From 873112ea9924074212f1dd667d345850c6dce789 Mon Sep 17 00:00:00 2001
From: Gabe Black <gblack@eecs.umich.edu>
Date: Wed, 29 Jul 2009 00:14:43 -0700
Subject: ARM: Make sure the target process doesn't run away from statetrace.

---
 src/arch/arm/nativetrace.cc            |  7 ++++++-
 util/statetrace/arch/tracechild_arm.cc | 36 ++++++++++++++++++++--------------
 2 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/src/arch/arm/nativetrace.cc b/src/arch/arm/nativetrace.cc
index 7c9c70e0c..90c5e5c25 100644
--- a/src/arch/arm/nativetrace.cc
+++ b/src/arch/arm/nativetrace.cc
@@ -104,8 +104,13 @@ Trace::ArmNativeTrace::ThreadState::update(ThreadContext *tc)
 void
 Trace::ArmNativeTrace::check(NativeTraceRecord *record)
 {
+    ThreadContext *tc = record->getThread();
+    // This area is read only on the target. It can't stop there to tell us
+    // what's going on, so we should skip over anything there also.
+    if (tc->readNextPC() > 0xffff0000)
+        return;
     nState.update(this);
-    mState.update(record->getThread());
+    mState.update(tc);
 
     bool errorFound = false;
     // Regular int regs
diff --git a/util/statetrace/arch/tracechild_arm.cc b/util/statetrace/arch/tracechild_arm.cc
index 4687b46f0..b52c0694a 100644
--- a/util/statetrace/arch/tracechild_arm.cc
+++ b/util/statetrace/arch/tracechild_arm.cc
@@ -203,22 +203,28 @@ ostream & ARMTraceChild::outputStartState(ostream & os)
 
 bool ARMTraceChild::step()
 {
+    const uint32_t bkpt_inst = 0xe7f001f0;
 
-    const uint32_t bkpt_inst = 0xE1200070;
-    const uint32_t bkpt_mask = 0xFFF000F0;
-
-    const uint32_t swi_inst = 0x0F0000000;
-    const uint32_t swi_mask = 0x0F0000000;
-
-    uint32_t next_op = ptrace(PTRACE_PEEKDATA, pid, getPC(), 0);
-    if ((next_op & swi_mask) == swi_inst) {
-        ptrace(PTRACE_POKEDATA, pid, next_op + sizeof(uint32_t), bkpt_inst);
-        ptraceSingleStep();
-        ptrace(PTRACE_POKEDATA, pid, next_op + sizeof(uint32_t), next_op);
-    } 
-    else
-    {
-        ptraceSingleStep();
+    uint32_t lr = getRegVal(14);
+    uint32_t pc = getPC();
+    uint32_t lrOp;
+
+    // Since ARM uses software breakpoints behind the scenes, they don't work
+    // in read only areas like the page of routines provided by the kernel. The
+    // link register generally holds the address the process wants to the
+    // kernel to return to after it's done, so we'll install a software
+    // breakpoint there. If the lr happens to point to the next instruction
+    // we'll leave out our breakpoint to avoid an infinite loop. This isn't a
+    // fool proof strategy, but it should work well in all the reasonable
+    // scenarios I can think of right now.
+
+    if (pc != lr) {
+        lrOp = ptrace(PTRACE_PEEKDATA, pid, lr, 0);
+        ptrace(PTRACE_POKEDATA, pid, lr, bkpt_inst);
+    }
+    ptraceSingleStep();
+    if (pc != lr) {
+        ptrace(PTRACE_POKEDATA, pid, lr, lrOp);
     }
 }
 
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