[Bug libc/29662] s390 glibc wrongly assumes GOT[0] is _DYNAMIC

["i at maskray dot me" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29662

Fangrui Song <i at maskray dot me> changed:

           What    |Removed                     |Added
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                 CC|                            |i at maskray dot me

--- Comment #3 from Fangrui Song <i at maskray dot me> ---
(In reply to Andreas Krebbel from comment #1)
> Actually our ABI document defines that the first GOT entry has to point at
> _DYNAMIC:
> 
> The format and interpretation of the Global Offset Table is processor
> specific. For s390x the symbol _GLOBAL_OFFSET_TABLE_ may be used to access
> the table. The symbol refers to the start of the .got section. Two words in
> the GOT are reserved:
> 
> • The word at _GLOBAL_OFFSET_TABLE_[0] is set by the linkage editor to hold
> the address of the dynamic structure, referenced with the symbol _DYNAMIC.
> This  allows a program, such as the dynamic linker, to find its own dynamic
> structure without having yet processed its relocation entries. This is
> especially important for the dynamic linker, because it must initialize
> itself without relying on other programs to relocate its memory image.
> • The word at _GLOBAL_OFFSET_TABLE_[1] is reserved for future use.
> 
> Please see:
> https://github.com/IBM/s390x-abi/releases/download/v1.6/lzsabi_s390x.pdf

This is the legacy SunOS 4.x ld.so way of getting `__DYNAMIC`, which was copied
by NetBSD ldso in 1993, and glibc.

`_GLOBAL_OFFSET_TABLE_[0] = _DYNAMIC` is not really necessary for ldso to get
_DYNAMIC.  It can use plain C with a hidden visibility of _DYNAMIC.

Even on x86-32 with no PC-relative relocations, I managed to change the glibc
port to avoid `_GLOBAL_OFFSET_TABLE_[0] = _DYNAMIC` assumption (commit
91e92272caefad4b6156572fc41671dcbd93afe5)

/* Return the link-time address of _DYNAMIC.  */
static inline Elf32_Addr __attribute__ ((unused))
elf_machine_dynamic (void)
{
  extern Elf32_Dyn _DYNAMIC[] attribute_hidden;
  return (Elf32_Addr) _DYNAMIC - elf_machine_load_address ();
 }

It would be nice to drop the assumption for s390x as well. It's very odd to
define `_GLOBAL_OFFSET_TABLE_[0] = _DYNAMIC` in a psABI, at least I haven't
seen it in other psABI documents.

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