[lldb-dev] dlsym() and RTLD_FIRST

Wed Aug 27 11:51:43 PDT 2014

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 <zturner at 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 <gclayton at apple.com> wrote:
>
>>
>> > 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.
>>
>> 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 <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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