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

Zachary Turner zturner at google.com
Tue Dec 2 15:10:27 PST 2014

Sounds good.  I tested with calling ModulesDidLoad() directly and it seems
to resolve the breakpoints, so now that I know that that was the issue
blocking me, I can try to do it the "right" way via a DynamicLoader plugin.

One thing I'm uncertain about though, is that I get the notification
asynchronously instead of going through this breakpoint / callback
mechanism.  So I can send a notification from my event listener thread to
the DynamicLoader plugin, but it's not going to be on the main thread.
Will this cause a problem?

On Tue Dec 02 2014 at 2:58:57 PM Greg Clayton <gclayton at apple.com> wrote:

> You must implement a DynamicLoaderWindows. Shared library
> loading/unloading won't work otherwise.
> The theory is simple: after launching or attaching, the plug-in will find
> the list of shared libraries to get the initial state. Also when you
> program dynamically loads/unloads DLLs, you need to update anything that
> changed (load/unload sections for things that got loaded/unloaded).
> Please do NOT call ModulesDidLoad directly. You can do this temporarily,
> but you really do need a dynamic loader.
> The MacOSX version finds the global list of shared libraries that are
> loaded, iterates though them, searches for and adds any modules that are in
> the target, removes any images from the target that aren't loaded, then
> sets the section load addresses for all sections in all modules to the
> correct value and then calls ModulesDidLoad(). This causes all breakpoints
> to get resolved.
> We then set a breakpoint at a location that gets hit after /usr/lib/dyld
> loads/unloads new shared libraries so we can keep up. This is a synchronous
> breakpoint where we detect the new shared libraries that were
> loaded/unloaded, we add/remove modules and set them to the loaded or
> unloaded and then continue. So it is a very easy plug-in to write and is
> required so that dynamic plug-in loading/unloaded can track breakpoints
> correctly.
> Greg
> > On Dec 2, 2014, at 2:29 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > 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?
> > > > _______________________________________________
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> > >
> >
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