> 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 > > _______________________________________________ lldb-dev mailing list lldb-dev@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
