I have some code (below) which segfaults because xmlXPathEvalExpression
returns:
(gdb) print *labelObj
$1 = {type = XPATH_NODESET, nodesetval = 0x0, boolval = 0, floatval = 0,
stringval = 0x0, user = 0x0, index = 0, user2 = 0x0, index2 = 0}
I have previously thought it was impossible for xmlXPathEvalExpression
to return something != NULL but with nodesetval == NULL, but obviously
it can do this.
But what does it mean?
It doesn't seem to mean "no nodes", because when I use the standalone
'xpath' program (which AIUI uses libxml2) it returns a single matching
node.
Rich.
libxml2-2.9.0-3.fc18.x86_64
XPath expr: /domain/seclabel[@model='selinux']/label
----------------------------------------------------------------------
Code:
static int
libvirt_selinux_label (guestfs_h *g, xmlDocPtr doc,
char **label_rtn, char **imagelabel_rtn)
{
CLEANUP_XMLXPATHFREECONTEXT xmlXPathContextPtr xpathCtx = NULL;
CLEANUP_XMLXPATHFREEOBJECT xmlXPathObjectPtr labelObj = NULL;
CLEANUP_XMLXPATHFREEOBJECT xmlXPathObjectPtr imagelabelObj = NULL;
xmlNodeSetPtr nodes;
size_t nr_nodes;
xmlNodePtr node;
xpathCtx = xmlXPathNewContext (doc);
if (xpathCtx == NULL) {
error (g, _("unable to create new XPath context"));
return -1;
}
const char *label_xpath =
"/domain/seclabel[@model='selinux']/label";
const char *imagelabel_xpath =
"/domain/seclabel[@model='selinux']/imagelabel";
labelObj = xmlXPathEvalExpression (BAD_CAST label_xpath, xpathCtx);
if (labelObj == NULL) {
error (g, _("unable to evaluate XPath expression"));
return -1;
}
/* Process <label> */
nodes = labelObj->nodesetval;
nr_nodes = nodes->nodeNr; /**** segfault here ****/
if (nr_nodes == 0)
return 0;
if (nr_nodes > 1) {
debug (g, _("ignoring multiple nodes matching '%s'"), label_xpath);
return 0;
}
node = nodes->nodeTab[0];
if (node->type != XML_ELEMENT_NODE) {
error (g, _("expected <label/> to be an XML element"));
return -1;
}
*label_rtn = (char *) xmlNodeGetContent (node);
/* Same as above for <imagelabel> */
imagelabelObj = xmlXPathEvalExpression (BAD_CAST imagelabel_xpath, xpathCtx);
if (imagelabelObj == NULL) {
error (g, _("unable to evaluate XPath expression"));
return -1;
}
nodes = imagelabelObj->nodesetval;
nr_nodes = nodes->nodeNr;
if (nr_nodes == 0)
return 0;
if (nr_nodes > 1) {
debug (g, _("ignoring multiple nodes matching '%s'"), imagelabel_xpath);
return 0;
}
node = nodes->nodeTab[0];
if (node->type != XML_ELEMENT_NODE) {
error (g, _("expected <imagelabel/> to be an XML element"));
return -1;
}
*imagelabel_rtn = (char *) xmlNodeGetContent (node);
return 0;
}
----------------------------------------------------------------------
XML:
<domain type='kvm' id='151'>
<name>f19rawhidex64</name>
<uuid>2697b7dd-4bcc-c828-2fc7-8a717f19ee80</uuid>
<memory unit='KiB'>2097152</memory>
<currentMemory unit='KiB'>2097152</currentMemory>
<vcpu placement='static'>1</vcpu>
<os>
<type arch='x86_64' machine='pc-1.2'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<cpu mode='custom' match='exact'>
<model fallback='allow'>SandyBridge</model>
<vendor>Intel</vendor>
<feature policy='require' name='tm2'/>
<feature policy='require' name='est'/>
<feature policy='require' name='monitor'/>
<feature policy='require' name='osxsave'/>
<feature policy='require' name='smx'/>
<feature policy='require' name='ss'/>
<feature policy='require' name='vme'/>
<feature policy='require' name='dtes64'/>
<feature policy='require' name='ht'/>
<feature policy='require' name='ds'/>
<feature policy='require' name='pbe'/>
<feature policy='require' name='tm'/>
<feature policy='require' name='pdcm'/>
<feature policy='require' name='vmx'/>
<feature policy='require' name='ds_cpl'/>
<feature policy='require' name='xtpr'/>
<feature policy='require' name='acpi'/>
</cpu>
<clock offset='utc'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/qemu-system-x86_64</emulator>
<disk type='block' device='disk'>
<driver name='qemu' type='raw'/>
<source dev='/dev/fedora/f19rawhidex64'/>
<target dev='vda' bus='virtio'/>
<alias name='virtio-disk0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04'
function='0x0'/>
</disk>
<disk type='block' device='cdrom'>
<driver name='qemu' type='raw'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<alias name='ide0-1-0'/>
<address type='drive' controller='0' bus='1' target='0' unit='0'/>
</disk>
<controller type='usb' index='0'>
<alias name='usb0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01'
function='0x2'/>
</controller>
<controller type='ide' index='0'>
<alias name='ide0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01'
function='0x1'/>
</controller>
<interface type='network'>
<mac address='52:54:00:a8:ba:f0'/>
<source network='default'/>
<target dev='vnet0'/>
<model type='virtio'/>
<alias name='net0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03'
function='0x0'/>
</interface>
<serial type='pty'>
<source path='/dev/pts/16'/>
<target port='0'/>
<alias name='serial0'/>
</serial>
<console type='pty' tty='/dev/pts/16'>
<source path='/dev/pts/16'/>
<target type='serial' port='0'/>
<alias name='serial0'/>
</console>
<input type='tablet' bus='usb'>
<alias name='input0'/>
</input>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='5900' autoport='yes' listen='127.0.0.1'>
<listen type='address' address='127.0.0.1'/>
</graphics>
<video>
<model type='cirrus' vram='9216' heads='1'/>
<alias name='video0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02'
function='0x0'/>
</video>
<memballoon model='virtio'>
<alias name='balloon0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05'
function='0x0'/>
</memballoon>
</devices>
<seclabel type='dynamic' model='selinux' relabel='yes'>
<label>system_u:system_r:svirt_t:s0:c117,c281</label>
<imagelabel>system_u:object_r:svirt_image_t:s0:c117,c281</imagelabel>
</seclabel>
</domain>
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines. Supports shell scripting,
bindings from many languages. http://libguestfs.org
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