_REPORT_PIDFD;
-	struct file *f = NULL;
+	struct file *f = NULL, *pidfd_file = NULL;
 	int ret, pidfd = FAN_NOPIDFD, fd = FAN_NOFD;
 
 	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
@@ -718,7 +718,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 		    !pid_has_task(event->pid, PIDTYPE_TGID)) {
 			pidfd = FAN_NOPIDFD;
 		} else {
-			pidfd = pidfd_create(event->pid, 0);
+			pidfd = pidfd_prepare(event->pid, 0, &pidfd_file);
 			if (pidfd < 0)
 				pidfd = FAN_EPIDFD;
 		}
@@ -751,6 +751,9 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 	if (f)
 		fd_install(fd, f);
 
+	if (pidfd_file)
+		fd_install(pidfd, pidfd_file);
+
 	return metadata.event_len;
 
 out_close_fd:
@@ -759,8 +762,10 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 		fput(f);
 	}
 
-	if (pidfd >= 0)
-		close_fd(pidfd);
+	if (pidfd >= 0) {
+		put_unused_fd(pidfd);
+		fput(pidfd_file);
+	}
 
 	return ret;
 }

-- 
2.34.1

Re: [PATCH 3/3] fanotify: use pidfd_prepare()

[Jan Kara]

On Mon 27-03-23 20:22:53, Christian Brauner wrote:
> We generally try to avoid installing a file descriptor into the caller's
> file descriptor table just to close it again via close_fd() in case an
> error occurs. Instead we reserve a file descriptor but don't install it
> into the caller's file descriptor table yet. If we fail for other,
> unrelated reasons we can just close the reserved file descriptor and if
> we make it past all meaningful error paths we just install it. Fanotify
> gets this right already for one fd type but not for pidfds.
> 
> Use the new pidfd_prepare() helper to reserve a pidfd and a pidfd file
> and switch to the more common fd allocation and installation pattern.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>

Thanks for the improvement! It looks good to me. Feel free to add:

Acked-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/notify/fanotify/fanotify_user.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 8f430bfad487..22fb1cf7e1fc 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -663,7 +663,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>  	struct fanotify_info *info = fanotify_event_info(event);
>  	unsigned int info_mode = FAN_GROUP_FLAG(group, FANOTIFY_INFO_MODES);
>  	unsigned int pidfd_mode = info_mode & FAN_REPORT_PIDFD;
> -	struct file *f = NULL;
> +	struct file *f = NULL, *pidfd_file = NULL;
>  	int ret, pidfd = FAN_NOPIDFD, fd = FAN_NOFD;
>  
>  	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
> @@ -718,7 +718,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>  		    !pid_has_task(event->pid, PIDTYPE_TGID)) {
>  			pidfd = FAN_NOPIDFD;
>  		} else {
> -			pidfd = pidfd_create(event->pid, 0);
> +			pidfd = pidfd_prepare(event->pid, 0, &pidfd_file);
>  			if (pidfd < 0)
>  				pidfd = FAN_EPIDFD;
>  		}
> @@ -751,6 +751,9 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>  	if (f)
>  		fd_install(fd, f);
>  
> +	if (pidfd_file)
> +		fd_install(pidfd, pidfd_file);
> +
>  	return metadata.event_len;
>  
>  out_close_fd:
> @@ -759,8 +762,10 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>  		fput(f);
>  	}
>  
> -	if (pidfd >= 0)
> -		close_fd(pidfd);
> +	if (pidfd >= 0) {
> +		put_unused_fd(pidfd);
> +		fput(pidfd_file);
> +	}
>  
>  	return ret;
>  }
> 
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 1/3] pid: add pidfd_prepare()

[Jan Kara]

On Mon 27-03-23 20:22:51, Christian Brauner wrote:
> Add a new helper that allows to reserve a pidfd and allocates a new
> pidfd file that stashes the provided struct pid. This will allow us to
> remove places that either open code this function or that call
> pidfd_create() but then have to call close_fd() because there are still
> failure points after pidfd_create() has been called.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  include/linux/pid.h |  1 +
>  kernel/pid.c        | 69 +++++++++++++++++++++++++++++++++++++++++++----------
>  2 files changed, 58 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 343abf22092e..b75de288a8c2 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -80,6 +80,7 @@ extern struct pid *pidfd_pid(const struct file *file);
>  struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
>  struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
>  int pidfd_create(struct pid *pid, unsigned int flags);
> +int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret);
>  
>  static inline struct pid *get_pid(struct pid *pid)
>  {
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 3fbc5e46b721..95e7e01574c8 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -576,6 +576,56 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
>  	return task;
>  }
>  
> +/**
> + * pidfd_prepare - allocate a new pidfd_file and reserve a pidfd
> + * @pid:   the struct pid for which to create a pidfd
> + * @flags: flags of the new @pidfd
> + * @pidfd: the pidfd to return
> + *
> + * Allocate a new file that stashes @pid and reserve a new pidfd number in the
> + * caller's file descriptor table. The pidfd is reserved but not installed yet.
> + *
> + * If this function returns successfully the caller is responsible to either
> + * call fd_install() passing the returned pidfd and pidfd file as arguments in
> + * order to install the pidfd into its file descriptor table or they must use
> + * put_unused_fd() and fput() on the returned pidfd and pidfd file
> + * respectively.
> + *
> + * This function is useful when a pidfd must already be reserved but there
> + * might still be points of failure afterwards and the caller wants to ensure
> + * that no pidfd is leaked into its file descriptor table.
> + *
> + * Return: On success, a reserved pidfd is returned from the function and a new
> + *         pidfd file is returned in the last argument to the function. On
> + *         error, a negative error code is returned from the function and the
> + *         last argument remains unchanged.
> + */
> +int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
> +{
> +	int pidfd;
> +	struct file *pidfd_file;
> +
> +	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
> +		return -EINVAL;
> +
> +	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
> +		return -EINVAL;
> +
> +	pidfd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
> +	if (pidfd < 0)
> +		return pidfd;
> +
> +	pidfd_file = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
> +					flags | O_RDWR | O_CLOEXEC);
> +	if (IS_ERR(pidfd_file)) {
> +		put_unused_fd(pidfd);
> +		return PTR_ERR(pidfd_file);
> +	}
> +	get_pid(pid); /* held by pidfd_file now */
> +	*ret = pidfd_file;
> +	return pidfd;
> +}
> +
>  /**
>   * pidfd_create() - Create a new pid file descriptor.
>   *
> @@ -594,20 +644,15 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
>   */
>  int pidfd_create(struct pid *pid, unsigned int flags)
>  {
> -	int fd;
> +	int pidfd;
> +	struct file *pidfd_file;
>  
> -	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
> -		return -EINVAL;
> -
> -	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
> -		return -EINVAL;
> -
> -	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
> -			      flags | O_RDWR | O_CLOEXEC);
> -	if (fd < 0)
> -		put_pid(pid);
> +	pidfd = pidfd_prepare(pid, flags, &pidfd_file);
> +	if (pidfd < 0)
> +		return pidfd;
>  
> -	return fd;
> +	fd_install(pidfd, pidfd_file);
> +	return pidfd;
>  }
>  
>  /**
> 
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH net-next v2 2/3] net: core: add getsockopt SO_PEERPIDFD

[Christian Brauner]

On Wed, Mar 22, 2023 at 04:35:51PM +0100, Christian Brauner wrote:
> On Tue, Mar 21, 2023 at 07:33:41PM +0100, Alexander Mikhalitsyn wrote:
> > 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.
> > 
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Paolo Abeni <pabeni@redhat.com>
> > Cc: Leon Romanovsky <leon@kernel.org>
> > Cc: David Ahern <dsahern@kernel.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> > Cc: Lennart Poettering <mzxreary@0pointer.de>
> > Cc: linux-kernel@vger.kernel.org
> > Cc: netdev@vger.kernel.org
> > Cc: linux-arch@vger.kernel.org
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> > ---
> > v2:
> > 	According to review comments from Kuniyuki Iwashima and Christian Brauner:
> > 	- use pidfd_create(..) retval as a result
> > 	- whitespace change
> > ---
> >  arch/alpha/include/uapi/asm/socket.h    |  1 +
> >  arch/mips/include/uapi/asm/socket.h     |  1 +
> >  arch/parisc/include/uapi/asm/socket.h   |  1 +
> >  arch/sparc/include/uapi/asm/socket.h    |  1 +
> >  include/uapi/asm-generic/socket.h       |  1 +
> >  net/core/sock.c                         | 21 +++++++++++++++++++++
> >  tools/include/uapi/asm-generic/socket.h |  1 +
> >  7 files changed, 27 insertions(+)
> > 
> > diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> > index ff310613ae64..e94f621903fe 100644
> > --- a/arch/alpha/include/uapi/asm/socket.h
> > +++ b/arch/alpha/include/uapi/asm/socket.h
> > @@ -138,6 +138,7 @@
> >  #define SO_RCVMARK		75
> >  
> >  #define SO_PASSPIDFD		76
> > +#define SO_PEERPIDFD		77
> >  
> >  #if !defined(__KERNEL__)
> >  
> > diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> > index 762dcb80e4ec..60ebaed28a4c 100644
> > --- a/arch/mips/include/uapi/asm/socket.h
> > +++ b/arch/mips/include/uapi/asm/socket.h
> > @@ -149,6 +149,7 @@
> >  #define SO_RCVMARK		75
> >  
> >  #define SO_PASSPIDFD		76
> > +#define SO_PEERPIDFD		77
> >  
> >  #if !defined(__KERNEL__)
> >  
> > diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> > index df16a3e16d64..be264c2b1a11 100644
> > --- a/arch/parisc/include/uapi/asm/socket.h
> > +++ b/arch/parisc/include/uapi/asm/socket.h
> > @@ -130,6 +130,7 @@
> >  #define SO_RCVMARK		0x4049
> >  
> >  #define SO_PASSPIDFD		0x404A
> > +#define SO_PEERPIDFD		0x404B
> >  
> >  #if !defined(__KERNEL__)
> >  
> > diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> > index 6e2847804fea..682da3714686 100644
> > --- a/arch/sparc/include/uapi/asm/socket.h
> > +++ b/arch/sparc/include/uapi/asm/socket.h
> > @@ -131,6 +131,7 @@
> >  #define SO_RCVMARK               0x0054
> >  
> >  #define SO_PASSPIDFD             0x0055
> > +#define SO_PEERPIDFD             0x0056
> >  
> >  #if !defined(__KERNEL__)
> >  
> > diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> > index b76169fdb80b..8ce8a39a1e5f 100644
> > --- a/include/uapi/asm-generic/socket.h
> > +++ b/include/uapi/asm-generic/socket.h
> > @@ -133,6 +133,7 @@
> >  #define SO_RCVMARK		75
> >  
> >  #define SO_PASSPIDFD		76
> > +#define SO_PEERPIDFD		77
> >  
> >  #if !defined(__KERNEL__)
> >  
> > diff --git a/net/core/sock.c b/net/core/sock.c
> > index 3f974246ba3e..85c269ca9d8a 100644
> > --- a/net/core/sock.c
> > +++ b/net/core/sock.c
> > @@ -1763,6 +1763,27 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
> >  		goto lenout;
> >  	}
> >  
> > +	case SO_PEERPIDFD:
> > +	{
> > +		struct pid *peer_pid;
> > +		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);
> > +
> > +		pidfd = pidfd_create(peer_pid, 0);
> > +
> > +		put_pid(peer_pid);
> > +
> > +		if (copy_to_sockptr(optval, &pidfd, len))
> > +			return -EFAULT;
> 
> This leaks the pidfd. We could do:
> 
> 	if (copy_to_sockptr(optval, &pidfd, len)) {
> 		close_fd(pidfd);
> 		return -EFAULT;
> 	}
> 
> but it's a nasty anti-pattern to install the fd in the caller's fdtable
> and then close it again. So let's avoid it if we can. Since you can only
> set one socket option per setsockopt() sycall we should be able to
> reserve an fd and pidfd_file, do the stuff that might fail, and then
> call fd_install. So that would roughly be:
> 
> 	peer_pid = get_pid(sk->sk_peer_pid);
> 	pidfd_file = pidfd_file_create(peer_pid, 0, &pidfd);
> 	f (copy_to_sockptr(optval, &pidfd, len))
> 	       return -EFAULT;
> 	goto lenout:
> 	
> 	.
> 	.
> 	.
> 
> lenout:
> 	if (copy_to_sockptr(optlen, &len, sizeof(int)))
> 		return -EFAULT;
> 
> 	// Made it safely, install pidfd now.
> 	fd_install(pidfd, pidfd_file)
> 
> (See below for the associated api I'm going to publish independent of
> this as kernel/fork.c and fanotify both could use it.)

Sent out yesterday:
https://lore.kernel.org/lkml/20230328090026.b54a4jhccntfraey@quack3

Re: [PATCH 0/3] pidfd: add pidfd_prepare()

[Christian Brauner]

On Mon, 27 Mar 2023 20:22:50 +0200, Christian Brauner wrote:
> This adds the pidfd_prepare() helper which allows the caller to reserve
> a pidfd number and allocates a new pidfd file that stashes the provided
> struct pid.
> 
> This will allow us to remove places that either open code this
> functionality e.g., during copy_process() or that currently call
> pidfd_create() but then have to call close_fd() because there are still
> failure points after pidfd_create() has been called.
> 
> [...]

Jan, thanks for the reviews.

I've picked this up now. Please note that this series is considered stable and
has thus been tagged. The reason is that the SCM_PIDFD work in the networking
depends wants to depend on this work. So they'll get a stable tag,

tree: git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git
branch: pidfd.file.api
tag: pidfd.file.api.v6.4

[1/3] pid: add pidfd_prepare()
      commit: 7021c1b14f83d9151ecaf976eaa6c1d5c6bb5dc7
[2/3] fork: use pidfd_prepare()
      commit: 761ce43fda7ebcdf1b1aa8e797ec83fae0e34c47
[3/3] fanotify: use pidfd_prepare()
      commit: 909939fc167d82cf09cd93ae44e968be916b6e41

Thanks!
Christian