rtld: resolve incoming STT_GNU_IFUNC only after DT_INIT

[John Reiser]

When loading a shared library, either DT_NEEDED or via dlopen(), then the
processing by rtld of relocations in other modules which refer to STT_GNU_IFUNC
symbols in the current library causes problems for me.  Relocating
a reference from outside to a STT_GNU_IFUNC symbol causes the IFUNC code
to be called before the DT_INIT executes.  This is not nice.  The _addresses_
in the relocation struct and symbol table are correct, but the instructions
are not there until DT_INIT executes.  My DT_INIT is a decompressor of the
tail of PT_LOAD[0], and the IFUNC code often is in the compressed tail.
[A reference to an STT_GNU_IFUNC symbol from inside the same library
is handled via an R_*_IRELATIVE relocation which the compressor hides
from rtld, and the decompressor handles separately.  A similar dodge
cannot be done for references from other modules.]

It seems to me that rtld could change the order of evaluation, so that
DT_INIT is called before any other code in the library is executed,
including the code for resolving incoming references to STT_GNU_IFUNC
symbols in the library.  Comments?

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