I'm personally okay with this on the Linux side, with the caveat as always that if we find an issue with this, we back out the change.
-Todd On Wed, Aug 27, 2014 at 11:51 AM, Zachary Turner <ztur...@google.com> wrote: > Combined with the fact that plugins are essentially a dead codepath (going > by Enrico's earlier comment), I think it's probably ok to just remove this > and use LLVM's default DynamicLibrary loader. Any objections? > > > On Wed, Aug 27, 2014 at 11:49 AM, Zachary Turner <ztur...@google.com> > wrote: > >> The documentation of dlsym() says this: >> >> ------ >> The *dlsym*() function shall search for the named symbol in all objects >> loaded automatically *as a result of loading the object referenced by * >> *handle* (see *dlopen*() >> <http://pubs.opengroup.org/onlinepubs/009695399/functions/dlopen.html>). >> Load ordering is used in *dlsym*() operations upon the global symbol >> object. The symbol resolution algorithm used shall be dependency order as >> described in *dlopen*() >> <http://pubs.opengroup.org/onlinepubs/009695399/functions/dlopen.html>. >> ------ >> >> So, it appears that this is perhaps a non-issue. In other words, if you >> dlopen plugin A and get back handle A, and then you dlopen plugin B and get >> back handle B, there is no chance that dlsym against B would find a symbol >> in plugin A. It will search only B and B's direct and indirect >> dependencies. >> >> There is a remote possibility that a plugin A could link against *another >> plugin B. *However, as long as A provides a LLDBPluginInitialize >> method, it will always be found first as described by the section on >> "dependency ordering" in the documentation of dlopen(). >> >> ----- >> Dependency ordering uses a breadth-first order *starting with a given >> object*, then all of its dependencies, then any dependents of those, >> iterating until all dependencies are satisfied. >> >> >> On Wed, Aug 27, 2014 at 11:32 AM, Greg Clayton <gclay...@apple.com> >> wrote: >> >>> >>> > 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. >>> >>> Bingo. We don't want to call any other version of LLDBPluginInitialize >>> or LLDBPluginTerminate from any other plug-in. It isn't clear that the >>> dynamic linker sticks to dependent modules, I would need to check on that. >>> It might search all loaded shared libraries in the current process. Not >>> sure if that differs between Mac and Linux. Probably not. >>> >>> > >>> > 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. >>> >>> Again, we _only_ want to call LLDBPluginInitialize or >>> LLDBPluginTerminate from the exact shared library we are trying to ask for >>> the symbols from, not from anywhere else. >>> >>> Greg >>> > >>> > >>> > 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 >>> > >>> > >>> >>> >> > -- Todd Fiala | Software Engineer | tfi...@google.com | 650-943-3180
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