lldb/examples/python/scripted_step.py

#############################################################################
# This script contains two trivial examples of simple "scripted step" classes.
# To fully understand how the lldb "Thread Plan" architecture works, read the
# comments at the beginning of ThreadPlan.h in the lldb sources.  The python
# interface is a reduced version of the full internal mechanism, but captures
# most of the power with a much simpler interface.
#
# But I'll attempt a brief summary here.
# Stepping in lldb is done independently for each thread.  Moreover, the stepping
# operations are stackable.  So for instance if you did a "step over", and in
# the course of stepping over you hit a breakpoint, stopped and stepped again,
# the first "step-over" would be suspended, and the new step operation would
# be enqueued.  Then if that step over caused the program to hit another breakpoint,
# lldb would again suspend the second step and return control to the user, so
# now there are two pending step overs.  Etc. with all the other stepping
# operations.  Then if you hit "continue" the bottom-most step-over would complete,
# and another continue would complete the first "step-over".
#
# lldb represents this system with a stack of "Thread Plans".  Each time a new
# stepping operation is requested, a new plan is pushed on the stack.  When the
# operation completes, it is pushed off the stack.
#
# The bottom-most plan in the stack is the immediate controller of stepping,
# most importantly, when the process resumes, the bottom most plan will get
# asked whether to set the program running freely, or to instruction-single-step
# the current thread.  In the scripted interface, you indicate this by returning
# False or True respectively from the should_step method.
#
# Each time the process stops the thread plan stack for each thread that stopped
# "for a reason", Ii.e. a single-step completed on that thread, or a breakpoint
# was hit), is queried to determine how to proceed, starting from the most
# recently pushed plan, in two stages:
#
# 1) Each plan is asked if it "explains" the stop.  The first plan to claim the
#    stop wins.  In scripted Thread Plans, this is done by returning True from
#    the "explains_stop method.  This is how, for instance, control is returned
#    to the User when the "step-over" plan hits a breakpoint.  The step-over
#    plan doesn't explain the breakpoint stop, so it returns false, and the
#    breakpoint hit is propagated up the stack to the "base" thread plan, which
#    is the one that handles random breakpoint hits.
#
# 2) Then the plan that won the first round is asked if the process should stop.
#    This is done in the "should_stop" method.  The scripted plans actually do
#    three jobs in should_stop:
#      a) They determine if they have completed their job or not.  If they have
#         they indicate that by calling SetPlanComplete on their thread plan.
#      b) They decide whether they want to return control to the user or not.
#         They do this by returning True or False respectively.
#      c) If they are not done, they set up whatever machinery they will use
#         the next time the thread continues.
#
#    Note that deciding to return control to the user, and deciding your plan
#    is done, are orthgonal operations.  You could set up the next phase of
#    stepping, and then return True from should_stop, and when the user next
#    "continued" the process your plan would resume control.  Of course, the
#    user might also "step-over" or some other operation that would push a
#    different plan, which would take control till it was done.
#
#    One other detail you should be aware of, if the plan below you on the
#    stack was done, then it will be popped and the next plan will take control
#    and its "should_stop" will be called.
#
#    Note also, there should be another method called when your plan is popped,
#    to allow you to do whatever cleanup is required.  I haven't gotten to that
#    yet.  For now you should do that at the same time you mark your plan complete.
#
# 3) After the round of negotiation over whether to stop or not is done, all the
#    plans get asked if they are "stale".  If they are say they are stale
#    then they will get popped.  This question is asked with the "is_stale" method.
#
#    This is useful, for instance, in the FinishPrintAndContinue plan.  What might
#    happen here is that after continuing but before the finish is done, the program
#    could hit another breakpoint and stop.  Then the user could use the step
#    command repeatedly until they leave the frame of interest by stepping.
#    In that case, the step plan is the one that will be responsible for stopping,
#    and the finish plan won't be asked should_stop, it will just be asked if it
#    is stale.  In this case, if the step_out plan that the FinishPrintAndContinue
#    plan is driving is stale, so is ours, and it is time to do our printing.
#
# Both examples show stepping through an address range for 20 bytes from the
# current PC.  The first one does it by single stepping and checking a condition.
# It doesn't, however handle the case where you step into another frame while
# still in the current range in the starting frame.
#
# That is better handled in the second example by using the built-in StepOverRange
# thread plan.
#
# To use these stepping modes, you would do:
#
#     (lldb) command script import scripted_step.py
#     (lldb) thread step-scripted -C scripted_step.SimpleStep
# or
#
#     (lldb) thread step-scripted -C scripted_step.StepWithPlan

import lldb


class SimpleStep:

    def __init__(self, thread_plan, dict):
        self.thread_plan = thread_plan
        self.start_address = thread_plan.GetThread().GetFrameAtIndex(0).GetPC()

    def explains_stop(self, event):
        # We are stepping, so if we stop for any other reason, it isn't
        # because of us.
        if self.thread_plan.GetThread().GetStopReason() == lldb.eStopReasonTrace:
            return True
        else:
            return False

    def should_stop(self, event):
        cur_pc = self.thread_plan.GetThread().GetFrameAtIndex(0).GetPC()

        if cur_pc < self.start_address or cur_pc >= self.start_address + 20:
            self.thread_plan.SetPlanComplete(True)
            return True
        else:
            return False

    def should_step(self):
        return True


class StepWithPlan:

    def __init__(self, thread_plan, dict):
        self.thread_plan = thread_plan
        self.start_address = thread_plan.GetThread().GetFrameAtIndex(0).GetPCAddress()
        self.step_thread_plan = thread_plan.QueueThreadPlanForStepOverRange(
            self.start_address, 20)

    def explains_stop(self, event):
        # Since all I'm doing is running a plan, I will only ever get askedthis
        # if myplan doesn't explain the stop, and in that caseI don'teither.
        return False

    def should_stop(self, event):
        if self.step_thread_plan.IsPlanComplete():
            self.thread_plan.SetPlanComplete(True)
            return True
        else:
            return False

    def should_step(self):
        return False

# Here's another example which does "step over" through the current function,
# and when it stops at each line, it checks some condition (in this example the
# value of a variable) and stops if that condition is true.


class StepCheckingCondition:

    def __init__(self, thread_plan, dict):
        self.thread_plan = thread_plan
        self.start_frame = thread_plan.GetThread().GetFrameAtIndex(0)
        self.queue_next_plan()

    def queue_next_plan(self):
        cur_frame = self.thread_plan.GetThread().GetFrameAtIndex(0)
        cur_line_entry = cur_frame.GetLineEntry()
        start_address = cur_line_entry.GetStartAddress()
        end_address = cur_line_entry.GetEndAddress()
        line_range = end_address.GetFileAddress() - start_address.GetFileAddress()
        self.step_thread_plan = self.thread_plan.QueueThreadPlanForStepOverRange(
            start_address, line_range)

    def explains_stop(self, event):
        # We are stepping, so if we stop for any other reason, it isn't
        # because of us.
        return False

    def should_stop(self, event):
        if not self.step_thread_plan.IsPlanComplete():
            return False

        frame = self.thread_plan.GetThread().GetFrameAtIndex(0)
        if not self.start_frame.IsEqual(frame):
            self.thread_plan.SetPlanComplete(True)
            return True

        # This part checks the condition.  In this case we are expecting
        # some integer variable called "a", and will stop when it is 20.
        a_var = frame.FindVariable("a")

        if not a_var.IsValid():
            print "A was not valid."
            return True

        error = lldb.SBError()
        a_value = a_var.GetValueAsSigned(error)
        if not error.Success():
            print "A value was not good."
            return True

        if a_value == 20:
            self.thread_plan.SetPlanComplete(True)
            return True
        else:
            self.queue_next_plan()
            return False

    def should_step(self):
        return True

# Here's an example that steps out of the current frame, gathers some information
# and then continues.  The information in this case is rax.  Currently the thread
# plans are not a safe place to call lldb command-line commands, so the information
# is gathered through SB API calls.


class FinishPrintAndContinue:

    def __init__(self, thread_plan, dict):
        self.thread_plan = thread_plan
        self.step_out_thread_plan = thread_plan.QueueThreadPlanForStepOut(
            0, True)
        self.thread = self.thread_plan.GetThread()

    def is_stale(self):
        if self.step_out_thread_plan.IsPlanStale():
            self.do_print()
            return True
        else:
            return False

    def explains_stop(self, event):
        return False

    def should_stop(self, event):
        if self.step_out_thread_plan.IsPlanComplete():
            self.do_print()
            self.thread_plan.SetPlanComplete(True)
        return False

    def do_print(self):
        frame_0 = self.thread.frames[0]
        rax_value = frame_0.FindRegister("rax")
        if rax_value.GetError().Success():
            print "RAX on exit: ", rax_value.GetValue()
        else:
            print "Couldn't get rax value:", rax_value.GetError().GetCString()