Handle very large stack traces

The main motivation for this change is to handle very large stack
traces, normally the result of infinite recursion. This part is
actually fairly simple, relaxing a few self-imposed limits on how
many frames we can unwind and the max size for stack memory.

Relaxing these limits requires stricter and more consistent checks for
stack unwinding. There are a number of unwinding invariants that apply
to all the platforms:

1. stack pointer (and frame pointer) must be within the stack memory
   (frame pointer, if preset, must point to the right frame too)
2. unwinding must monotonically increase SP
   (except for the first frame unwind, this must be a strict increase)
3. Instruction pointer (return address) must point to a valid location
4. stack pointer (and frame pointer) must be appropriately aligned

This change is focused on 2), which is enough to guarantee that the
unwinding doesn't get stuck in an infinite loop.

1) is implicitly validated part of accessing the stack memory
   (explicit checks might be nice though).
4) is ABI specific and while it may be valuable in catching suspicious
   frames is not in the scope of this change.
3) is also an interesting check but thanks to just-in-time compilation
   it's more complex than just calling 
   StackWalker::InstructionAddressSeemsValid() 
   and we don't want to drop parts of the callstack due to an overly
   conservative check.

Bug: chromium:735989

Change-Id: I9aaba77c7fd028942d77c87d51b5e6f94e136ddd
Reviewed-on: https://chromium-review.googlesource.com/563771
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Ivan Penkov <ivanpe@chromium.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/processor/stackwalker_ppc.cc b/src/processor/stackwalker_ppc.cc
index 7e208844..c9ab144f 100644
--- a/src/processor/stackwalker_ppc.cc
+++ b/src/processor/stackwalker_ppc.cc
@@ -130,6 +130,14 @@ StackFrame* StackwalkerPPC::GetCallerFrame(const CallStack* stack,
                             StackFramePPC::CONTEXT_VALID_GPR1;
   frame->trust = StackFrame::FRAME_TRUST_FP;
 
+  // Should we terminate the stack walk? (end-of-stack or broken invariant)
+  if (TerminateWalk(instruction,
+                    stack_pointer,
+                    last_frame->context.gpr[1],
+                    stack->frames()->size() == 1)) {
+    return NULL;
+  }
+
   // frame->context.srr0 is the return address, which is one instruction
   // past the branch that caused us to arrive at the callee.  Set
   // frame_ppc->instruction to four less than that.  Since all ppc