Re: Duplicate data objects in shared libraries

["H . J . Lu" <hjl at lucon dot org>]

On Mon, May 20, 2002 at 08:57:35PM +0100, Jason Merrill wrote:
> >>>>> "H" == H J Lu <hjl@lucon.org> writes:
> 
> > On Mon, May 20, 2002 at 05:12:19PM +0100, Jason Merrill wrote:
> >> 
> >> Interestingly, loading C.so first as RTLD_LOCAL causes both A.so and B.so
> >> to resolve to different addresses from C.so on Linux, but on Solaris it
> >> produces the desired result.
> 
> > It seems like a Linux bug. I will look into it if no one else does.
> 
> >> #4 as written above could have the effect of causing B.so to refer to
> >> a definition in A.so, which would be problematic if we try to unload A.so.
> >> Perhaps the right approach is
> >> 
> >> 5) Do not allow an object loaded with RTLD_LOCAL to override symbols from a
> >> dependency.
> >> 
> >> This rule is easily stated; it would cause both A.so and B.so to refer to
> >> the definition in C.so, regardless of the order of loading.  I like it.
> 
> > If you were saying:
> 
> > 1. Load C.so with RTLD_LOCAL.
> > 2. Load A.so with RTLD_LOCAL.
> > 3. Load B.so with RTLD_LOCAL.
> 
> > both A.so and B.so should resolve to C.so, I think it makes sense.
> 
> Yes, that is what I was saying.  A possible refinement would be
> 
> 6) #5, but if the definition in the RTLD_LOCAL object is strong, use it in
>    the object.

I believe Linux is trying to move away from special treatment of weak
symbol in ld.so.

> 
> Which would produce the current Linux semantics described above if the
> definitions in A.so and B.so are strong, and the current Solaris semantics
> described above if they are weak.  This would allow a plugin writer to

Do you have a testcase in C to show the Linux behavior? I believe it is
a Linux bug.


H.J.