Meta objects should be bundle-able into the output binary

[stephenrkell]

Keeping metadata in a /usr/lib/meta hierarchy has its pros and cons. As an alternative, it should be possible to bundle the metadata with the output binary. I have prototyped some hacks which provide a basis for this, in my elftin repo (https://github.com/stephenrkell/elftin). More specifically, the "embed-loadable" example shows how to bundle an ELF file opaquely into a containing ELF file.

This embedded ELF file can itself be a loadable .so file... this is directly loadable so long as the ld.so supports dlopening from a file descriptor. Sadly glibc's ld.so doesn't expose this, but internally it more-or-less implements it; elftin also has some non-robust hacks that make this work on a particular build of the now-outdated glibc 2.19 (in the ldso-helper.c file). The way to make this more robust, over multiple versions of glibc, is to use reflection on the ld.so itself. We already generate metadata for the ld.so; it is probably possible to use that to obtain the entry points we need, instead of the hacky offset-based code in ldso-helper.c.

[stephenrkell]

I'm no longer sure this is a good idea. If bundled, the meta-DSO would become opaque to an attached debugger, because it cannot be identified by a path in the filesystem.

Perhaps the right approach is on-demand creation of missing meta-DSOs? This could use debuginfod (#75).

Also, remember the idea of never bundling uniqtypes or other metadata into base (non-meta) DSOs (in #69), instead having then depend on the meta-DSO, perhaps using weak references and a DT_USEFUL extension. Could allocsld rewrite DT_USEFUL into DT_NEEDED or something ignorable, according to whether the meta-DSO is findable-or-generable?