RE: [MIPS] Check info->executable to create DT_MIPS_RLD_MAP

["Maciej W. Rozycki" <macro at codesourcery dot com>]

Chao-ying,

 I failed to cc the binutils list with my reply for some reason, so here 
it is again, with a further note.

On Sat, 22 Jun 2013, Maciej W. Rozycki wrote:

> On Sat, 22 Jun 2013, Chao-Ying Fu wrote:
> 
> > David Daney wrote:
> > > >    When using "-pie", LD doesn't create the DT_MIPS_RLD_MAP entry,
> > > > because LD checks "!info->shared" and the "-pie" flag sets 
> > > "info->shared" to true.
> > > > Note that LD checks "info->executable" to create DT_DEBUG, and
> > > > creating DT_MIPS_RLD_MAP should be similar to creating DT_DEBUG.
> > > > Ex:
> > > >        /* SGI object has the equivalence of DT_DEBUG in the
> > > >           DT_MIPS_RLD_MAP entry.  This must come first because glibc
> > > >           only fills in DT_MIPS_RLD_MAP (not DT_DEBUG) and 
> > > some tools
> > > >           may only look at the first one they see.  */
> > > >        if (!info->shared
> > > >            && !MIPS_ELF_ADD_DYNAMIC_ENTRY (info, 
> > > DT_MIPS_RLD_MAP, 0))
> > > >          return FALSE;
> > > >
> > > 
> > > Probably you want info->executable here too.
> > > 
> > > I have tried simple programs with -pie and they seem to work as is. 
> > > What failures are you seeing?  Does the libgcc unwinder use this?
> 
>  David, just as the comments say -- these tags are used by debuggers and 
> both GDB (in the native mode) and gdbserver make use of them.  The 
> comments also explain why DT_DEBUG is not sufficient (on the 
> MIPS/Linux/glibc target) and getting DT_MIPS_RLD_MAP right is important.
> 
> >   I hit an issue in gdbserver that fails to return a shared-library list,
> > because gdbserver tries to find the DT_MIPS_RLD_MAP entry from
> > a program that is built with -pie.
> 
>  Chao-ying, your consideration appears right to me, please offer a patch.

 Please make sure ld.so handles it correctly first though as I suspect it 
may segfault.  The thing is in a PIE the segment containing the word 
pointed to by DT_MIPS_RLD_MAP may get mapped to an arbitrary memory area 
and it does not immediately appear to me that ld.so relocates the value of 
the DT_MIPS_RLD_MAP tag by the base address, which it has to if it wants 
to use it (it may be enough to add l->l_addr to the value retrieved in 
ELF_MACHINE_DEBUG_SETUP).  But perhaps I'm missing something here.

  Maciej