libgl-elf-tricks-demo
There are several ELF-level problems mentioned in my new Linux OpenGL ABI straw-man proposal. The files here demonstrate some ways those problems could be solved.
Problem 1: Symbol Collisions
OpenGL ES is speced such that most of its entry points have the same names as entry points in (non-ES) OpenGL. The OpenGL and OpenGL ES symbols are provided by separate libraries (e.g., libOpenGL.so.1 and libGLESv2.so.1, respectively). It may be possible for applications to have both libraries loaded at the same time (often indirectly, through chains of DSO dependencies). Multiple symbols with the same name will cause ambiguity over which symbol should be used where. Calling the wrong symbol could lead to incorrect behavior.
One solution to this problem is to use ELF symbol versioning to stamp each symbol with a version that indicates which library provides it. When an application is linked with -lOpenGL or -lGLESv2, the symbol's version will be recorded in the application. At run time, only the symbol with the correct version will be used to resolve the symbol in the application.
This solution is not entirely satisfying: the version of the symbol is defined at link time, so each linked object can only reference symbols from one of libOpenGL or libGLESv2. For an application to have run-time selection of OpenGL vs OpenGL ES, it would need to abstract its usage of those two APIs into separate helper DSOs.
The demo here creates dumby libOpenGL.so.1 and libGLESv2.so.1 DSOs with versioned symbols. Then it shows how an application would link against or the other or both and always get the intended symbol.
To see this in action, run:
make symbol-version-tests && sh ./run-symbol-version-tests.sh
See Maintaining APIs and ABIs in Ulrich Drepper's DSO Howto: www.akkadia.org/drepper/dsohowto.pdf
Problem 2: one DSO as a "pass-thru" to another DSO
For backwards compatibility there will need to be a /usr/lib/libGL.so.1, but it would be nice if that could just be an empty stub that routes to the new OpenGL implementation in libOpenGL.so.1.
ELF filters can be used such that symbols advertised by libGL.so.1 are actually satisfied by libOpenGL.so.1.
To see this in action, run:
make filter-tests && sh ./run-filter-tests.sh
See:
- http://docs.oracle.com/cd/E23824_01/html/819-0690/chapter4-4.html
- GNU ld(1)'s '--filter' command line option