ssh-proxy: add ssh ProxyCommand tool that can connect to AF_UNIX + AF_VSOCK sockets

This adds a tiny binary that is hooked into SSH client config via
ProxyCommand and which simply connects to an AF_UNIX or AF_VSOCK socket
of choice.

The syntax is as simple as this:

     ssh unix/some/path     # (this connects to AF_UNIX socket /some/path)

or:

     ssh vsock/4711

I used "/" as separator of the protocol ID and the value since ":" is
already taken by SSH itself when doing sftp. And "@" is already taken
for separating the user name.

src/ssh-generator/ssh-proxy.c

@@ -0,0 +1,102 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <net/if_arp.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include "fd-util.h"
+#include "iovec-util.h"
+#include "log.h"
+#include "main-func.h"
+#include "missing_socket.h"
+#include "parse-util.h"
+#include "socket-util.h"
+#include "string-util.h"
+#include "strv.h"
+
+static int process_vsock(const char *host, const char *port) {
+        int r;
+
+        assert(host);
+        assert(port);
+
+        union sockaddr_union sa = {
+                .vm.svm_family = AF_VSOCK,
+        };
+
+        r = vsock_parse_cid(host, &sa.vm.svm_cid);
+        if (r < 0)
+                return log_error_errno(r, "Failed to parse vsock cid: %s", host);
+
+        r = vsock_parse_port(port, &sa.vm.svm_port);
+        if (r < 0)
+                return log_error_errno(r, "Failed to parse vsock port: %s", port);
+
+        _cleanup_close_ int fd = socket(AF_VSOCK, SOCK_STREAM|SOCK_CLOEXEC, 0);
+        if (fd < 0)
+                return log_error_errno(errno, "Failed to allocate AF_VSOCK socket: %m");
+
+        if (connect(fd, &sa.sa, SOCKADDR_LEN(sa)) < 0)
+                return log_error_errno(errno, "Failed to connect to vsock:%u:%u: %m", sa.vm.svm_cid, sa.vm.svm_port);
+
+        /* OpenSSH wants us to send a single byte along with the file descriptor, hence do so */
+        r = send_one_fd_iov(STDOUT_FILENO, fd, &IOVEC_NUL_BYTE, /* n_iovec= */ 1, /* flags= */ 0);
+        if (r < 0)
+                return log_error_errno(r, "Failed to send socket via STDOUT: %m");
+
+        log_debug("Successfully sent AF_VSOCK socket via STDOUT.");
+        return 0;
+}
+
+static int process_unix(const char *path) {
+        int r;
+
+        assert(path);
+
+        /* We assume the path is absolute unless it starts with a dot (or is already explicitly absolute) */
+        _cleanup_free_ char *prefixed = NULL;
+        if (!STARTSWITH_SET(path, "/", "./")) {
+                prefixed = strjoin("/", path);
+                if (!prefixed)
+                        return log_oom();
+
+                path = prefixed;
+        }
+
+        _cleanup_close_ int fd = socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0);
+        if (fd < 0)
+                return log_error_errno(errno, "Failed to allocate AF_UNIX socket: %m");
+
+        r = connect_unix_path(fd, AT_FDCWD, path);
+        if (r < 0)
+                return log_error_errno(r, "Failed to connect to AF_UNIX socket %s: %m", path);
+
+        r = send_one_fd_iov(STDOUT_FILENO, fd, &IOVEC_NUL_BYTE, /* n_iovec= */ 1, /* flags= */ 0);
+        if (r < 0)
+                return log_error_errno(r, "Failed to send socket via STDOUT: %m");
+
+        log_debug("Successfully sent AF_UNIX socket via STDOUT.");
+        return 0;
+}
+
+static int run(int argc, char* argv[]) {
+
+        log_setup();
+
+        if (argc != 3)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Expected two arguments: host and port.");
+
+        const char *host = argv[1], *port = argv[2];
+
+        const char *p = startswith(host, "vsock/");
+        if (p)
+                return process_vsock(p, port);
+
+        p = startswith(host, "unix/");
+        if (p)
+                return process_unix(p);
+
+        return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Don't know how to parse host name specification: %s", host);
+}
+
+DEFINE_MAIN_FUNCTION(run);