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

On Mon, 24 Jun 2013, Chao-Ying Fu wrote:

> >  Please make sure 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 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.
>   As your suggestion, here is a patch that can fix seg fault inside 
> for a simple "hello" test.
> Ex:
> fu@debian6:~/dev/test$ ./  ./hello-pie-new
> Segmentation fault
> fu@debian6:~/dev/test$ ./  ./hello-pie-new
> Hello
> Ex:
> Index: mips/dl-machine.h
> ===================================================================
> --- mips.orig/dl-machine.h	2012-12-14 03:50:06.000000000 -0800
> +++ mips/dl-machine.h	2013-06-24 13:49:59.622179000 -0700
> @@ -69,7 +69,8 @@
>     with the run-time address of the r_debug structure  */
>  #define ELF_MACHINE_DEBUG_SETUP(l,r) \
>  do { if ((l)->l_info[DT_MIPS (RLD_MAP)]) \
> -       *(ElfW(Addr) *)((l)->l_info[DT_MIPS (RLD_MAP)]->d_un.d_ptr) = \
> +       *(ElfW(Addr) *)((l)->l_info[DT_MIPS (RLD_MAP)]->d_un.d_ptr \
> +                       + (l)->l_addr) = \
>         (ElfW(Addr)) (r); \
>     } while (0)

 Please make sure this actually works for native GDB and gdbserver.

 You may have to relocate the tag itself, as stored in memory.  Please 
take care though as the lone reason for DT_MIPS_RLD_MAP's existence is so 
that the dynamic segment can be mapped read-only.  Therefore you may have 
to tweak map permissions temporarily (and then preferably only if the base 
address is non-zero).

 A better approach might be adding the base address in GDB/gdbserver so 
that the dynamic segment remains unchanged, but having no experience with 
PIE executables I'm not sure offhand if these programs will have this 
information available -- hopefully yes.

>   The question is how to deploy the changes in and GNU LD, without
> breaking existing systems.  Thanks!

 Well, this should have been addressed from the beginning by whoever added 
PIE support -- then we wouldn't have this problem now, sigh.  
Unfortunately nothing clever comes immediately to my mind, however I think 
it should be safe to fix the side right away, and then we can 
think what to do with the binutils side.


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