[Xen-devel] [PATCH] X server crashes Xen on xeno-unstable.bk

[Leendert van Doorn <leendert@xxxxxxxxxxxxxx>]

[My platform is a VMX machine, but this is a generic problem in
xeno-unstable.bk. I use FC3 as a base and some onboard graphics
controller chipset]

Starting an X server on xeno-unstable.bk causes Xen to crash with
a page fault generated in Xen. It turns out that the fault is
generated by put_page_from_l1e().

The culprit is the following dereference:

     struct domain   *e = page->u.inuse.domain;

Where the value of page is undefined if the condition

     if ( !(l1v & _PAGE_PRESENT) || !pfn_is_ram(pfn) )

does hold. Unfortunately, this condition is checked after the dereference.

The fix is straightforward, move the dereference after the condition test.

        Leendert

--- xeno-unstable.bk/xen/arch/x86/memory.c.orig 2005-01-06 14:19:33.000000000 
-0500
+++ xeno-unstable.bk/xen/arch/x86/memory.c      2005-01-06 15:02:54.000000000 
-0500
@@ -467,11 +467,12 @@
     unsigned long    l1v  = l1_pgentry_val(l1e);
     unsigned long    pfn  = l1_pgentry_to_pagenr(l1e);
     struct pfn_info *page = &frame_table[pfn];
-    struct domain   *e = page->u.inuse.domain;
+    struct domain   *e;
 
     if ( !(l1v & _PAGE_PRESENT) || !pfn_is_ram(pfn) )
         return;
 
+    e = page->u.inuse.domain;
     if ( unlikely(e != d) )
     {
         /*


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