vsock-util: allow VMADDR_CID_ANY as local CID if enabled in hwdb

On Hyper-V guests, `VMADDR_CID_ANY` is valid as per implementation in
kernel driver: net/vmw_vsock/hyperv_transport.c.

Fixes https://github.com/systemd/systemd/issues/42496

Follow-up for 83359c4da0

src/shared/meson.build

@@ -261,6 +261,7 @@ shared_sources = files(
         'vlan-util.c',
         'volatile-util.c',
         'vpick.c',
+        'vsock-util.c',
         'wall.c',
         'watchdog.c',
         'web-util.c',

src/shared/vsock-util.c

@@ -0,0 +1,92 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <fcntl.h>
+#include <sys/ioctl.h>
+
+#include "sd-hwdb.h"
+
+#include "fd-util.h"
+#include "fileio.h"
+#include "log.h"
+#include "parse-util.h"
+#include "string-util.h"
+#include "vsock-util.h"
+
+static int smbios_get_modalias(char **ret) {
+        int r;
+
+        assert(ret);
+
+        _cleanup_free_ char *modalias = NULL;
+        r = read_virtual_file("/sys/devices/virtual/dmi/id/modalias", SIZE_MAX, &modalias, /* ret_size= */ NULL);
+        if (r < 0)
+                return r;
+
+        truncate_nl(modalias);
+
+        *ret = TAKE_PTR(modalias);
+        return 0;
+}
+
+static int smbios_get_accepts_any(void) {
+        static int accepts_any = -1;
+        int r;
+
+        if (accepts_any >= 0)
+                return accepts_any;
+
+        _cleanup_free_ char *modalias = NULL;
+        r = smbios_get_modalias(&modalias);
+        if (r == -ENOENT)
+                return (accepts_any = false);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to read DMI modalias: %m");
+
+        _cleanup_(sd_hwdb_unrefp) sd_hwdb *hwdb = NULL;
+        r = sd_hwdb_new(&hwdb);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to open hwdb: %m");
+
+        const char *value;
+        r = sd_hwdb_get(hwdb, modalias, "VSOCK_ACCEPT_VMADDR_CID_ANY", &value);
+        if (r < 0) {
+                if (r != -ENOENT)
+                        log_debug_errno(r, "Failed to get VSOCK_ACCEPT_VMADDR_CID_ANY, ignoring: %m");
+                return (accepts_any = false);
+        }
+        log_debug("VSOCK_ACCEPT_VMADDR_CID_ANY: %s", value);
+
+        r = parse_boolean(value);
+        if (r < 0) {
+                log_debug_errno(r, "Failed to parse VSOCK_ACCEPT_VMADDR_CID_ANY, ignoring: %m");
+                return (accepts_any = false);
+        }
+
+        return (accepts_any = r);
+}
+
+int vsock_get_local_cid(unsigned *ret) {
+        _cleanup_close_ int vsock_fd = -EBADF;
+
+        vsock_fd = open("/dev/vsock", O_RDONLY|O_CLOEXEC);
+        if (vsock_fd < 0)
+                return log_debug_errno(errno, "Failed to open %s: %m", "/dev/vsock");
+
+        unsigned tmp;
+        if (ioctl(vsock_fd, IOCTL_VM_SOCKETS_GET_LOCAL_CID, &tmp) < 0)
+                return log_debug_errno(errno, "Failed to query local AF_VSOCK CID: %m");
+        log_debug("Local AF_VSOCK CID: %u", tmp);
+
+        /* If ret == NULL, we just want to check if AF_VSOCK is available, so accept any
+         * address. Otherwise, filter out special addresses that cannot be used to identify
+         * _this_ machine from the outside. */
+        if (ret &&
+            (IN_SET(tmp, VMADDR_CID_LOCAL, VMADDR_CID_HOST) ||
+             (tmp == VMADDR_CID_ANY && smbios_get_accepts_any() <= 0)))
+                return log_debug_errno(SYNTHETIC_ERRNO(EADDRNOTAVAIL),
+                                       "IOCTL_VM_SOCKETS_GET_LOCAL_CID returned special value (%u), ignoring.", tmp);
+
+        if (ret)
+                *ret = tmp;
+        return 0;
+}

src/shared/vsock-util.h

@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <linux/vm_sockets.h> /* IWYU pragma: export */
+
+int vsock_get_local_cid(unsigned *ret);