[Tinycc-devel] Support for indirect function in external modules

[Thomas Preud'homme]

Hi all,

last version of GNU libc uses symbol with indirect function type (also known as 
STT_GNU_IFUNC) in its recent versions on x86_64 architecture.

This straightforward patch (pushed on mob with short sha1 bcc9137) allow tinycc 
to build against libraries using such type for symbol. Notes that this patch 
does not allow tcc to build executable or library with such symbol type. It 
only supports linking against such libraries.

Important note: The documentation I read about STT_GNU_IFUNC symbols says that 
when a code *load* (which is different from jump and calls) such a symbol, the 
value used should be either its GOTPLT (.got.plt) entry or its GOT entry 
depending on the context. I quickly look at the code and saw that whenever a 
STT_FUNC (or also STT_IFUNC now) is used, a GOT *and* a PLTGOT entry is created 
but I don't know which of the two entries is used depending on the context. But 
for what I saw, whenever there is a load, it's loaded from the same place (I 
guess it's loaded from GOT, according to first comment in load() in 
x86_64-gen.c).

I'm still lacking some tinycc knowledge about the link between source parsing 
and code generation so it would be quite long for me to add the necessary 
support. So could one of the tinycc guru add the code to handle this 
differentiation ? The rules about the value to load (GOTPLT entry or GOT entry) 
are 
explained in this document:

http://generic-abi.googlegroups.com/web/ifunc.txt?gda=xtGavTsAAADkRlZw3fTn8WF8pRDgf6aLfN_Atkb_cwXe6cZOcg5qrUgoJ4G7--yvLPBq95D3B7sGRdr3QrylPkw2aRbXD_gF

I quote:

"Note:

Instead of branching to an STT_GNU_IFUNC symbol directly, we always
branch to its PLT entry, which simply loads its GOTPLT entry and branch
to it.  Its GOTPLT entry has the real function address.  It has an
optional GOT entry for the function pointer value of the symbol.  To
load an STT_GNU_IFUNC symbol function pointer value,

1. Use its GOTPLT entry in a shared object if it is forced local or not
dynamic.
2. Use its GOTPLT entry in a non-shared object if pointer equality isn't
needed.
3. Use its GOTPLT entry in a position independent executable (PIE).
4. Use its GOTPLT entry if no normal GOT, other than GOTPLT, is used.
5. Otherwise use its GOT entry.  We only need to relocate its GOT entry
in a shared object."

Best regards,

Thomas Preud'homme

indirect_function.patch
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