[lldb-dev] dlsym() and RTLD_FIRST

Tue Aug 26 16:53:21 PDT 2014

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.
>>>>> >
>>>>>
>>>>>
>>>>
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>>>>
>>>>
>>>
>>>
>>> --
>>>  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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