[PATCH v2 1/1] qemu: Allow sockets in long or deep paths.

[Nick Guenther <nick.guenther@xxxxxxxxxx>]

The qemu driver creates IPC sockets using absolute paths,
but under POSIX socket paths are constrained pretty tightly.
On systems with homedirs on an unusual mount point, like
network homedirs, or just particularly long usernames, this
could make starting VMs under qemu:///session impossible.

Resolves https://gitlab.com/libvirt/libvirt/-/issues/466

Signed-off-by: Nick Guenther <nick.guenther@xxxxxxxxxx>
---
 src/qemu/qemu_command.c | 72 ++++++++++++++++++++++++++++++++---------
 1 file changed, 57 insertions(+), 15 deletions(-)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 4ca93bf3dc..4bedbb515f 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -4860,12 +4860,64 @@ qemuBuildSCSIHostdevDevProps(const virDomainDef *def,
     return g_steal_pointer(&props);
 }
 
+struct qemuBindSocketData {
+    int fd;
+    char* path;
+};
+
+static int
+qemuBindSocket(pid_t ppid G_GNUC_UNUSED, void *opaque)
+{
+    /* The path length of a unix socket is limited to what fits in sockaddr_un.
+     * It's pretty short: 108 on Linux, and this is too easy to hit.
+     *
+     * Work around this limit by using a *relative path*, by chdir()ing first.
+     * But chdir() isn't thread-safe, so run it in a *subprocess* (this function)
+     * where the chdir() will be instantly forgotten once it has helped configure fd.
+     *
+     * background: https://stackoverflow.com/questions/34829600/why-is-the-maximal-path-length-allowed-for-unix-sockets-on-linux-108
+     */
+
+    g_autofree char *dir = NULL;
+    g_autofree char *name = NULL;
+
+    struct sockaddr_un addr;
+    struct qemuBindSocketData *data = opaque;
+
+    dir = g_path_get_dirname(data->path);
+    name = g_path_get_basename(data->path);
+
+    memset(&addr, 0, sizeof(addr));
+    addr.sun_family = AF_UNIX;
+    if (virStrcpyStatic(addr.sun_path, name) < 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("UNIX socket name '%1$s' too long"),
+                       name);
+        return -1;
+    }
+
+    if (chdir(dir) < 0) {
+        virReportSystemError(errno,
+                             _("Unable to chdir to containing directory '%1$s' while binding UNIX socket '%2$s'"),
+                             dir, name);
+        return -1;
+    }
+
+    if (bind(data->fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
+        virReportSystemError(errno,
+                             _("Unable to bind UNIX socket '%1$s/%2$s'"),
+                             dir, name);
+        return -1;
+    }
+
+    return 0;
+}
+
 int
 qemuOpenChrChardevUNIXSocket(const virDomainChrSourceDef *dev)
 {
-    struct sockaddr_un addr;
-    socklen_t addrlen = sizeof(addr);
     int fd;
+    struct qemuBindSocketData bindData;
 
     if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {
         virReportSystemError(errno, "%s",
@@ -4873,15 +4925,6 @@ qemuOpenChrChardevUNIXSocket(const virDomainChrSourceDef *dev)
         goto error;
     }
 
-    memset(&addr, 0, sizeof(addr));
-    addr.sun_family = AF_UNIX;
-    if (virStrcpyStatic(addr.sun_path, dev->data.nix.path) < 0) {
-        virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("UNIX socket path '%1$s' too long"),
-                       dev->data.nix.path);
-        goto error;
-    }
-
     if (unlink(dev->data.nix.path) < 0 && errno != ENOENT) {
         virReportSystemError(errno,
                              _("Unable to unlink %1$s"),
@@ -4889,10 +4932,9 @@ qemuOpenChrChardevUNIXSocket(const virDomainChrSourceDef *dev)
         goto error;
     }
 
-    if (bind(fd, (struct sockaddr *)&addr, addrlen) < 0) {
-        virReportSystemError(errno,
-                             _("Unable to bind to UNIX socket path '%1$s'"),
-                             dev->data.nix.path);
+    bindData.fd = fd;
+    bindData.path = dev->data.nix.path;
+    if (virProcessRunInFork(qemuBindSocket, &bindData) < 0) {
         goto error;
     }
 
-- 
2.34.1