[patch] Check for SYSCALL on x86_64 after "spurious" step/trap

[patch] Check for SYSCALL on x86_64 after "spurious" step/trap

[Mark Wielaard]

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Hi,

On some x86_64 kernels you will get a trapped event after a step out of
a syscall, but the stepping flag isn't set in that case. This is at
least the case at least on 2.6.18-8.1.6.el5, but not on any of the
Fedora kernels I ever tested with, currently 2.6.20-1.2962.fc6.

Luckily the Isa has a way to signal such an event and this patch updates
the X86_64 Isa as follows to handle this:

2007-07-06  Mark Wielaard  <mwielaard@redhat.com>

    * IsaX8664.java hasExecutedSpuriousTrap): Check for SYSCALL
    instruction.

This makes the following testcases PASS on such kernels and doesn't
change the results on any other system I tested on:

testSteppingtestHitAndRun(frysk.proc.TestBreakpoints) ...PASS
testSteppingtestInsertRemove(frysk.proc.TestBreakpoints) ...PASS
testSteppingAddLots(frysk.proc.TestBreakpoints) ...PASS
testStepSigReturn(frysk.proc.TestTaskObserverInstructionSigReturn) ...PASS

Cheers,

Mark

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Index: frysk-core/frysk/proc/IsaX8664.java
===================================================================
RCS file: /cvs/frysk/frysk-core/frysk/proc/IsaX8664.java,v
retrieving revision 1.18
diff -u -r1.18 IsaX8664.java
--- frysk-core/frysk/proc/IsaX8664.java	5 Jul 2007 12:40:28 -0000	1.18
+++ frysk-core/frysk/proc/IsaX8664.java	6 Jul 2007 14:49:25 -0000
@@ -336,12 +336,15 @@
    * attributed to entering a signal handler or a normal step
    * instruction notification.
    * 
-   * x86_64 doesn't generate spurious trap events and this method
-   * always returns false on this architecture.
+   * On some kernels x86_64 doesn't generate spurious trap events (or
+   * rather doesn't set the stepping flag) after returning from a
+   * SYSCALL instruction.
    */
   public boolean hasExecutedSpuriousTrap(Task task)
   {
-    return false;
+    long address = pc(task);
+    return (task.getMemory().getByte(address - 1) == (byte) 0x05
+            && task.getMemory().getByte(address - 2) == (byte) 0x0f);
   }
 
   /**