Re: Minimal program crashes, depending on linker script

[Paul Pluzhnikov]

Carsten Fuchs <CarstenFuchs@T-Online.de> writes:

> I'd be very grateful for any advice about this problem or any help to
> understand what causes it!

The program crashes because (as can be seen in your "info shared"
output), libGL.so.1 is loaded before your dso.so, and because it
*defines* a (non-writable) .text symbol glActiveStencilFaceEXT().

The rules for UNIX symbol resolution are that the first instance
of a symbol "wins", and all subsequent uses of that symbol bind to
that first definition.

This allows one to redefine e.g. malloc(), and have all calls to
malloc() from other DSOs go to that "custom definition".

So here is what happens:

1. 0x4009b69c glActiveStencilFaceEXT .text definition in libGL
2. 0xNNNNNNNN glActiveStencilFaceEXT=NULL; .bss definition  in dso.so; ignored

void GetRenderer() {

 glActiveStencilFaceEXT= ...; // 3. here you *intend* to assign value to symbol 
2,
                              // but the linker binds reference to 
"glActiveStencilFaceEXT"
                              // to symbol 1, because it came *first*
}

Since symbol 1 is in a non-writable .text section, you loose.

In effect, your crash is equivalent to:

  extern int *malloc;
  int main() { int i; malloc = &i; return 0; }

Naturally, if you localize all symbols in dso with a linker script,
then at point 3 the reference is to local glActiveStencilFaceEXT,
and you win. The -Bsymbolic has much the same effect.

If you symply rename "glActiveStencilFaceEXT" to
e.g. "pGlActiveStencilFaceEXT" at points 2 and 3, the problem will
also go away.

You may also wish to run your (crashing) test like this:

  LD_DEBUG=symbols,bindings ./test 2>&1 | grep glActiveStencilFaceEXT

and study the output.

Cheers,
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