[libvirt] [PATCH] util: avoid symbol clash between json libraries

[Daniel P. Berrangé]

On Tue, Jul 31, 2018 at 11:28:02AM -0500, Eric Blake wrote:
> On 07/31/2018 09:55 AM, Daniel P. Berrangé wrote:
> > The jansson and json-glib libraries both export symbols with a json_
> > name prefix and json_object_iter_next() clashes between them.
> > 
> > Unfortunately json_glib is linked in by GTK, so any app using GTK and
> > libvirt will get a clash, resulting in SEGV. This also affects the NSS
> > module provided by libvirt
> > 
> > Instead of directly linking to jansson, use dlopen() with the RTLD_LOCAL
> > flag which allows us to hide the symbols from the application that loads
> > libvirt or the NSS module.
> > 
> > Some preprocessor black magic and wrapper functions are used to redirect
> > calls into the dlopen resolved symbols.
> 
> Would using dlmopen() instead of dlopen() make this task any easier?

Not available on *BSD

> 
> Otherwise, this looks reasonable to me, and is preferable to Jan's proposal
> to revert jansson support.
> 
> > +++ b/src/util/virjsoncompat.c
> 
> > +
> > +static int virJSONJanssonOnceInit(void)
> > +{
> > +    void *handle = dlopen("libjansson.so.4", RTLD_LAZY|RTLD_LOCAL|RTLD_DEEPBIND|RTLD_NODELETE);
> 
> RTLD_DEEPBIND might be specific to glibc; is this going to cause any
> compilation issues on BSD machines?

Yeah it broke BSD, so removed in v2.

> > +int json_array_append_new_impl(json_t *array, json_t *value)
> > +{
> > +    return json_array_append_new_ptr(array, value);
> > +}
> 
> Would it be possible with __typeof__ to write a macro to make this
> forwarding more compact (one line per stem, instead of open-coding each
> renamed function)?  Looking something like:
> 
> #define FORWARD(name, params, args) \
>   __typeof__(name # _impl) name # _impl params { \
>     return name # _ptr args; \
>   }
> 
> FORWARD(json_array, (void), ())
> FORWARD(json_array_append_new, (json_t * array, json_t *value),
>          (array, value))
> 
> (hmm, that's still a bit verbose; I'm not sure if the preprocessor could be
> cajoled into even less repetition of parameter names)

I'm not too fussed about the verbosity as this is write-once code (i hope).

Regards,
Daniel
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