[Xen-changelog] [xen-unstable] x86: Prevent an infinite series of traps

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User kfraser@xxxxxxxxxxxxxxxxxxxxx
# Date 1179999568 -3600
# Node ID e49b110cbb4afa7186a5d666ac73b44a55584145
# Parent  96915ca8d5f239062f889970279c5d90296a8a96
x86: Prevent an infinite series of traps

In some cases, we can end up in a vicious cycle of fatal_trap()s
within fatal_trap()s. Panic after a certain number of attempts.

Signed-off-by: Nils Nieuwejaar <nils.nieuwejaar@xxxxxxx>

Use a per-cpu depth variable.

Signed-off-by: Keir Fraser <keir@xxxxxxxxxxxxx>
---
 xen/arch/x86/traps.c |   30 ++++++++++++++++++++----------
 1 files changed, 20 insertions(+), 10 deletions(-)

diff -r 96915ca8d5f2 -r e49b110cbb4a xen/arch/x86/traps.c
--- a/xen/arch/x86/traps.c      Thu May 24 10:18:55 2007 +0100
+++ b/xen/arch/x86/traps.c      Thu May 24 10:39:28 2007 +0100
@@ -345,16 +345,26 @@ char *trapstr(int trapnr)
  */
 asmlinkage void fatal_trap(int trapnr, struct cpu_user_regs *regs)
 {
-    watchdog_disable();
-    console_start_sync();
-
-    show_execution_state(regs);
-
-    if ( trapnr == TRAP_page_fault )
-    {
-        unsigned long cr2 = read_cr2();
-        printk("Faulting linear address: %p\n", _p(cr2));
-        show_page_walk(cr2);
+    static DEFINE_PER_CPU(char, depth);
+
+    /*
+     * In some cases, we can end up in a vicious cycle of fatal_trap()s
+     * within fatal_trap()s. We give the problem a couple of iterations to
+     * bottom out, and then we just panic.
+     */
+    if ( ++this_cpu(depth) < 3 )
+    {
+        watchdog_disable();
+        console_start_sync();
+
+        show_execution_state(regs);
+
+        if ( trapnr == TRAP_page_fault )
+        {
+            unsigned long cr2 = read_cr2();
+            printk("Faulting linear address: %p\n", _p(cr2));
+            show_page_walk(cr2);
+        }
     }
 
     panic("FATAL TRAP: vector = %d (%s)\n"

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