[wasm] trap handlers: Factor out landing pad search code

This is the first of a series of refactoring CLs to make way for
Windows trap handling support.

See https://chromium-review.googlesource.com/c/v8/v8/+/626558 as well.

Bug: 
Change-Id: I5fe9ef9c1cec58a81e51fcffbbe4419e0e298ab7
Reviewed-on: https://chromium-review.googlesource.com/644104Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48191}

src/trap-handler/handler-inside.cc

@@ -98,7 +98,23 @@ bool TryHandleSignal(int signum, siginfo_t* info, ucontext_t* context) {
     SigUnmaskStack unmask(sigs);
 
     uintptr_t fault_addr = context->uc_mcontext.gregs[REG_RIP];
+    uintptr_t landing_pad = 0;
+    if (TryFindLandingPad(fault_addr, &landing_pad)) {
+      // Tell the caller to return to the landing pad.
+      context->uc_mcontext.gregs[REG_RIP] = landing_pad;
+      return true;
+    }
+  }  // end signal mask scope
+
+  // If we get here, it's not a recoverable wasm fault, so we go to the next
+  // handler.
+  g_thread_in_wasm_code = true;
+  return false;
+}
 
+// This function contains the platform independent portions of fault
+// classification.
+bool TryFindLandingPad(uintptr_t fault_addr, uintptr_t* landing_pad) {
   // TODO(eholk): broad code range check
 
   // Taking locks in a signal handler is risky because a fault in the signal
@@ -122,10 +138,8 @@ bool TryHandleSignal(int signum, siginfo_t* info, ucontext_t* context) {
 
       for (unsigned i = 0; i < data->num_protected_instructions; ++i) {
         if (data->instructions[i].instr_offset == offset) {
-            // Hurray again, we found the actual instruction. Tell the caller to
-            // return to the landing pad.
-            context->uc_mcontext.gregs[REG_RIP] =
-                data->instructions[i].landing_offset + base;
+          // Hurray again, we found the actual instruction.
+          *landing_pad = data->instructions[i].landing_offset + base;
 
           gRecoveredTrapCount.store(
               gRecoveredTrapCount.load(std::memory_order_relaxed) + 1,
@@ -136,11 +150,6 @@ bool TryHandleSignal(int signum, siginfo_t* info, ucontext_t* context) {
       }
     }
   }
-  }  // end signal mask scope
-
-  // If we get here, it's not a recoverable wasm fault, so we go to the next
-  // handler.
-  g_thread_in_wasm_code = true;
   return false;
 }
 #endif  // V8_TRAP_HANDLER_SUPPORTED && V8_OS_LINUX

src/trap-handler/trap-handler-internal.h

@@ -62,6 +62,12 @@ extern CodeProtectionInfoListEntry* gCodeObjects;
 
 extern std::atomic_size_t gRecoveredTrapCount;
 
+// Searches the fault location table for an entry matching fault_addr. If found,
+// returns true and sets landing_pad to the address of a fragment of code that
+// can recover from this fault. Otherwise, returns false and leaves offset
+// unchanged.
+bool TryFindLandingPad(uintptr_t fault_addr, uintptr_t* landing_pad);
+
 }  // namespace trap_handler
 }  // namespace internal
 }  // namespace v8