lldb/examples/python/scripted_step.py

   1 #############################################################################
   2 # This script contains two trivial examples of simple "scripted step" classes.
   3 # To fully understand how the lldb "Thread Plan" architecture works, read the
   4 # comments at the beginning of ThreadPlan.h in the lldb sources.  The python
   5 # interface is a reduced version of the full internal mechanism, but captures
   6 # most of the power with a much simpler interface.
   7 #
   8 # But I'll attempt a brief summary here.
   9 # Stepping in lldb is done independently for each thread.  Moreover, the stepping
  10 # operations are stackable.  So for instance if you did a "step over", and in
  11 # the course of stepping over you hit a breakpoint, stopped and stepped again,
  12 # the first "step-over" would be suspended, and the new step operation would
  13 # be enqueued.  Then if that step over caused the program to hit another breakpoint,
  14 # lldb would again suspend the second step and return control to the user, so
  15 # now there are two pending step overs.  Etc. with all the other stepping
  16 # operations.  Then if you hit "continue" the bottom-most step-over would complete,
  17 # and another continue would complete the first "step-over".
  18 #
  19 # lldb represents this system with a stack of "Thread Plans".  Each time a new
  20 # stepping operation is requested, a new plan is pushed on the stack.  When the
  21 # operation completes, it is pushed off the stack.
  22 #
  23 # The bottom-most plan in the stack is the immediate controller of stepping,
  24 # most importantly, when the process resumes, the bottom most plan will get
  25 # asked whether to set the program running freely, or to instruction-single-step
  26 # the current thread.  In the scripted interface, you indicate this by returning
  27 # False or True respectively from the should_step method.
  28 #
  29 # Each time the process stops the thread plan stack for each thread that stopped
  30 # "for a reason", Ii.e. a single-step completed on that thread, or a breakpoint
  31 # was hit), is queried to determine how to proceed, starting from the most
  32 # recently pushed plan, in two stages:
  33 #
  34 # 1) Each plan is asked if it "explains" the stop.  The first plan to claim the
  35 #    stop wins.  In scripted Thread Plans, this is done by returning True from
  36 #    the "explains_stop method.  This is how, for instance, control is returned
  37 #    to the User when the "step-over" plan hits a breakpoint.  The step-over
  38 #    plan doesn't explain the breakpoint stop, so it returns false, and the
  39 #    breakpoint hit is propagated up the stack to the "base" thread plan, which
  40 #    is the one that handles random breakpoint hits.
  41 #
  42 # 2) Then the plan that won the first round is asked if the process should stop.
  43 #    This is done in the "should_stop" method.  The scripted plans actually do
  44 #    three jobs in should_stop:
  45 #      a) They determine if they have completed their job or not.  If they have
  46 #         they indicate that by calling SetPlanComplete on their thread plan.
  47 #      b) They decide whether they want to return control to the user or not.
  48 #         They do this by returning True or False respectively.
  49 #      c) If they are not done, they set up whatever machinery they will use
  50 #         the next time the thread continues.
  51 #
  52 #    Note that deciding to return control to the user, and deciding your plan
  53 #    is done, are orthgonal operations.  You could set up the next phase of
  54 #    stepping, and then return True from should_stop, and when the user next
  55 #    "continued" the process your plan would resume control.  Of course, the
  56 #    user might also "step-over" or some other operation that would push a
  57 #    different plan, which would take control till it was done.
  58 #
  59 #    One other detail you should be aware of, if the plan below you on the
  60 #    stack was done, then it will be popped and the next plan will take control
  61 #    and its "should_stop" will be called.
  62 #
  63 #    Note also, there should be another method called when your plan is popped,
  64 #    to allow you to do whatever cleanup is required.  I haven't gotten to that
  65 #    yet.  For now you should do that at the same time you mark your plan complete.
  66 #
  67 # 3) After the round of negotiation over whether to stop or not is done, all the
  68 #    plans get asked if they are "stale".  If they are say they are stale
  69 #    then they will get popped.  This question is asked with the "is_stale" method.
  70 #
  71 #    This is useful, for instance, in the FinishPrintAndContinue plan.  What might
  72 #    happen here is that after continuing but before the finish is done, the program
  73 #    could hit another breakpoint and stop.  Then the user could use the step
  74 #    command repeatedly until they leave the frame of interest by stepping.
  75 #    In that case, the step plan is the one that will be responsible for stopping,
  76 #    and the finish plan won't be asked should_stop, it will just be asked if it
  77 #    is stale.  In this case, if the step_out plan that the FinishPrintAndContinue
  78 #    plan is driving is stale, so is ours, and it is time to do our printing.
  79 #
  80 # Both examples show stepping through an address range for 20 bytes from the
  81 # current PC.  The first one does it by single stepping and checking a condition.
  82 # It doesn't, however handle the case where you step into another frame while
  83 # still in the current range in the starting frame.
  84 #
  85 # That is better handled in the second example by using the built-in StepOverRange
  86 # thread plan.
  87 #
  88 # To use these stepping modes, you would do:
  89 #
  90 #     (lldb) command script import scripted_step.py
  91 #     (lldb) thread step-scripted -C scripted_step.SimpleStep
  92 # or
  93 #
  94 #     (lldb) thread step-scripted -C scripted_step.StepWithPlan
  95
  96 from __future__ import print_function
  97
  98 import lldb
  99
 100
 101 class SimpleStep:
 102
 103     def __init__(self, thread_plan, dict):
 104         self.thread_plan = thread_plan
 105         self.start_address = thread_plan.GetThread().GetFrameAtIndex(0).GetPC()
 106
 107     def explains_stop(self, event):
 108         # We are stepping, so if we stop for any other reason, it isn't
 109         # because of us.
 110         if self.thread_plan.GetThread().GetStopReason() == lldb.eStopReasonTrace:
 111             return True
 112         else:
 113             return False
 114
 115     def should_stop(self, event):
 116         cur_pc = self.thread_plan.GetThread().GetFrameAtIndex(0).GetPC()
 117
 118         if cur_pc < self.start_address or cur_pc >= self.start_address + 20:
 119             self.thread_plan.SetPlanComplete(True)
 120             return True
 121         else:
 122             return False
 123
 124     def should_step(self):
 125         return True
 126
 127
 128 class StepWithPlan:
 129
 130     def __init__(self, thread_plan, dict):
 131         self.thread_plan = thread_plan
 132         self.start_address = thread_plan.GetThread().GetFrameAtIndex(0).GetPCAddress()
 133         self.step_thread_plan = thread_plan.QueueThreadPlanForStepOverRange(
 134             self.start_address, 20)
 135
 136     def explains_stop(self, event):
 137         # Since all I'm doing is running a plan, I will only ever get askedthis
 138         # if myplan doesn't explain the stop, and in that caseI don'teither.
 139         return False
 140
 141     def should_stop(self, event):
 142         if self.step_thread_plan.IsPlanComplete():
 143             self.thread_plan.SetPlanComplete(True)
 144             return True
 145         else:
 146             return False
 147
 148     def should_step(self):
 149         return False
 150
 151 # Here's another example which does "step over" through the current function,
 152 # and when it stops at each line, it checks some condition (in this example the
 153 # value of a variable) and stops if that condition is true.
 154
 155
 156 class StepCheckingCondition:
 157
 158     def __init__(self, thread_plan, dict):
 159         self.thread_plan = thread_plan
 160         self.start_frame = thread_plan.GetThread().GetFrameAtIndex(0)
 161         self.queue_next_plan()
 162
 163     def queue_next_plan(self):
 164         cur_frame = self.thread_plan.GetThread().GetFrameAtIndex(0)
 165         cur_line_entry = cur_frame.GetLineEntry()
 166         start_address = cur_line_entry.GetStartAddress()
 167         end_address = cur_line_entry.GetEndAddress()
 168         line_range = end_address.GetFileAddress() - start_address.GetFileAddress()
 169         self.step_thread_plan = self.thread_plan.QueueThreadPlanForStepOverRange(
 170             start_address, line_range)
 171
 172     def explains_stop(self, event):
 173         # We are stepping, so if we stop for any other reason, it isn't
 174         # because of us.
 175         return False
 176
 177     def should_stop(self, event):
 178         if not self.step_thread_plan.IsPlanComplete():
 179             return False
 180
 181         frame = self.thread_plan.GetThread().GetFrameAtIndex(0)
 182         if not self.start_frame.IsEqual(frame):
 183             self.thread_plan.SetPlanComplete(True)
 184             return True
 185
 186         # This part checks the condition.  In this case we are expecting
 187         # some integer variable called "a", and will stop when it is 20.
 188         a_var = frame.FindVariable("a")
 189
 190         if not a_var.IsValid():
 191             print("A was not valid.")
 192             return True
 193
 194         error = lldb.SBError()
 195         a_value = a_var.GetValueAsSigned(error)
 196         if not error.Success():
 197             print("A value was not good.")
 198             return True
 199
 200         if a_value == 20:
 201             self.thread_plan.SetPlanComplete(True)
 202             return True
 203         else:
 204             self.queue_next_plan()
 205             return False
 206
 207     def should_step(self):
 208         return True
 209
 210 # Here's an example that steps out of the current frame, gathers some information
 211 # and then continues.  The information in this case is rax.  Currently the thread
 212 # plans are not a safe place to call lldb command-line commands, so the information
 213 # is gathered through SB API calls.
 214
 215
 216 class FinishPrintAndContinue:
 217
 218     def __init__(self, thread_plan, dict):
 219         self.thread_plan = thread_plan
 220         self.step_out_thread_plan = thread_plan.QueueThreadPlanForStepOut(
 221             0, True)
 222         self.thread = self.thread_plan.GetThread()
 223
 224     def is_stale(self):
 225         if self.step_out_thread_plan.IsPlanStale():
 226             self.do_print()
 227             return True
 228         else:
 229             return False
 230
 231     def explains_stop(self, event):
 232         return False
 233
 234     def should_stop(self, event):
 235         if self.step_out_thread_plan.IsPlanComplete():
 236             self.do_print()
 237             self.thread_plan.SetPlanComplete(True)
 238         return False
 239
 240     def do_print(self):
 241         frame_0 = self.thread.frames[0]
 242         rax_value = frame_0.FindRegister("rax")
 243         if rax_value.GetError().Success():
 244             print("RAX on exit: ", rax_value.GetValue())
 245         else:
 246             print("Couldn't get rax value:", rax_value.GetError().GetCString())