Re: Proposal for STT_GNU_IFUNC and R_*_IRELATIVE

["H.J. Lu" <hjl dot tools at gmail dot com>]

Hi,

Here is the current proposal implemented in the Linux binutils
2.19.51.0.7.

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 */

All references to a STT_GNU_IFUNC symbol, including function call and
function pointer, will go through a PLT slot, which jumps to the address
stored in the GOT entry.  If the STT_GNU_IFUNC symbol is locally defined,
a R_*_IRELATIVE relocation will be applied to the GOT entry at load time.
Otherwise, dynamic linker will lookup the symbol at the first call to the
function and update the GOT entry. This applies to all usages of
STT_GNU_IFUNC symbols in shared library, dynamic executable and static
executable.