Libguestfs - Feb 2013 - [PATCH v2 0/5] Fix SELinux security contexts so we can access shared disks (RHBZ#912499).

Link to version 1:
https://www.redhat.com/archives/libguestfs/2013-February/thread.html#00122

Changes since version 1:

 - I've pushed two (of the three) code refactoring patches.  The third
   one proved rather hard to move.

 - selinuxnorelabel option is no more.  Instead there is a second
   internal API (internal_set_libvirt_selinux_norelabel_disks).

 - fixed bogus commit message

 - various other very minor fixes

Rich.

From: "Richard W.M. Jones" <rjones at redhat.com>

This internal API sets two SELinux labels in the handle (the process
label and the image label -- they are closely related).

If using the libvirt attach-method with SELinux and sVirt, then this
will cause the following XML to be added to the appliance definition:

<seclabel type=static model=selinux relabel=yes>
  <label>[LABEL HERE]</label>
  <imagelabel>[IMAGELABEL HERE]</imagelabel>
</seclabel>

It is ignored by other attach-methods.
---
 generator/actions.ml   | 12 ++++++++++++
 src/guestfs-internal.h |  2 ++
 src/handle.c           |  2 ++
 src/launch-libvirt.c   | 36 ++++++++++++++++++++++++++++++++++++
 4 files changed, 52 insertions(+)

diff --git a/generator/actions.ml b/generator/actions.ml
index 8a8e3ff..59e667d 100644
--- a/generator/actions.ml
+++ b/generator/actions.ml
@@ -2694,6 +2694,18 @@ the default.  Else C</var/tmp> is the
default." };
     longdesc = "\
 Get the directory used by the handle to store the appliance cache." };
 
+  { defaults with
+    name = "internal_set_libvirt_selinux_label";
+    style = RErr, [String "label"; String "imagelabel"],
[];
+    blocking = false;
+    visibility = VInternal;
+    shortdesc = "set SELinux label used by the libvirt attach
method";
+    longdesc = "\
+This internal function sets the SELinux security label (in
+reality, two labels: the process label and the image label)
+used by the appliance when the libvirt attach method is selected
+(it is ignored by other attach methods)." };
+
 ]
 
 (* daemon_functions are any functions which cause some action
diff --git a/src/guestfs-internal.h b/src/guestfs-internal.h
index e1a7d31..78e2bf5 100644
--- a/src/guestfs-internal.h
+++ b/src/guestfs-internal.h
@@ -322,6 +322,8 @@ struct guestfs_h
     virDomainPtr dom;           /* libvirt domain */
   } virt;
 #endif
+  char *virt_selinux_label;
+  char *virt_selinux_imagelabel;
 };
 
 /* Per-filesystem data stored for inspect_os. */
diff --git a/src/handle.c b/src/handle.c
index c630daf..2f44632 100644
--- a/src/handle.c
+++ b/src/handle.c
@@ -326,6 +326,8 @@ guestfs_close (guestfs_h *g)
 
   if (g->pda)
     hash_free (g->pda);
+  free (g->virt_selinux_label);
+  free (g->virt_selinux_imagelabel);
   free (g->tmpdir);
   free (g->env_tmpdir);
   free (g->int_tmpdir);
diff --git a/src/launch-libvirt.c b/src/launch-libvirt.c
index 7db2ce5..68d875e 100644
--- a/src/launch-libvirt.c
+++ b/src/launch-libvirt.c
@@ -855,6 +855,31 @@ construct_libvirt_xml_seclabel (guestfs_h *g,
                                            BAD_CAST "none"));
     XMLERROR (-1, xmlTextWriterEndElement (xo));
   }
+  else if (g->virt_selinux_label && g->virt_selinux_imagelabel) {
+    /* Enable sVirt and pass a custom <seclabel/> inherited from the
+     * original libvirt domain (when guestfs_add_domain was called).
+     * https://bugzilla.redhat.com/show_bug.cgi?id=912499#c7
+     */
+    XMLERROR (-1, xmlTextWriterStartElement (xo, BAD_CAST
"seclabel"));
+    XMLERROR (-1,
+              xmlTextWriterWriteAttribute (xo, BAD_CAST "type",
+                                           BAD_CAST "static"));
+    XMLERROR (-1,
+              xmlTextWriterWriteAttribute (xo, BAD_CAST "model",
+                                           BAD_CAST "selinux"));
+    XMLERROR (-1,
+              xmlTextWriterWriteAttribute (xo, BAD_CAST "relabel",
+                                           BAD_CAST "yes"));
+    XMLERROR (-1, xmlTextWriterStartElement (xo, BAD_CAST "label"));
+    XMLERROR (-1, xmlTextWriterWriteString (xo,
+                                            BAD_CAST
g->virt_selinux_label));
+    XMLERROR (-1, xmlTextWriterEndElement (xo));
+    XMLERROR (-1, xmlTextWriterStartElement (xo, BAD_CAST
"imagelabel"));
+    XMLERROR (-1, xmlTextWriterWriteString (xo,
+                                            BAD_CAST
g->virt_selinux_imagelabel));
+    XMLERROR (-1, xmlTextWriterEndElement (xo));
+    XMLERROR (-1, xmlTextWriterEndElement (xo));
+  }
 
   return 0;
 }
@@ -1603,3 +1628,14 @@ struct attach_ops attach_ops_libvirt = {
 };
 
 #endif /* no libvirt or libxml2 at compile time */
+
+int
+guestfs__internal_set_libvirt_selinux_label (guestfs_h *g, const char *label,
+                                             const char *imagelabel)
+{
+  free (g->virt_selinux_label);
+  g->virt_selinux_label = safe_strdup (g, label);
+  free (g->virt_selinux_imagelabel);
+  g->virt_selinux_imagelabel = safe_strdup (g, imagelabel);
+  return 0;
+}
-- 
1.8.1.2

From: "Richard W.M. Jones" <rjones at redhat.com>

If set, this causes <seclabel model=selinux relabel=no> to be added to
the disk element in the libvirt XML.

It has no effect *except* on the libvirt attach method when SELinux
and sVirt is being used.
---
 generator/actions.ml   | 11 +++++++++++
 src/guestfs-internal.h |  1 +
 src/launch-libvirt.c   | 30 ++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/generator/actions.ml b/generator/actions.ml
index 59e667d..f685e92 100644
--- a/generator/actions.ml
+++ b/generator/actions.ml
@@ -2706,6 +2706,17 @@ reality, two labels: the process label and the image
label)
 used by the appliance when the libvirt attach method is selected
 (it is ignored by other attach methods)." };
 
+  { defaults with
+    name = "internal_set_libvirt_selinux_norelabel_disks";
+    style = RErr, [Bool "norelabeldisks"], [];
+    blocking = false;
+    visibility = VInternal;
+    shortdesc = "tell libvirt attach method not to relabel disks";
+    longdesc = "\
+This internal function adds E<lt>seclabel model=selinux
relabel=noE<gt>
+to all application disks.  It is only used by the libvirt attach method
+and is ignored by other attach methods." };
+
 ]
 
 (* daemon_functions are any functions which cause some action
diff --git a/src/guestfs-internal.h b/src/guestfs-internal.h
index 78e2bf5..c25b893 100644
--- a/src/guestfs-internal.h
+++ b/src/guestfs-internal.h
@@ -324,6 +324,7 @@ struct guestfs_h
 #endif
   char *virt_selinux_label;
   char *virt_selinux_imagelabel;
+  bool virt_selinux_norelabel_disks;
 };
 
 /* Per-filesystem data stored for inspect_os. */
diff --git a/src/launch-libvirt.c b/src/launch-libvirt.c
index 68d875e..318847a 100644
--- a/src/launch-libvirt.c
+++ b/src/launch-libvirt.c
@@ -651,6 +651,7 @@ static int construct_libvirt_xml_lifecycle (guestfs_h *g,
const struct libvirt_x
 static int construct_libvirt_xml_devices (guestfs_h *g, const struct
libvirt_xml_params *params, xmlTextWriterPtr xo);
 static int construct_libvirt_xml_qemu_cmdline (guestfs_h *g, const struct
libvirt_xml_params *params, xmlTextWriterPtr xo);
 static int construct_libvirt_xml_disk (guestfs_h *g, xmlTextWriterPtr xo,
struct drive *drv, size_t drv_index);
+static int construct_libvirt_xml_disk_source_seclabel (guestfs_h *g,
xmlTextWriterPtr xo);
 static int construct_libvirt_xml_appliance (guestfs_h *g, const struct
libvirt_xml_params *params, xmlTextWriterPtr xo);
 
 /* Note this macro is rather specialized: It assumes that any local
@@ -1035,6 +1036,8 @@ construct_libvirt_xml_disk (guestfs_h *g,
     XMLERROR (-1,
               xmlTextWriterWriteAttribute (xo, BAD_CAST "file",
                                            BAD_CAST drv_priv->path));
+    if (construct_libvirt_xml_disk_source_seclabel (g, xo) == -1)
+      return -1;
     XMLERROR (-1, xmlTextWriterEndElement (xo));
   }
   else {
@@ -1046,6 +1049,8 @@ construct_libvirt_xml_disk (guestfs_h *g,
     XMLERROR (-1,
               xmlTextWriterWriteAttribute (xo, BAD_CAST "dev",
                                            BAD_CAST drv_priv->path));
+    if (construct_libvirt_xml_disk_source_seclabel (g, xo) == -1)
+      return -1;
     XMLERROR (-1, xmlTextWriterEndElement (xo));
   }
 
@@ -1131,6 +1136,24 @@ construct_libvirt_xml_disk (guestfs_h *g,
 }
 
 static int
+construct_libvirt_xml_disk_source_seclabel (guestfs_h *g,
+                                            xmlTextWriterPtr xo)
+{
+  if (g->virt_selinux_norelabel_disks) {
+    XMLERROR (-1, xmlTextWriterStartElement (xo, BAD_CAST
"seclabel"));
+    XMLERROR (-1,
+              xmlTextWriterWriteAttribute (xo, BAD_CAST "model",
+                                           BAD_CAST "selinux"));
+    XMLERROR (-1,
+              xmlTextWriterWriteAttribute (xo, BAD_CAST "relabel",
+                                           BAD_CAST "no"));
+    XMLERROR (-1, xmlTextWriterEndElement (xo));
+  }
+
+  return 0;
+}
+
+static int
 construct_libvirt_xml_appliance (guestfs_h *g,
                                  const struct libvirt_xml_params *params,
                                  xmlTextWriterPtr xo)
@@ -1639,3 +1662,10 @@ guestfs__internal_set_libvirt_selinux_label (guestfs_h
*g, const char *label,
   g->virt_selinux_imagelabel = safe_strdup (g, imagelabel);
   return 0;
 }
+
+int
+guestfs__internal_set_libvirt_selinux_norelabel_disks (guestfs_h *g, int flag)
+{
+  g->virt_selinux_norelabel_disks = flag;
+  return 0;
+}
-- 
1.8.1.2

From: "Richard W.M. Jones" <rjones at redhat.com>

When a disk is opened readonly, the libvirt attach-method privately
creates a qcow2 overlay on top.

This commit lets that overlay get an SELinux label, and sets it to the
imagelabel specified by guestfs_internal_set_libvirt_selinux_label.

The above only applies to the libvirt attach-method.
---
 src/launch-libvirt.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/src/launch-libvirt.c b/src/launch-libvirt.c
index 318847a..b692fd6 100644
--- a/src/launch-libvirt.c
+++ b/src/launch-libvirt.c
@@ -133,8 +133,8 @@ static int is_custom_qemu (guestfs_h *g);
 static int is_blk (const char *path);
 static int random_chars (char *ret, size_t len);
 static void ignore_errors (void *ignore, virErrorPtr ignore2);
-static char *make_qcow2_overlay (guestfs_h *g, const char *path, const char
*format);
-static int make_qcow2_overlay_for_drive (guestfs_h *g, struct drive *drv);
+static char *make_qcow2_overlay (guestfs_h *g, const char *path, const char
*format, const char *selinux_imagelabel);
+static int make_qcow2_overlay_for_drive (guestfs_h *g, struct drive *drv, const
char *selinux_imagelabel);
 static void drive_free_priv (void *);
 static void set_socket_create_context (guestfs_h *g);
 static void clear_socket_create_context (guestfs_h *g);
@@ -235,13 +235,13 @@ launch_libvirt (guestfs_h *g, const char *libvirt_uri)
    * Note that appliance can be NULL if using the old-style appliance.
    */
   if (appliance) {
-    params.appliance_overlay = make_qcow2_overlay (g, appliance,
"raw");
+    params.appliance_overlay = make_qcow2_overlay (g, appliance,
"raw", NULL);
     if (!params.appliance_overlay)
       goto cleanup;
   }
 
   ITER_DRIVES (g, i, drv) {
-    if (make_qcow2_overlay_for_drive (g, drv) == -1)
+    if (make_qcow2_overlay_for_drive (g, drv, g->virt_selinux_imagelabel) ==
-1)
       goto cleanup;
   }
 
@@ -1353,7 +1353,8 @@ ignore_errors (void *ignore, virErrorPtr ignore2)
 
 /* Create a temporary qcow2 overlay on top of 'path'. */
 static char *
-make_qcow2_overlay (guestfs_h *g, const char *path, const char *format)
+make_qcow2_overlay (guestfs_h *g, const char *path, const char *format,
+                    const char *selinux_imagelabel)
 {
   char *tmpfile = NULL;
   CLEANUP_CMD_CLOSE struct command *cmd = guestfs___new_command (g);
@@ -1384,6 +1385,15 @@ make_qcow2_overlay (guestfs_h *g, const char *path, const
char *format)
     goto error;
   }
 
+#if HAVE_LIBSELINUX
+  if (selinux_imagelabel) {
+    debug (g, "setting SELinux label on %s to %s",
+           tmpfile, selinux_imagelabel);
+    if (setfilecon (tmpfile, (security_context_t) selinux_imagelabel) == -1)
+      selinux_warning (g, __func__, "setfilecon", tmpfile);
+  }
+#endif
+
   return tmpfile;               /* caller frees */
 
  error:
@@ -1393,7 +1403,8 @@ make_qcow2_overlay (guestfs_h *g, const char *path, const
char *format)
 }
 
 static int
-make_qcow2_overlay_for_drive (guestfs_h *g, struct drive *drv)
+make_qcow2_overlay_for_drive (guestfs_h *g, struct drive *drv,
+                              const char *selinux_imagelabel)
 {
   char *path;
   struct drive_libvirt *drv_priv;
@@ -1417,7 +1428,8 @@ make_qcow2_overlay_for_drive (guestfs_h *g, struct drive
*drv)
     drv_priv->format = drv->format ? safe_strdup (g, drv->format) :
NULL;
   }
   else {
-    drv_priv->path = make_qcow2_overlay (g, path, drv->format);
+    drv_priv->path = make_qcow2_overlay (g, path, drv->format,
+                                         selinux_imagelabel);
     free (path);
     if (!drv_priv->path)
       return -1;
@@ -1534,7 +1546,7 @@ hot_add_drive_libvirt (guestfs_h *g, struct drive *drv,
size_t drv_index)
   /* Create overlay for read-only drive.  This works around lack of
    * support for <transient/> disks in libvirt.
    */
-  if (make_qcow2_overlay_for_drive (g, drv) == -1)
+  if (make_qcow2_overlay_for_drive (g, drv, g->virt_selinux_imagelabel) ==
-1)
     return -1;
 
   /* Create the XML for the new disk. */
-- 
1.8.1.2

From: "Richard W.M. Jones" <rjones at redhat.com>

When adding a domain (ie. guestfs_add_domain), read the SELinux
<label/> and <imagelabel/> from the guest and use them for the
appliance.  The appliance is statically labelled the same as the
guest, so it is able to read its disks.

However tell libvirt not to try relabelling the disks, to prevent
libvirt from disturbing the existing labels on the disks (in
particular when the libvirt connection is closed, we don't want
libvirt to try to restore some other label on the disks).
---
 src/libvirt-domain.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/src/libvirt-domain.c b/src/libvirt-domain.c
index 3f5b1ed..768ed78 100644
--- a/src/libvirt-domain.c
+++ b/src/libvirt-domain.c
@@ -20,6 +20,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdbool.h>
 #include <assert.h>
 
 #ifdef HAVE_LIBVIRT
@@ -42,6 +43,7 @@
 
 static xmlDocPtr get_domain_xml (guestfs_h *g, virDomainPtr dom);
 static ssize_t for_each_disk (guestfs_h *g, xmlDocPtr doc, int (*f) (guestfs_h
*g, const char *filename, const char *format, int readonly, void *data), void
*data);
+static int libvirt_selinux_label (guestfs_h *g, xmlDocPtr doc, char
**label_rtn, char **imagelabel_rtn);
 
 static void
 ignore_errors (void *ignore, virErrorPtr ignore2)
@@ -169,6 +171,7 @@ guestfs___add_libvirt_dom (guestfs_h *g, virDomainPtr dom,
   size_t ckp;
   struct add_disk_data data;
   CLEANUP_XMLFREEDOC xmlDocPtr doc = NULL;
+  CLEANUP_FREE char *label = NULL, *imagelabel = NULL;
 
   readonly      optargs->bitmask &
GUESTFS___ADD_LIBVIRT_DOM_READONLY_BITMASK
@@ -234,6 +237,16 @@ guestfs___add_libvirt_dom (guestfs_h *g, virDomainPtr dom,
   if ((doc = get_domain_xml (g, dom)) == NULL)
     return -1;
 
+  /* Find and pass the SELinux security label to the libvirt back end. */
+  if (libvirt_selinux_label (g, doc, &label, &imagelabel) == -1)
+    return -1;
+  if (label && imagelabel) {
+    guestfs_internal_set_libvirt_selinux_label (g, label, imagelabel);
+    guestfs_internal_set_libvirt_selinux_norelabel_disks (g, 1);
+  }
+  else
+    guestfs_internal_set_libvirt_selinux_norelabel_disks (g, 0);
+
   /* Add the disks. */
   data.optargs.bitmask = 0;
   data.readonly = readonly;
@@ -374,6 +387,66 @@ connect_live (guestfs_h *g, virDomainPtr dom)
   return guestfs_set_attach_method (g, attach_method);
 }
 
+/* Find the <seclabel/> element in the libvirt XML, and if it exists
+ * get the SELinux process label and image label from it.
+ *
+ * The reason for all this is because of sVirt:
+ * https://bugzilla.redhat.com/show_bug.cgi?id=912499#c7
+ */
+static int
+libvirt_selinux_label (guestfs_h *g, xmlDocPtr doc,
+                       char **label_rtn, char **imagelabel_rtn)
+{
+  CLEANUP_XMLXPATHFREECONTEXT xmlXPathContextPtr xpathCtx = NULL;
+  CLEANUP_XMLXPATHFREEOBJECT xmlXPathObjectPtr xpathObj = NULL;
+  xmlNodeSetPtr nodes;
+  size_t nr_nodes;
+  xmlNodePtr node, child;
+  bool gotlabel = 0, gotimagelabel = 0;
+
+  xpathCtx = xmlXPathNewContext (doc);
+  if (xpathCtx == NULL) {
+    error (g, _("unable to create new XPath context"));
+    return -1;
+  }
+
+  /* This gives us a set of all the <seclabel/> nodes, hopefully only
one. */
+  xpathObj = xmlXPathEvalExpression (BAD_CAST "/domain/seclabel",
+                                     xpathCtx);
+  if (xpathObj == NULL) {
+    error (g, _("unable to evaluate XPath expression"));
+    return -1;
+  }
+
+  nodes = xpathObj->nodesetval;
+  nr_nodes = nodes->nodeNr;
+
+  if (nr_nodes == 0 || nr_nodes > 1)
+    return 0;
+
+  node = nodes->nodeTab[0];
+  if (node->type != XML_ELEMENT_NODE) {
+    error (g, _("expected <seclabel/> to be an XML element"));
+    return -1;
+  }
+
+  /* Find the <label/> and <imagelabel/> child nodes. */
+  for (child = node->children; child != NULL; child = child->next) {
+    if (!gotlabel && STREQ ((char *) child->name,
"label")) {
+      /* Get the label content. */
+      *label_rtn = (char *) xmlNodeGetContent (child);
+      gotlabel = 1;
+    }
+    if (!gotimagelabel && STREQ ((char *) child->name,
"imagelabel")) {
+      /* Get the imagelabel content. */
+      *imagelabel_rtn = (char *) xmlNodeGetContent (child);
+      gotimagelabel = 1;
+    }
+  }
+
+  return 0;
+}
+
 /* The callback function 'f' is called once for each disk.
  *
  * Returns number of disks, or -1 if there was an error.
-- 
1.8.1.2

From: "Richard W.M. Jones" <rjones at redhat.com>

This is just code motion.
---
 src/libvirt-domain.c | 138 +++++++++++++++++++++++++--------------------------
 1 file changed, 69 insertions(+), 69 deletions(-)

diff --git a/src/libvirt-domain.c b/src/libvirt-domain.c
index 768ed78..e2068dd 100644
--- a/src/libvirt-domain.c
+++ b/src/libvirt-domain.c
@@ -318,75 +318,6 @@ add_disk (guestfs_h *g,
   return guestfs__add_drive_opts (g, filename, &optargs);
 }
 
-static int
-connect_live (guestfs_h *g, virDomainPtr dom)
-{
-  int i;
-  CLEANUP_XMLFREEDOC xmlDocPtr doc = NULL;
-  CLEANUP_XMLXPATHFREECONTEXT xmlXPathContextPtr xpathCtx = NULL;
-  CLEANUP_XMLXPATHFREEOBJECT xmlXPathObjectPtr xpathObj = NULL;
-  CLEANUP_FREE char *path = NULL, *attach_method = NULL;
-  xmlNodeSetPtr nodes;
-
-  /* Domain XML. */
-  if ((doc = get_domain_xml (g, dom)) == NULL)
-    return -1;
-
-  xpathCtx = xmlXPathNewContext (doc);
-  if (xpathCtx == NULL) {
-    error (g, _("unable to create new XPath context"));
-    return -1;
-  }
-
-  /* This gives us a set of all the <channel> nodes related to the
-   * guestfsd virtio-serial channel.
-   */
-  xpathObj = xmlXPathEvalExpression (BAD_CAST
-      "//devices/channel[@type=\"unix\" and "
-                        "./source/@mode=\"bind\" and "
-                        "./source/@path and "
-                        "./target/@type=\"virtio\" and "
-                       
"./target/@name=\"org.libguestfs.channel.0\"]",
-                                     xpathCtx);
-  if (xpathObj == NULL) {
-    error (g, _("unable to evaluate XPath expression"));
-    return -1;
-  }
-
-  nodes = xpathObj->nodesetval;
-  for (i = 0; i < nodes->nodeNr; ++i) {
-    CLEANUP_XMLXPATHFREEOBJECT xmlXPathObjectPtr xppath = NULL;
-    xmlAttrPtr attr;
-
-    /* See note in function above. */
-    xpathCtx->node = nodes->nodeTab[i];
-
-    /* The path is in <source path=..> attribute. */
-    xppath = xmlXPathEvalExpression (BAD_CAST "./source/@path",
xpathCtx);
-    if (xppath == NULL ||
-        xppath->nodesetval == NULL ||
-        xppath->nodesetval->nodeNr == 0) {
-      xmlXPathFreeObject (xppath);
-      continue;                 /* no type attribute, skip it */
-    }
-    assert (xppath->nodesetval->nodeTab[0]);
-    assert (xppath->nodesetval->nodeTab[0]->type ==
XML_ATTRIBUTE_NODE);
-    attr = (xmlAttrPtr) xppath->nodesetval->nodeTab[0];
-    path = (char *) xmlNodeListGetString (doc, attr->children, 1);
-    break;
-  }
-
-  if (path == NULL) {
-    error (g, _("this guest has no libvirt <channel> definition for
guestfsd\n"
-                "See ATTACHING TO RUNNING DAEMONS in guestfs(3) for
further information."));
-    return -1;
-  }
-
-  /* Got a path. */
-  attach_method = safe_asprintf (g, "unix:%s", path);
-  return guestfs_set_attach_method (g, attach_method);
-}
-
 /* Find the <seclabel/> element in the libvirt XML, and if it exists
  * get the SELinux process label and image label from it.
  *
@@ -580,6 +511,75 @@ for_each_disk (guestfs_h *g,
   return nr_added;
 }
 
+static int
+connect_live (guestfs_h *g, virDomainPtr dom)
+{
+  int i;
+  CLEANUP_XMLFREEDOC xmlDocPtr doc = NULL;
+  CLEANUP_XMLXPATHFREECONTEXT xmlXPathContextPtr xpathCtx = NULL;
+  CLEANUP_XMLXPATHFREEOBJECT xmlXPathObjectPtr xpathObj = NULL;
+  CLEANUP_FREE char *path = NULL, *attach_method = NULL;
+  xmlNodeSetPtr nodes;
+
+  /* Domain XML. */
+  if ((doc = get_domain_xml (g, dom)) == NULL)
+    return -1;
+
+  xpathCtx = xmlXPathNewContext (doc);
+  if (xpathCtx == NULL) {
+    error (g, _("unable to create new XPath context"));
+    return -1;
+  }
+
+  /* This gives us a set of all the <channel> nodes related to the
+   * guestfsd virtio-serial channel.
+   */
+  xpathObj = xmlXPathEvalExpression (BAD_CAST
+      "//devices/channel[@type=\"unix\" and "
+                        "./source/@mode=\"bind\" and "
+                        "./source/@path and "
+                        "./target/@type=\"virtio\" and "
+                       
"./target/@name=\"org.libguestfs.channel.0\"]",
+                                     xpathCtx);
+  if (xpathObj == NULL) {
+    error (g, _("unable to evaluate XPath expression"));
+    return -1;
+  }
+
+  nodes = xpathObj->nodesetval;
+  for (i = 0; i < nodes->nodeNr; ++i) {
+    CLEANUP_XMLXPATHFREEOBJECT xmlXPathObjectPtr xppath = NULL;
+    xmlAttrPtr attr;
+
+    /* See note in function above. */
+    xpathCtx->node = nodes->nodeTab[i];
+
+    /* The path is in <source path=..> attribute. */
+    xppath = xmlXPathEvalExpression (BAD_CAST "./source/@path",
xpathCtx);
+    if (xppath == NULL ||
+        xppath->nodesetval == NULL ||
+        xppath->nodesetval->nodeNr == 0) {
+      xmlXPathFreeObject (xppath);
+      continue;                 /* no type attribute, skip it */
+    }
+    assert (xppath->nodesetval->nodeTab[0]);
+    assert (xppath->nodesetval->nodeTab[0]->type ==
XML_ATTRIBUTE_NODE);
+    attr = (xmlAttrPtr) xppath->nodesetval->nodeTab[0];
+    path = (char *) xmlNodeListGetString (doc, attr->children, 1);
+    break;
+  }
+
+  if (path == NULL) {
+    error (g, _("this guest has no libvirt <channel> definition for
guestfsd\n"
+                "See ATTACHING TO RUNNING DAEMONS in guestfs(3) for
further information."));
+    return -1;
+  }
+
+  /* Got a path. */
+  attach_method = safe_asprintf (g, "unix:%s", path);
+  return guestfs_set_attach_method (g, attach_method);
+}
+
 static xmlDocPtr
 get_domain_xml (guestfs_h *g, virDomainPtr dom)
 {
-- 
1.8.1.2