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 >> > >> > >> >> >
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