[PLUGIN] dlopen and RTLD_NOW

[PLUGIN] dlopen and RTLD_NOW

[Romain Geissler]

Hi

Is there any particular reason to load plugin with the RTLD_NOW option?
This option force .so symbol resolution to be completely made at load time,
but this could be done only when a symbol is needed (RTLD_NOW).

Here is the dlopen line in plugin.c:
dl_handle = dlopen (plugin->full_name, RTLD_NOW | RTLD_GLOBAL);

My issue is, I want to load the same plugin.so in both cc1 and cc1plus, but
in the C++ case, I may need to reference some cc1plus specific symbols. I can
check whether cc1 or cc1plus loaded the plugin and thus use custom C++
symbols only when present. With RTLD_NOW, the plugin fails to load in cc1 as
symbol resolution is forced at load time.

If RTLD_NOW is removed, dlopen falls back to the RTLD_LAZY mode which fits
my need. Moreover, if one can force the complete symbol resolution at load time
by defining the environment LD_BIND_NOW variable.

So, is RTLD_NOW use justified ?

Romain Geissler

Re: [PLUGIN] dlopen and RTLD_NOW

[Richard Guenther]

On Mon, Sep 5, 2011 at 9:50 AM, Romain Geissler
<romain.geissler@gmail.com> wrote:
> Hi
>
> Is there any particular reason to load plugin with the RTLD_NOW option?
> This option force .so symbol resolution to be completely made at load time,
> but this could be done only when a symbol is needed (RTLD_NOW).
>
> Here is the dlopen line in plugin.c:
> dl_handle = dlopen (plugin->full_name, RTLD_NOW | RTLD_GLOBAL);
>
> My issue is, I want to load the same plugin.so in both cc1 and cc1plus, but
> in the C++ case, I may need to reference some cc1plus specific symbols. I can
> check whether cc1 or cc1plus loaded the plugin and thus use custom C++
> symbols only when present. With RTLD_NOW, the plugin fails to load in cc1 as
> symbol resolution is forced at load time.
>
> If RTLD_NOW is removed, dlopen falls back to the RTLD_LAZY mode which fits
> my need. Moreover, if one can force the complete symbol resolution at load time
> by defining the environment LD_BIND_NOW variable.
>
> So, is RTLD_NOW use justified ?

Not really, neither RTLD_GLOBAL.

Richard.

> Romain Geissler
>

Re: [PLUGIN] dlopen and RTLD_NOW

[Jakub Jelinek]

On Mon, Sep 05, 2011 at 09:51:34AM +0200, Richard Guenther wrote:
> > My issue is, I want to load the same plugin.so in both cc1 and cc1plus, but
> > in the C++ case, I may need to reference some cc1plus specific symbols. I can
> > check whether cc1 or cc1plus loaded the plugin and thus use custom C++
> > symbols only when present. With RTLD_NOW, the plugin fails to load in cc1 as
> > symbol resolution is forced at load time.
> >
> > If RTLD_NOW is removed, dlopen falls back to the RTLD_LAZY mode which fits
> > my need. Moreover, if one can force the complete symbol resolution at load time
> > by defining the environment LD_BIND_NOW variable.
> >
> > So, is RTLD_NOW use justified ?
> 
> Not really, neither RTLD_GLOBAL.

That said, relying on lazy binding is terribly bad design.

	Jakub

Re: [PLUGIN] dlopen and RTLD_NOW

[Andrew Pinski]

On Mon, Sep 5, 2011 at 1:10 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> That said, relying on lazy binding is terribly bad design.

In fact I was going to say why can't those symbols be marked as weak
in your plugin?  You don't even need to change the GCC headers, just
have an extra header that does:
#pargma weak

Thanks,
Andrew Pinski

Re: [PLUGIN] dlopen and RTLD_NOW

[Jakub Jelinek]

On Mon, Sep 05, 2011 at 10:22:10AM -0700, Andrew Pinski wrote:
> On Mon, Sep 5, 2011 at 1:10 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> > That said, relying on lazy binding is terribly bad design.
> 
> In fact I was going to say why can't those symbols be marked as weak
> in your plugin?  You don't even need to change the GCC headers, just
> have an extra header that does:
> #pargma weak

s/pargma/pragma/.  Yeah, making them weak will work just fine, independently
on whether it is RTLD_NOW or not, or, when program is directly linked
against it, with LD_BIND_NOW=1 or not.

	Jakub

Re: [PLUGIN] dlopen and RTLD_NOW

[Romain Geissler]

2011/9/5 Jakub Jelinek <jakub@redhat.com>:
> On Mon, Sep 05, 2011 at 10:22:10AM -0700, Andrew Pinski wrote:
>> On Mon, Sep 5, 2011 at 1:10 AM, Jakub Jelinek <jakub@redhat.com> wrote:
>> > That said, relying on lazy binding is terribly bad design.
>>
>> In fact I was going to say why can't those symbols be marked as weak
>> in your plugin?  You don't even need to change the GCC headers, just
>> have an extra header that does:
>> #pargma weak
>
> s/pargma/pragma/.  Yeah, making them weak will work just fine, independently
> on whether it is RTLD_NOW or not, or, when program is directly linked
> against it, with LD_BIND_NOW=1 or not.
>
>        Jakub
>

Thanks, it works fine. I didn't know about weak symbols.

Romain Geissler

Re: [PLUGIN] dlopen and RTLD_NOW

[David Daney]

On 09/05/2011 12:50 AM, Romain Geissler wrote:
> Hi
>
> Is there any particular reason to load plugin with the RTLD_NOW option?
> This option force .so symbol resolution to be completely made at load time,
> but this could be done only when a symbol is needed (RTLD_NOW).
>
> Here is the dlopen line in plugin.c:
> dl_handle = dlopen (plugin->full_name, RTLD_NOW | RTLD_GLOBAL);
>
> My issue is, I want to load the same plugin.so in both cc1 and cc1plus, but
> in the C++ case, I may need to reference some cc1plus specific symbols. I can
> check whether cc1 or cc1plus loaded the plugin and thus use custom C++
> symbols only when present. With RTLD_NOW, the plugin fails to load in cc1 as
> symbol resolution is forced at load time.
>

Can you supply weak binding implementations for the missing functions? 
That might allow the linking to succeed.

David Daney

Re: [PLUGIN] dlopen and RTLD_NOW

[David Daney]

On 09/06/2011 10:55 AM, David Daney wrote:
> On 09/05/2011 12:50 AM, Romain Geissler wrote:
>> Hi
>>
>> Is there any particular reason to load plugin with the RTLD_NOW option?
>> This option force .so symbol resolution to be completely made at load
>> time,
>> but this could be done only when a symbol is needed (RTLD_NOW).
>>
>> Here is the dlopen line in plugin.c:
>> dl_handle = dlopen (plugin->full_name, RTLD_NOW | RTLD_GLOBAL);
>>
>> My issue is, I want to load the same plugin.so in both cc1 and
>> cc1plus, but
>> in the C++ case, I may need to reference some cc1plus specific
>> symbols. I can
>> check whether cc1 or cc1plus loaded the plugin and thus use custom C++
>> symbols only when present. With RTLD_NOW, the plugin fails to load in
>> cc1 as
>> symbol resolution is forced at load time.
>>
>
> Can you supply weak binding implementations for the missing functions?
> That might allow the linking to succeed.
>

... And if I read the entire thread before responding, I would have seen 
that others had already suggested the same thing.

Sorry for the noise.

David Daney