Re: Problem in rtld.c with gcc-2.95pre

[Geoff Keating <geoffk@ozemail.com.au>]

> Mailing-List: contact libc-alpha-help@sourceware.cygnus.com; run by ezmlm
> From: Franz Sirl <Franz.Sirl-kernel@lauterbach.com>
> Date: Thu, 10 Jun 1999 22:07:28 +0200
> 
> Hi,
> 
> in rtld.c is the following code:
> 
>   /* Relocate ourselves so we can do normal function calls and
>      data access using the global offset table.  */
> 
>   ELF_DYNAMIC_RELOCATE (&bootstrap_map, 0, 0);
> 
>   /* Please note that we don't allow profiling of this object and
>      therefore need not test whether we have to allocate the array
>      for the relocation results (as done in dl-reloc.c).  */
> 
>   /* Now life is sane; we can call functions and access global data.
>      Set up to use the operating system facilities, and find out from
>      the operating system's program loader where to find the program
>      header table in core.  */
> 
>   if (HP_TIMING_AVAIL)
>     {
>       /* If it hasn't happen yet record the startup time.  */
>       if (! HP_TIMING_INLINE)
>         HP_TIMING_NOW (start_time);
> 
>       /* Initialize the timing functions.  */
>       HP_TIMING_DIFF_INIT ();
>     }
> 
>   /* Transfer data about ourselves to the permanent link_map structure.  */
>   _dl_rtld_map.l_addr = bootstrap_map.l_addr;
> 
> 
> With gcc-2.95pre the loading of the address of _dl_rtld_map into a register is
> moved _before_ the call to ELF_DYNAMIC_RELOCATE()! This lets ld.so crash
> immediately when it tries to access _dl_rtld_map.l_addr, because it still uses
> the address loaded out of GOT before the relocation.
> I tried to surround ELF_DYNAMIC_RELOCATE() with 
> 
> asm volatile ("" : : : "memory");
> 
> but this had no effect, maybe because gcc doesn't consider the GOT as normal
> memory?
> 
> My platform is as usual powerpc-linux-gnu with glibc-2.1.1.

Aha!  I've been waiting for that to cause trouble for years now.

Indeed, gcc doesn't consider a reference through the GOT as a memory
reference.

I think the change that is needed is something like switching, in
rs6000.md, from:

(define_expand "movsi_got"
  [(set (match_operand:SI 0 "register_operand" "")
        (unspec [(match_operand:SI 1 "got_operand" "")
                 (match_dup 2)] 8))]

(or whatever it now is in gcc)

to:

(define_expand "movsi_got"
  [(set (match_operand:SI 0 "register_operand" "")
        (mem (plus (unspec [(match_operand:SI 1 "got_operand" "")] 8)
                 (match_dup 2))))]

in all the places where an 'unspec 8' appears.  This will inform gcc
of what is going on, in particular that this is a memory reference,
and then the `asm volatile' will work.  At least, I hope so.  Tell me
how it goes.

-- 
Geoffrey Keating <geoffk@ozemail.com.au>