Hi all,
one of the archer branches of gdb allows to turn gdb into a python interpreter
and allow scripting.
My idea was to use this to track all refcounted objects in WebCore and answer
questions like. How many objects are alive? How many objects changed from
point a to b? What are the refcounts? Where was the ref count increased?
I have a slightly modified release build to turn RefCountedBase into a non
inline class and only compile the RefCounted.cpp with -ggdb3 debug support to
easily set a breakpoint there.
With the attached python script I'm able to keep track of RefCountedBase
objects, have commands to print the python dictionary from a gdb cli. I have
no other exploring functions in place.
I wonder if I could develop this tool inside a subdirectory
WebKitTools/Scripts without the usual review process?
Due some limitations in the python support the workflow is currently like this:
gdb -P script GtKLauncher
(gdb) webkit-setup
... questions about pending breakpoints...
(gdb) r
CTRL+C
(gdb) webkit-objects
WebCore has 2891 live objects of 0 kinds
0xb3e05784
0xb3d99378
0xb3e1e6ac
"""
WebKit object tracking system. This
"""
import gdb
import sys
# all allocated and non freed objects
live_objects = {}
class live_object:
"""
An object of WebCore with the RefCountedBase base class
"""
def __init__(self, initial_backtrace, addr):
self.addr = addr
self.initial_backtrace = initial_backtrace
self.obj_type = live_object.determine_type(initial_backtrace)
@staticmethod
def determine_type(backtrace):
"""
In general we are not building with RTTI so we can not determine
the WebCore type from the given pointer value. We will need to
look at the backtrace to determine the type of this object.
"""
return None
def find_this_pointer(frame):
for sym in frame.block():
if not sym.is_argument:
continue
if sym.print_name == "this":
return frame.read_var(sym)
return None
def hash_value(gdb_value):
return str(gdb_value)
# Functions for gdb as a replacement for callbacks
class WebKitObjectCreated(gdb.Function):
"""
A new RefCountedBase object was created... keep track of it
"""
def __init__(self):
super(WebKitObjectCreated, self).__init__("webkit_new_object")
def invoke(self):
ptr = find_this_pointer(gdb.selected_frame())
if ptr:
assert hash_value(ptr) not in live_objects
live_objects[hash_value(ptr)] = live_object(gdb.selected_frame, ptr)
return False
class WebKitObjectDeleted(gdb.Function):
"""
A RefCountedBase object was destroyed, clear references to it.
"""
def __init__(self):
super(WebKitObjectDeleted, self).__init__("webkit_delete_object")
def invoke(self):
ptr = find_this_pointer(gdb.selected_frame())
if ptr:
assert hash_value(ptr) in live_objects
del live_objects[hash_value(ptr)]
return False
class WebKitSetup(gdb.Command):
"""
Setup breakpoints
"""
def __init__(self):
super(WebKitSetup, self).__init__("webkit-setup",
gdb.COMMAND_SUPPORT, gdb.COMPLETE_NONE, True)
def invoke(self, arg, from_tty):
gdb.execute("break WTF::RefCountedBase::RefCountedBase() if $webkit_new_object()")
gdb.execute("break WTF::RefCountedBase::~RefCountedBase() if $webkit_delete_object()")
class WebKitLiveObjects(gdb.Command):
"""
List the live objects in webkit...
"""
def __init__(self):
super(WebKitLiveObjects, self).__init__("webkit-objects",
gdb.COMMAND_SUPPORT, gdb.COMPLETE_NONE, True)
def invoke(self, arg, from_tty):
# TODO have some param to filter for a given backtrace/type...
sys.stdout.write("WebCore has %d live objects of %d kinds\n" % (len(live_objects), 0))
for live_object in live_objects.keys():
print live_object
def webkit_register():
"""
Register commands for GDB to use.
"""
WebKitObjectCreated()
WebKitObjectDeleted()
#WebKitObjectRef()
#WebKitObjectUnref()
WebKitSetup()
WebKitLiveObjects()
def webkit_setup():
"""
Setup the
"""
print sys.argv
gdb.execute("file %s" % sys.argv[0])
webkit_register()
webkit_setup()
print "The executable was loaded. Please invoke the webkit-setup command"
gdb.cli()
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