[libvirt PATCH] 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 | 52 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 49 insertions(+), 3 deletions(-)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 4ca93bf3dc..3f180d5fb6 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -53,6 +53,7 @@
 #include "virmdev.h"
 #include "virutil.h"
 
+#include <libgen.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 
@@ -4866,6 +4867,37 @@ qemuOpenChrChardevUNIXSocket(const virDomainChrSourceDef *dev)
     struct sockaddr_un addr;
     socklen_t addrlen = sizeof(addr);
     int fd;
+    char* wd = NULL;
+    char* socket_dir_c = NULL;
+    char* socket_name_c = NULL;
+    char* socket_dir = NULL;
+    char* socket_name = NULL;
+
+    /* The path length 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*.
+     *
+     * background: https://stackoverflow.com/questions/34829600/why-is-the-maximal-path-length-allowed-for-unix-sockets-on-linux-108
+     *
+     * docker added a different workaround: https://github.com/moby/moby/pull/13408
+     */
+    if ((wd = getcwd(NULL, 0)) == NULL) {
+        virReportSystemError(errno, "%s",
+                             _("Unable to get working directory"));
+	goto error;
+    }
+
+    socket_dir_c = strdup(dev->data.nix.path); // dirname edits the string given it, so it must be copied
+    socket_name_c = strdup(dev->data.nix.path);
+
+    socket_dir = dirname(socket_dir_c);
+    socket_name = basename(socket_name_c);
+
+    if (chdir(socket_dir) < 0) {
+        virReportSystemError(errno, "%s",
+                             _("Unable to get change to socket directory"));
+	goto error;
+    }
 
     if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {
         virReportSystemError(errno, "%s",
@@ -4875,10 +4907,10 @@ qemuOpenChrChardevUNIXSocket(const virDomainChrSourceDef *dev)
 
     memset(&addr, 0, sizeof(addr));
     addr.sun_family = AF_UNIX;
-    if (virStrcpyStatic(addr.sun_path, dev->data.nix.path) < 0) {
+    if (virStrcpyStatic(addr.sun_path, socket_name) < 0) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("UNIX socket path '%1$s' too long"),
-                       dev->data.nix.path);
+                       _("UNIX socket name '%1$s' too long"),
+                       socket_name);
         goto error;
     }
 
@@ -4909,9 +4941,23 @@ qemuOpenChrChardevUNIXSocket(const virDomainChrSourceDef *dev)
     if (virFileUpdatePerm(dev->data.nix.path, 0002, 0664) < 0)
         goto error;
 
+    /* restore working directory */
+    if (chdir(wd) < 0) {
+        virReportSystemError(errno, "%s",
+                             _("Unable to restore working directory"));
+        goto error;
+    }
+
+    free(socket_name_c);
+    free(socket_dir_c);
+    free(wd);
+
     return fd;
 
  error:
+    if (socket_name_c != NULL) { free(socket_name_c); }
+    if (socket_dir_c != NULL) { free(socket_dir_c); }
+    if (wd != NULL) { free(wd); }
     VIR_FORCE_CLOSE(fd);
     return -1;
 }
-- 
2.34.1