That was actually one of the main questions I wanted answered: *Who is using them?* If the answer is truly nobody, which I kind of suspected, then I don't think there's any harm in removing this optimization from the linux side.
On Tue, Aug 26, 2014 at 5:46 PM, Enrico Granata <egran...@apple.com> wrote: > I don’t know if anyone is actually relying on the plugin feature at this > point > > I believe when I first wrote it it was meant to allow people to write LLDB > commands using C/C++ libraries that they already had available (e.g. for > purposes like data analysis) > I also believe it didn’t find practical usage, and that it is also not > documented anywhere (except looking at source code, that is!) - the One > True Way (TM) these days is to use Python commands > > Ideally, I have always been hoping to extend this to also allow loading > other kind of plugin entities (data formatters, for instance - but if one > were to dream big, also frame/thread format keywords), but never gotten > around to it > > *tl;dr* I don’t think anyone is actually using these plugins, would not > want them to be removed though, because I have plans for their future > evolution > > On Aug 26, 2014, at 5:09 PM, Zachary Turner <ztur...@google.com> wrote: > > I think my understanding of what it does is correct, but maybe Greg or > someone can confirm. Basically, it tries to dlopen() the module at the > path specified, and then search for the symbols LLDBPluginInitialize and > LLDBPluginTerminate. If it finds them, it calls them. If it doesn't, the > plugin load fails. According to the documentation of dladdr(), it appears > that the process for locating this symbol involves first searching the > module specified in the argument to dlopen(), and then searching any > dependent modules. If it is found in any of these, it succeeds. This > optimization (using RTLD_FIRST and the filename comparison), causes this > search to fail if the symbol is found in a dependent module, but not the > original module. > > I will try to verify that this is correct with someone who knows more than > me about Linux, Mac, and dynamic linking on these platforms, but if correct > then it doesn't seem like there is any risk to removing this. That said, > I'm interested in who is actually use these plugins. The best way to find > out if it's going to break something is to talk to the people who depend on > this code. > > > On Tue, Aug 26, 2014 at 5:01 PM, Todd Fiala <tfi...@google.com> wrote: > >> Ah ok. >> >> It's worth figuring out what it does (really) before we consider removing >> it. >> >> >> >> On Tue, Aug 26, 2014 at 4:53 PM, Zachary Turner <ztur...@google.com> >> wrote: >> >>> That part is a good question, which I don't totally understand. There's >>> a function Debugger::LoadPlugin() though, which accepts a path to a plugin >>> to load. It's called there. This also appears to be exposed through the >>> "plugin load" command. >>> >>> >>> On Tue, Aug 26, 2014 at 4:32 PM, Todd Fiala <tfi...@google.com> wrote: >>> >>>> I guess the thing to do is make sure we're certain we understand the >>>> behavior, which is perhaps best captured in a test. (i.e. test it with the >>>> RTLD_FIRST behavior where it does something, then verify it does something >>>> different without the flag. Then, once we agree it is not useful behavior >>>> for us, look at removing it). >>>> >>>> By valid plugin, you're referring to shared libraries, right? (What >>>> plugins are we referring to here, at what load point?) >>>> >>>> >>>> On Tue, Aug 26, 2014 at 4:29 PM, Zachary Turner <ztur...@google.com> >>>> wrote: >>>> >>>>> Just as a counterpoint, unless I'm misunderstanding this code, I don't >>>>> see it actually having a noticeable impact on stability. The search >>>>> limiting will only be a factor in a case where you attempt to load >>>>> something that *isn't a valid plugin*. It's already an error path. In >>>>> fact, this code worked fine before the change was made, and was only made >>>>> to imitate what appears to have been an optimization that was >>>>> Mac-specific. >>>>> The change for Mac doesn't seem to have been strictly necessary either, >>>>> but just an optimization. It's actually not an optimization for Linux, >>>>> because the dynamic loader will still search every module on linux, it >>>>> will >>>>> just fail anyway. >>>>> >>>>> >>>>> On Tue, Aug 26, 2014 at 4:21 PM, Todd Fiala <tfi...@google.com> wrote: >>>>> >>>>>> Probably the way I'd look at this right now is that support in Linux >>>>>> is a bit dicey and we're doing our best to stabilize (starting with >>>>>> single >>>>>> path for remote/local debugging, and making that stable and fast). In an >>>>>> effort to stabilize, I'd prefer to limit how much code change we do on >>>>>> the >>>>>> Linux end until we have a more stable product. >>>>>> >>>>>> So while we could potentially take that out, I'd rather avoid making >>>>>> changes just because it might be simpler, as it might also add yet >>>>>> another >>>>>> error scenario on the Linux side. Right now I value similarity to MacOSX >>>>>> execution over code reduction. Once we're a lot more stable on the Linux >>>>>> side, I'd be much more interested in revisiting with some actual use >>>>>> cases >>>>>> to see diffs in performance and scope of usage. >>>>>> >>>>>> Just my 2 cents... >>>>>> >>>>>> >>>>>> On Tue, Aug 26, 2014 at 3:47 PM, Zachary Turner <ztur...@google.com> >>>>>> wrote: >>>>>> >>>>>>> The review is up on the LLVM side. One point which was raised, and >>>>>>> which I agree with, is that the presence of the string makes the class >>>>>>> much >>>>>>> heavier. This string is only needed to mimic MacOSX's RTLD_FIRST >>>>>>> behavior >>>>>>> on other posix platforms. However, going back through the history of >>>>>>> when >>>>>>> this was added, I never actually saw a use case from anyone saying "we >>>>>>> *need* this on Linux". See the full original thread at [1]. But the >>>>>>> TL;DR >>>>>>> is that the flag is nice to have on MacOSX, and the filename comparison >>>>>>> was >>>>>>> added to Linux to maintain parity. >>>>>>> >>>>>>> If nobody actually knows of a specific example of why this is >>>>>>> necessary on Linux, can we just remove this behavior on Linux? My >>>>>>> understanding is that the only thing which will change by removing this >>>>>>> for >>>>>>> Linux is the following: Imagine a plugin X is loaded, and X has a >>>>>>> library >>>>>>> dependency on Y and Z. X doesn't contain the plugin Initialize or >>>>>>> Terminate symbol, but Y or Z does. With the filename comparison code, >>>>>>> LoadPlugin would fail, and without it, it would succeed and use the >>>>>>> symbol >>>>>>> found in Y or Z. I can understand that with the comparison the >>>>>>> algorithm >>>>>>> is a bit better, but it seems such an extremely unusual edge case that I >>>>>>> don't think it's a big deal to remove it from the Linux side. >>>>>>> >>>>>>> Thoughts? >>>>>>> >>>>>>> [1] - >>>>>>> http://thread.gmane.org/gmane.comp.debugging.lldb.devel/300/focus=302 >>>>>>> >>>>>>> >>>>>>> On Thu, Aug 21, 2014 at 5:27 PM, Greg Clayton <gclay...@apple.com> >>>>>>> wrote: >>>>>>> >>>>>>>> Sounds good to me. Hopefully if they don't want that they might >>>>>>>> accept an extra boolean argument that can specify to only look in the >>>>>>>> current shared library and then we can switch over to using LLVM's >>>>>>>> DynamicLibrary. >>>>>>>> >>>>>>>> > On Aug 21, 2014, at 4:22 PM, Zachary Turner <ztur...@google.com> >>>>>>>> wrote: >>>>>>>> > >>>>>>>> > This seems like the only case we ever want, so I'm going to post >>>>>>>> a patch to LLVM's DynamicLibrary class to use RTLD_FIRST on Apple, and >>>>>>>> a >>>>>>>> similar method of checking the module filespec on other platforms, and >>>>>>>> see >>>>>>>> if they accept it. If so, I will convert our Plugin code to use LLVM's >>>>>>>> DynamicLibrary and then delete our DynamicLibrary >>>>>>>> > >>>>>>>> > >>>>>>>> > On Thu, Aug 21, 2014 at 4:08 PM, Greg Clayton <gclay...@apple.com> >>>>>>>> wrote: >>>>>>>> > >>>>>>>> > > On Aug 21, 2014, at 3:31 PM, Zachary Turner <ztur...@google.com> >>>>>>>> wrote: >>>>>>>> > > >>>>>>>> > > Can someone explain this flag to me? >>>>>>>> > >>>>>>>> > It says "only look in this binary, don't look in any others. We >>>>>>>> are looking for a plug-in initialization function and we don't want to >>>>>>>> get >>>>>>>> one back from another dylib. >>>>>>>> > >>>>>>>> > As Enrico said, the email from a while back details this: >>>>>>>> > >>>>>>>> > http://comments.gmane.org/gmane.comp.debugging.lldb.devel/305 >>>>>>>> > >>>>>>>> > > I've read the documentation, but it's still not clear to me. >>>>>>>> If you ask dlsym() to search some module X, why would it ever search >>>>>>>> modules other than X? >>>>>>>> > >>>>>>>> > I don't know but it does. >>>>>>>> > >>>>>>>> > > >>>>>>>> > > The reason I ask about this is that llvm support library >>>>>>>> already has a DynamicLibrary class whose purpose almost exactly matches >>>>>>>> what we're using the Host::DynamicLibrary related functions for. >>>>>>>> However, >>>>>>>> it doesn't use the RTLD_FIRST flag, and so I'm not sure what the >>>>>>>> implications are of us using it and deleting our own DynamicLibrary >>>>>>>> code. >>>>>>>> > >>>>>>>> > It would be nice if we could specify this flag so we either find >>>>>>>> the symbol from libx.dylib or we don't. We don't want to find the >>>>>>>> symbol in >>>>>>>> liby.dylib and call it in our case. >>>>>>>> > >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> _______________________________________________ >>>>>>> lldb-dev mailing list >>>>>>> lldb-dev@cs.uiuc.edu >>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev >>>>>>> >>>>>>> >>>>>> >>>>>> >>>>>> -- >>>>>> Todd Fiala | Software Engineer | tfi...@google.com | 650-943-3180 >>>>>> >>>>> >>>>> >>>> >>>> >>>> -- >>>> Todd Fiala | Software Engineer | tfi...@google.com | 650-943-3180 >>>> >>> >>> >> >> >> -- >> Todd Fiala | Software Engineer | tfi...@google.com | 650-943-3180 >> > > _______________________________________________ > lldb-dev mailing list > lldb-dev@cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev > > > Thanks, > *- Enrico* > 📩 egranata@.com ☎️ 27683 > > > > >
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