Proposal for STT_GNU_IFUNC and R_*_IRELATIVE

[Jan Beulich]

I have to admit that I don't see why a new symbol type *and* a new
relocation type would be necessary here. If the to-be-relocated symbols'
name is to not appear in the original (dynamic) symbol table, then all that is
needed is a second relocation section referencing a second (in the case of
non-ET_REL objects perhaps the static) symbol table. Whether this split
would really be necessary in ET_REL objects I doubt in the first place, so
it's perhaps just a job of the static linker to separate the relocations
accordingly. The consequence would be the need for a new set of DT_*
tags, which, along with the symbol type, ought to be OS-specific (whereas
the new relocation type is OS-independent, and hence meaningless for
OSes not supporting STT_GNU_IFUNC).

Jan

>>> "H.J. Lu" <hjl.tools@gmail.com> 25.05.09 20:56 >>>
Hi,

Here is a proposal for STT_GNU_IFUNC and R_*_IRELATIVE. It
has been implemented in the Linux binutils 2.19.51.0.5.


H.J.
----
STT_GNU_IFUNC

      This symbol type is the same as STT_FUNC except that it always
      points to a function or piece of executable code which takes no
      arguments and returns a function pointer.  If an STT_GNU_IFUNC
      symbol is referred to by a relocation, then evaluation of that
      relocation is delayed until load-time.  The value used in the
      relocation is the function pointer returned by an invocation
      of the STT_GNU_IFUNC symbol.

      The purpose of this symbol type is to allow the run-time to
      select between multiple versions of the implementation of a
      specific function.  The selection made in general will take the
      currently available hardware into account and select the most
      appropriate version.

STT_GNU_IFUNC is defined in OS-specific range:

#define STT_LOOS	10	/* OS-specific semantics */
#define STT_GNU_IFUNC	10	/* Symbol is an indirect code object */
#define STT_HIOS	12	/* OS-specific semantics */

R_*_IRELATIVE

      This relocation is similar to R_*_RELATIVE except that the
      value used in this relocation is the program address returned
      by the function, which takes no arguments, at the address of
      the result of the corresponding R_*_RELATIVE relocation.

      The purpose of this relocation to avoid name lookup for locally
      defined STT_GNU_IFUNC symbols at load-time.

R_*_IRELATIVE is defined for i386 and x86-64:

#define R_386_IRELATIVE    42	/* Adjust indirectly by program base */
#define R_X86_64_IRELATIVE 37	/* Adjust indirectly by program base */