Use absolute pcs and load locations to symbolize on macOS.

tracing/bin/symbolize_trace

Index: tracing/bin/symbolize_trace
diff --git a/tracing/bin/symbolize_trace b/tracing/bin/symbolize_trace
index adb26739cce3c75165c50ab153e0cb2cde19d0c3..c5646e3aba5a430ca1ba202594c7d58a289d5792 100755
--- a/tracing/bin/symbolize_trace
+++ b/tracing/bin/symbolize_trace
@@ -96,7 +96,8 @@ class Symbolizer(object):
     chars_per_address = 19
 
     cmd_base = [self.symbolizer_path, '-arch', 'x86_64', '-l',
-                '0x0', '-o' , symfile.symbolizable_path]
+                '0x%x' % symfile.region_start_address, '-o',
+                symfile.symbolizable_path]
     chars_for_other_arguments = len(' '.join(cmd_base)) + 1
 
     # The maximum number of inputs that can be processed at once is limited by
@@ -110,7 +111,8 @@ class Symbolizer(object):
       keys_to_process = all_keys[0:input_count]
 
       cmd = list(cmd_base)
-      cmd.extend([hex(int(x)) for x in keys_to_process])
+      cmd.extend([hex(int(x) + symfile.region_start_address)
+          for x in keys_to_process])
       output_array = subprocess.check_output(cmd).split('\n')
       for i in range(len(keys_to_process)):
         for frame in symfile.frames_by_address.values()[i + processed_keys_count]:
@@ -318,6 +320,7 @@ class SymbolizableFile(object):
     self.path = file_path
     self.symbolizable_path = file_path # path to use for symbolization
     self.frames_by_address = collections.defaultdict(list)
+    self.region_start_address = 0
 
 
 def ResolveSymbolizableFiles(processes):
@@ -341,6 +344,7 @@ def ResolveSymbolizableFiles(processes):
         symfile_by_path[symfile.path] = symfile
 
       relative_pc = frame.pc - region.start_address
+      symfile.region_start_address = region.start_address

[DmitrySkiba, 2017/02/15 22:40:38]

Hmm, this overwrites the whatever value region_start_address previously had.

Does macOS have ASLR for the image itself? The reason we calculate relative_pc
here is because different absolute addresses can end up being single relative
address after translation.

I.e. browser mapped foo() to 0xA000, and renderer mapped it to 0xF000, but it's
relative address ends up being 0x1000 if we take into account load addresses.

With this change we end up with the same relative address 0x1000, but use the
last known start address 0xE000 to later (in _SymbolizeMac) translate 0x1000
back to 0xF000.

What am I missing?

       symfile.frames_by_address[relative_pc].append(frame)
   return symfile_by_path.values()