Revert "arm: Allow the first function to use linked register as return pc"

This reverts commit f2b3ab5e0a.

Reason for revert: Causes symbolization errors on ARM ChromeOS
devices crbug.com/1182948.

Original change's description:
> arm: Allow the first function to use linked register as return pc
>
> For a crash at the function entry with corrupted PC, the caller's PC
> could be lying in the link register. Using the PC from link register
> would be more effective than blindly scanning the stack immediately.
>
> Change-Id: I51673b7298e70faeeab2bfa97075e3c4793f94bc
> Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2678992
> Reviewed-by: Mike Frysinger <vapier@chromium.org>
> Reviewed-by: Joshua Peraza <jperaza@chromium.org>

Bug: 1182948
Change-Id: I2818b35ab1fb99012919cccc0fb80368e456ca15
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2765164
Reviewed-by: Joshua Peraza <jperaza@chromium.org>

src/processor/microdump_processor_unittest.cc

@@ -232,7 +232,7 @@ TEST_F(MicrodumpProcessorTest, TestProcessMultiple) {
   ASSERT_EQ("arm", state.system_info()->cpu);
   ASSERT_EQ("lge/p1_tmo_us/p1:6.0/MRA58K/1603210524c8d:user/release-keys",
             state.system_info()->os_version);
-  ASSERT_EQ(6U, state.threads()->at(0)->frames()->size());
+  ASSERT_EQ(5U, state.threads()->at(0)->frames()->size());
 }
 
 TEST_F(MicrodumpProcessorTest, TestProcessMips) {

src/processor/stackwalker_arm.cc

@@ -238,27 +238,6 @@ StackFrameARM* StackwalkerARM::GetCallerByFramePointer(
   return frame;
 }
 
-StackFrameARM* StackwalkerARM::GetCallerByLinkRegister(
-    const vector<StackFrame*>& frames) {
-  StackFrameARM* last_frame = static_cast<StackFrameARM*>(frames.back());
-  uint32_t last_lr = last_frame->context.iregs[MD_CONTEXT_ARM_REG_LR];
-
-  if (!(last_frame->context_validity & StackFrameARM::CONTEXT_VALID_LR) ||
-      !InstructionAddressSeemsValid(last_lr)) {
-    return NULL;
-  }
-
-  StackFrameARM* frame = new StackFrameARM();
-
-  frame->trust = StackFrame::FRAME_TRUST_SCAN;
-  frame->context = last_frame->context;
-  frame->context.iregs[MD_CONTEXT_ARM_REG_PC] = last_lr;
-  frame->context_validity =
-      context_frame_validity_ & (~StackFrameARM::CONTEXT_VALID_LR);
-
-  return frame;
-}
-
 StackFrame* StackwalkerARM::GetCallerFrame(const CallStack* stack,
                                            bool stack_scan_allowed) {
   if (!memory_ || !stack) {
@@ -285,12 +264,6 @@ StackFrame* StackwalkerARM::GetCallerFrame(const CallStack* stack,
   if (fp_register_ >= 0 && !frame.get())
     frame.reset(GetCallerByFramePointer(frames));
 
-  // For the first frame, return address may still be in the LR register at
-  // entry. Prefer to use LR register than scanning stack if LR register value
-  // points to a function range.
-  if (frames.size() == 1 && !frame.get())
-    frame.reset(GetCallerByLinkRegister(frames));
-
   // If everuthing failed, fall back to stack scanning.
   if (stack_scan_allowed && !frame.get())
     frame.reset(GetCallerByStackScan(frames));

src/processor/stackwalker_arm.h

@@ -82,11 +82,6 @@ class StackwalkerARM : public Stackwalker {
   // Return NULL on failure.
   StackFrameARM* GetCallerByFramePointer(const vector<StackFrame*>& frames);
 
-  // Use the link register if it seems to be a valid function adderss.
-  // The caller takes ownership of the returned frame. Return NULL on failure.
-  // This is useful when PC register is corrupted.
-  StackFrameARM* GetCallerByLinkRegister(const vector<StackFrame*>& frames);
-
   // Scan the stack for plausible return addresses. The caller takes ownership
   // of the returned frame. Return NULL on failure.
   StackFrameARM* GetCallerByStackScan(const vector<StackFrame*>& frames);