net: core: add getsockopt SO_PEERPIDFD

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: Luca Boccassi <bluca@debian.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: bpf@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Reviewed-by: Christian Brauner <brauner@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Tested-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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__)
 

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__)
 

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__)
 

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__)
 

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__)
 

net/core/sock.c

@@ -1758,6 +1758,39 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
 		goto lenout;
 	}
 
+	case SO_PEERPIDFD:
+	{
+		struct pid *peer_pid;
+		struct file *pidfd_file = NULL;
+		int pidfd;
+
+		if (len > sizeof(pidfd))
+			len = sizeof(pidfd);
+
+		spin_lock(&sk->sk_peer_lock);
+		peer_pid = get_pid(sk->sk_peer_pid);
+		spin_unlock(&sk->sk_peer_lock);
+
+		if (!peer_pid)
+			return -ESRCH;
+
+		pidfd = pidfd_prepare(peer_pid, 0, &pidfd_file);
+		put_pid(peer_pid);
+		if (pidfd < 0)
+			return pidfd;
+
+		if (copy_to_sockptr(optval, &pidfd, len) ||
+		    copy_to_sockptr(optlen, &len, sizeof(int))) {
+			put_unused_fd(pidfd);
+			fput(pidfd_file);
+
+			return -EFAULT;
+		}
+
+		fd_install(pidfd, pidfd_file);
+		return 0;
+	}
+
 	case SO_PEERGROUPS:
 	{
 		const struct cred *cred;

net/unix/af_unix.c

@@ -921,11 +921,26 @@ static void unix_unhash(struct sock *sk)
 	 */
 }
 
+static bool unix_bpf_bypass_getsockopt(int level, int optname)
+{
+	if (level == SOL_SOCKET) {
+		switch (optname) {
+		case SO_PEERPIDFD:
+			return true;
+		default:
+			return false;
+		}
+	}
+
+	return false;
+}
+
 struct proto unix_dgram_proto = {
 	.name			= "UNIX",
 	.owner			= THIS_MODULE,
 	.obj_size		= sizeof(struct unix_sock),
 	.close			= unix_close,
+	.bpf_bypass_getsockopt	= unix_bpf_bypass_getsockopt,
 #ifdef CONFIG_BPF_SYSCALL
 	.psock_update_sk_prot	= unix_dgram_bpf_update_proto,
 #endif
@@ -937,6 +952,7 @@ struct proto unix_stream_proto = {
 	.obj_size		= sizeof(struct unix_sock),
 	.close			= unix_close,
 	.unhash			= unix_unhash,
+	.bpf_bypass_getsockopt	= unix_bpf_bypass_getsockopt,
 #ifdef CONFIG_BPF_SYSCALL
 	.psock_update_sk_prot	= unix_stream_bpf_update_proto,
 #endif

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__)