Use __ehdr_start rather than _begin in _dl_start_final

[Alan Modra]

On Wed, Apr 27, 2022 at 03:54:27PM -0300, Adhemerval Zanella wrote:
> 
> 
> On 27/04/2022 09:35, Alan Modra via Libc-alpha wrote:
> > __ehdr_start is already used in rltld.c:dl_main, and can serve the
> > same purpose as _begin.  Besides tidying the code, using linker
> > defined section relative symbols rather than "-defsym _begin=0" better
> > reflects the intent of _dl_start_final use of _begin, which is to
> > refer to the load address of ld.so rather than absolute address zero.
> > 
> > The motive for this patch is to finally tidy ppc32 GNU ld treatment of
> > absolute symbols.  On ppc32, the reference to _begin generates a GOT
> > entry.  A GOT entry for an absolute symbol shouldn't be dynamically
> > relocated, so this usage in glibc of an absolute _begin will fail once
> > I fix ppc32 ld.  Note that on many other targets, including ppc64, gcc
> > generates pc relative or got-pointer relative addressing for _begin.
> > ie. the compiler assumes _begin is *not* at an absolute address, and
> > generally linkers go along with that.
> 
> Does it mean that once you fix it, newer binutils will start to fail
> to build older glibc releases for powerpc32?

Not necessarily.  I found these issues with absolute symbols in glibc
by inspecting object files.  Possibly dl_rtld_map.l_map_start == 0
causes no problem, I'll know when I have set up a test environment for
ppc32 that lets me build and run a ppc32 glibc.

-- 
Alan Modra
Australia Development Lab, IBM