Patch needed for RELENG_2_1 XENHVM on Xen 4.2 dom0

[A Former User]

I've run into a problem with using Xen PVHVM drivers on RELENG_2_1 on freeBSD 8.3.

I consistently get a "Fatal trap 12" error booting a XENHVM kernel on my Xen 4.2.3 dom0.  For reference, I'm using the default (current) Centos 6.4 Xen packages, installed from the Centos Base repository using the instructions http://wiki.centos.org/HowTos/Xen/Xen4QuickStart - so this is likely to affect other users.  One forum user here, sabrewarrior, has it running okay on the latest Xen 4.3, but I don't know which OS s/he's running for the dom0 (probably not Centos).

The solution to the "Fatal trap 12 error"  is to patch /usr/pfSensesrc/src/sys/xen/xenstore/xenstore.c with the following:

@@ -306,6 +306,10 @@
 {
 	const char **ret;

+	/* :XXX: */
+	if (len < 1)
+		return (NULL);
+
 	/* Protect against unterminated buffers. */
 	strings[len - 1] = '\0';

Here's the error and trace:

Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address   = 0xffffff010294c2ef
fault code              = supervisor write data, page not present
instruction pointer     = 0x20:0xffffffff80a56a68
stack pointer           = 0x28:0xffffffff816b19f0
frame pointer           = 0x28:0xffffffff816b1a30
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = trace trap, interrupt enabled, resume, IOPL = 0
current process         = 0 (swapper)

[A Former User]

Just an update - switching to a newer Xen 4.3 dom0 (Fedora 20) still has the same issue.

Patching xenstore.c using the above patch fixes the issue.