[PATCH] Allow memset local PLT reference for RISC-V.

[Maciej W. Rozycki]

On Fri, 10 Jul 2020, Alistair Francis wrote:

> > > Anyways, if you want to know where the PLT call is coming from, you
> > > can't rely on the relocs.  R_RISCV_CALL_PLT is not necessarily a plt
> > > call.
> >
> >  If a PLT entry has been created, then the linker must have considered the
> > symbol referred preemptible, whether legitimately or not.
> >
> >  Rather than scratching one's head I would suggest running the relevant LD
> > invocation under GDB to find out what really happens there, which may be
> > as easy as setting a breakpoint on `riscv_elf_finish_dynamic_symbol' with
> > the right condition on the hash entry so as to stop on `memset' only, and
> > then working backwards with a watchpoint (on a host system that does not
> > use ASLR) to find out what sets `h->plt.offset'.  There'll be the answer.
> 
> I tried to look into this but haven't made much progress here. If
> anyone has more ideas they would be very welcome otherwise I'll keep
> digging into this.

 The `h->needs_plt' flag is set by this `riscv_elf_check_relocs' code in 
bfd/elfnn-riscv.c:
        
	case R_RISCV_CALL_PLT:
	  /* This symbol requires a procedure linkage table entry.  We
	     actually build the entry in adjust_dynamic_symbol,
	     because this might be a case of linking PIC code without
	     linking in any dynamic objects, in which case we don't
	     need to generate a procedure linkage table after all.  */

	  if (h != NULL)
	    {
	      h->needs_plt = 1;
	      h->plt.refcount += 1;
	    }
	  break;

due to a reference from `addtf3.o', from `libgcc.a'.  There are further 20 
references for a total of 21.  There is a later opportunity for the flag 
to be cleared in `riscv_elf_adjust_dynamic_symbol', as observed in the 
comment, however that only happens for symbols that bind locally, or which 
are not referenced, and for `memset' obviously neither is the case (and 
`allocate_dynrelocs' sets `h->plt.offset' in place of `h->plt.refcount' 
then).

 I don't think we have a way to redirect the reference to `__GI_memset'.

  Maciej