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

Tue Dec 2 16:06:58 PST 2014

You really want your shared library loads to be synchronous. There has to be a way to stop your target when a shared library loads? If not, you might miss your breakpoint if it is in a "PluginInitialize()" call and you stop the target after receiving the shared library load/unload notification. So try as hard as you can to make this happen synchronously...

> On Dec 2, 2014, at 3:10 PM, Zachary Turner <zturner at google.com> wrote:
> 
> 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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