Android: Support unwinding function prolog

If the sampler stoped a thread at prolog of a chrome function, use the
LR register to find the next return address.

BUG=859260

Change-Id: Ic20d793ee11a8b0fe3edc9438c0f12ccc19f487c
Reviewed-on: https://chromium-review.googlesource.com/c/1265560
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602077}

base/trace_event/cfi_backtrace_android.cc

@@ -197,11 +197,12 @@ size_t CFIBacktraceAndroid::Unwind(const void** out_trace, size_t max_depth) {
   asm volatile("mov %0, pc" : "=r"(pc));
   asm volatile("mov %0, sp" : "=r"(sp));
 
-  return Unwind(pc, sp, out_trace, max_depth);
+  return Unwind(pc, sp, /*lr=*/0, out_trace, max_depth);
 }
 
 size_t CFIBacktraceAndroid::Unwind(uintptr_t pc,
                                    uintptr_t sp,
+                                   uintptr_t lr,
                                    const void** out_trace,
                                    size_t max_depth) {
   if (!can_unwind_stack_frames())
@@ -214,8 +215,19 @@ size_t CFIBacktraceAndroid::Unwind(uintptr_t pc,
     // The offset of function from the start of the chrome.so binary:
     uintptr_t func_addr = pc - executable_start_addr();
     CFIRow cfi{};
-    if (!FindCFIRowForPC(func_addr, &cfi))
+    if (!FindCFIRowForPC(func_addr, &cfi)) {
+      if (depth == 1 && lr != 0 && pc != lr) {
+        // If CFI data is not found for the frame, then we stopped in prolog of
+        // a function. The return address is stored in LR when in function
+        // prolog. So, update the PC with address in LR and do not update SP
+        // since SP was not updated by the prolog yet.
+        // TODO(ssid): Write tests / add info to detect if we are actually in
+        // function prolog. https://crbug.com/898276
+        pc = lr;
+        continue;
+      }
       break;
+    }
 
     // The rules for unwinding using the CFI information are:
     // SP_prev = SP_cur + cfa_offset and

base/trace_event/cfi_backtrace_android.h

@@ -62,12 +62,13 @@ class BASE_EXPORT CFIBacktraceAndroid {
   // Initialize() returns success.
   size_t Unwind(const void** out_trace, size_t max_depth);
 
-  // Same as above function, but starts from a given program counter |pc| and
-  // stack pointer |sp|. This can be from current thread or any other thread.
-  // But the caller must make sure that the thread's stack segment is not racy
-  // to read.
+  // Same as above function, but starts from a given program counter |pc|,
+  // stack pointer |sp| and link register |lr|. This can be from current thread
+  // or any other thread. But the caller must make sure that the thread's stack
+  // segment is not racy to read.
   size_t Unwind(uintptr_t pc,
                 uintptr_t sp,
+                uintptr_t lr,
                 const void** out_trace,
                 size_t max_depth);
 

components/tracing/common/stack_unwinder_android.cc

@@ -123,8 +123,10 @@ size_t TraceStackWithContext(unw_cursor_t* cursor,
   if (CFIBacktraceAndroid::is_chrome_address(ip)) {
     // Continue unwinding CFI unwinder if we found stack frame from chrome
     // library.
-    return depth +
-           cfi_unwinder->Unwind(ip, sp, out_trace + depth, max_depth - depth);
+    uintptr_t lr = 0;
+    unw_get_reg(cursor, UNW_ARM_LR, &lr);
+    return depth + cfi_unwinder->Unwind(ip, sp, lr, out_trace + depth,
+                                        max_depth - depth);
   } else if (depth == 0) {
     RecordUnwindResult(SamplingProfilerUnwindResult::kFirstFrameUnmapped);
   }
@@ -368,7 +370,7 @@ size_t StackUnwinderAndroid::TraceStack(base::PlatformThreadId tid,
 
   // Do not use libunwind if we stopped at chrome frame.
   if (CFIBacktraceAndroid::is_chrome_address(ip))
-    return cfi_unwinder->Unwind(ip, sp, out_trace, max_depth);
+    return cfi_unwinder->Unwind(ip, sp, 0, out_trace, max_depth);
 
   // Reset the unwind cursor to previous function and continue with libunwind.
   // TODO(ssid): Dynamic allocation functions might require registers to be