[lldb-dev] dlsym() and RTLD_FIRST

Zachary Turner zturner at google.com
Tue Aug 26 18:55:51 PDT 2014 

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 <egranata at 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 <zturner at 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 <tfiala at 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 <zturner at 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 <tfiala at 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 <zturner at 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 <tfiala at 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 <zturner at 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 <gclayton at 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 <zturner at 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 <gclayton at apple.com>
>>>>>>>> wrote:
>>>>>>>> >
>>>>>>>> > > On Aug 21, 2014, at 3:31 PM, Zachary Turner <zturner at 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 at cs.uiuc.edu
>>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>>  Todd Fiala | Software Engineer |  tfiala at google.com |  650-943-3180
>>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Todd Fiala | Software Engineer |  tfiala at google.com |  650-943-3180
>>>>
>>>
>>>
>>
>>
>> --
>> Todd Fiala | Software Engineer |  tfiala at google.com |  650-943-3180
>>
>
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>
> Thanks,
> *- Enrico*
> 📩 egranata@.com ☎️ 27683
>
>
>
>
>
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