#############################################################################
# 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.
#
# 4) If you implement the "stop_description(SBStream stream)" method in your
# python class, then that will show up as the "plan completed" reason when
# your thread plan is complete.
#
# 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
def stop_description(self, stream):
stream.Print("Simple step completed")
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
def stop_description(self, stream):
self.step_thread_plan.GetDescription(stream, lldb.eDescriptionLevelBrief)
# 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
def stop_description(self, stream):
stream.Print(f"Stepped until a == 20")
# 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())
def stop_description(self, stream):
self.step_out_thread_plan.GetDescription(stream, lldb.eDescriptionLevelBrief)
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