[lldb-dev] Creating a breakpoint on a target with no process

Zachary Turner zturner at google.com
Tue Dec 2 14:29:18 PST 2014

Thanks.  For now I'll experiment with your suggestion of just calling
ModulesDidLoad directly in the callback, since getting the actual
notification that a library is loaded is trivial on Windows and all the
work is done for us by the OS.  Is it safe to update the module list from a
thread other than the main thread?  All threads of the inferior will be
stopped while I process this notification, but I know for example that with
thread creation / thread exit, I have to maintain this thread list, and
then only in UpdateThreadList do I actually update the thread list on the
target.  Is this restriction not the same with the module list?

One more question, how do I find the module that is loaded at a specific
address?  When this shared library is unloaded, the only information I have
is its load address, but the only method for getting a Module from the
target is to call GetSharedModule() with a ModuleSpec, which I won't have.
Is there a way to search based only on the load address?

On Tue Dec 02 2014 at 2:19:33 PM <jingham at apple.com> wrote:

> The dynamic loader plugin has a couple of different jobs.
> The one that is relevant to your question is that it is responsible for
> hooking up the mechanism whereby lldb gets notified of new shared library
> loads.  It gets called when we attach or launch a process, at which point
> it is supposed to make whatever instrumentation is needed for tracking the
> loader.  On most platforms this is done by setting some breakpoint in the
> correct place in the loader code and then decoding the meaning of the event
> when the breakpoint to gets hit (load or unload, what got loaded, etc.)
> Since this is often a non-trivial bit of code, and one that changes as the
> versions of the OS go by, so it seemed worthwhile to have it be a separate
> module.  If you wanted to use this model for Windows, you would have your
> DynamicLoader plugin register the callback for the "Shared libraries
> changed" event that your main loop is getting, and then call into that to
> process the event.
> In the short term you can probably just call ModulesDidLoad in the code
> you have below.  Note, this isn't done in GetSharedModule because it is
> expensive to go looking through new modules for breakpoints, so you don't
> want to hang it off some call that might be called many times.  Instead we
> have an explicit "Okay here's the set of new libraries" type call.
> There isn't good documentation on this in the code, which we should fix.
> Also, it would arguably be cleaner to separate out the "discover new
> modules" part of the DynamicLoader, and the "Make these new modules work
> correctly" into separate steps within the Dynamic loader plugin.  The
> former is going to be specific to the various subclasses, but the latter
> job is pretty generic.  Then each port would know it had to call the
> DynamicLoader::RegisterNewModules or whatever it was when it was done
> with the platform specific side of registering them.  But since that job
> currently consists of calling Target::ModulesDidLoad, we haven't been
> motivated to move the code around to do this.
> The other main dynamic loader job is not relevant to your question, but
> for completeness sake is that it is also the place where knowledge of the
> library intercalling mechanism resides.  Most importantly, most
> inter-library calls are implemented using some sort of stub that
> trampolines over to the actual call. That stub generally doesn't have debug
> information, so the normal behavior of "next" when it lands in the stub
> would be to say "I've stepped into code with no debug information, so I'll
> step out".  But if the stub was going to resolve to a routine that did have
> debug info, that would be the wrong behavior.  So before we decide to step
> out of unknown code, we always ask the current dynamic loader plugin to
> "GetStepThroughTrampolinePlan" to see if it knows how to get from this PC
> to somewhere more interesting, and if so to return a plan that does that
> job.
> Jim
> > On Dec 2, 2014, at 1:15 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > I actually don't even have a dynamic loader plugin implemented at all.
> I wasn't completely sure what the purpose of it was.  I saw that Virgile
> had implemented one in his original patch to get debugging working on
> Windows [https://github.com/xen2/lldb/commit/
> 515956244784a9162183a6135068e893ba994532], but it did very little actual
> work, and in particular does not seem to do anything related to what you
> are suggesting above.
> >
> > As for adding new modules when they load, basically this is the entirety
> of what I do.
> >
> >     Error error;
> >     ModuleSP module = GetTarget().GetSharedModule(module_spec, &error);
> >     module->SetLoadAddress(GetTarget(), module_addr, false, false);
> >
> > However, as mentioned I don't do this from a DynamicLoader plugin.
> Instead I just run this code directly from the same background thread that
> gets other debug events from the process, such as thread creation,
> exceptions, etc.
> >
> > I guess can you elaborate a little bit on the interaction between the
> DynamicLoader plugin and the process plugin, and the responsibilities of
> each?
> >
> > On Tue Dec 02 2014 at 1:07:35 PM <jingham at apple.com> wrote:
> > It is the responsibility of the dynamic loader plugin to tell the
> breakpoints to re-scan for new locations when shared libraries get added to
> the process.  You should do this by collecting a list of the added
> libraries, and calling:
> >
> > m_process->GetTarget().ModulesDidLoad(added_list);
> >
> > How are you adding new modules as they get loaded?
> >
> > Jim
> >
> >
> > > On Dec 2, 2014, at 12:45 PM, Zachary Turner <zturner at google.com>
> wrote:
> > >
> > > In my effort to get tests working on Windows, I've run across an issue
> with test\expression_command\timeout\TestCallWithTimeout.py ::
> TestCallWithTimeout.ExprCommandWithTimeoutsTestCase
> > >
> > > This test creates a target and immediately puts a breakpoint on it
> before attempting to launch the process.  Is this something that is
> supposed to work?  BreakpointLocation::ResolveBreakpointSite() contains
> this line:
> > >
> > >     Process *process = m_owner.GetTarget().GetProcessSP().get();
> > >     if (process == NULL)
> > >         return false;
> > >
> > > So naturally the breakpoint site cannot be resolved because there is
> no process.  The end result of this is that this breakpoint never gets hit
> and the test fails.
> > >
> > > Presumably this test works on other platforms, so any tips as to where
> I should look to track down this bug on Windows?
> > > _______________________________________________
> > > lldb-dev mailing list
> > > lldb-dev at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> >
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