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 > > > > > >
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