[lldb-dev] dlsym() and RTLD_FIRST

Wed Aug 27 11:49:43 PDT 2014

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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