[v2] aarch64: Make elf_machine_{load_address,dynamic} robust [BZ #28203]

  The AArch64 ABI is largely platform agnostic and does not specify
_GLOBAL_OFFSET_TABLE_[0] ([1]). glibc ld.so turns out to be probably the
only user of _GLOBAL_OFFSET_TABLE_[0] and GNU ld defines the value
to the link-time address _DYNAMIC. [2]

In 2012, __ehdr_start was implemented in GNU ld and gold in binutils
2.23.  Using adrp+add / (-mcmodel=tiny) adr to access
__ehdr_start/_DYNAMIC gives us a robust way to get the load address and
the link-time address of _DYNAMIC.

With https://sourceware.org/pipermail/libc-alpha/2021-August/129864.html,
this patch, and disabling traditional TLSGD tests (neither Clang nor
LLD's aarch64 port supports), LLD linked glibc has the same number of
`make check` failures.

[1]: From a psABI maintainer, https://bugs.llvm.org/show_bug.cgi?id=49672#c2
[2]: LLD's aarch64 port does not set _GLOBAL_OFFSET_TABLE_[0] to the
link-time address _DYNAMIC.
LLD is widely used on aarch64 Android and ChromeOS devices.  Software
just works without the need for _GLOBAL_OFFSET_TABLE_[0].

---
Changes from v1
* Use C instead of asm. -mcmodel=tiny adr is possible.
* Adjust commit message
---
 sysdeps/aarch64/dl-machine.h | 24 +++++++++---------------
 1 file changed, 9 insertions(+), 15 deletions(-)
  

The 08/09/2021 10:25, Fangrui Song via Libc-alpha wrote:
> The AArch64 ABI is largely platform agnostic and does not specify
> _GLOBAL_OFFSET_TABLE_[0] ([1]). glibc ld.so turns out to be probably the
> only user of _GLOBAL_OFFSET_TABLE_[0] and GNU ld defines the value
> to the link-time address _DYNAMIC. [2]
> 
> In 2012, __ehdr_start was implemented in GNU ld and gold in binutils
> 2.23.  Using adrp+add / (-mcmodel=tiny) adr to access
> __ehdr_start/_DYNAMIC gives us a robust way to get the load address and
> the link-time address of _DYNAMIC.
> 
> With https://sourceware.org/pipermail/libc-alpha/2021-August/129864.html,
> this patch, and disabling traditional TLSGD tests (neither Clang nor
> LLD's aarch64 port supports), LLD linked glibc has the same number of
> `make check` failures.
> 
> [1]: From a psABI maintainer, https://bugs.llvm.org/show_bug.cgi?id=49672#c2
> [2]: LLD's aarch64 port does not set _GLOBAL_OFFSET_TABLE_[0] to the
> link-time address _DYNAMIC.
> LLD is widely used on aarch64 Android and ChromeOS devices.  Software
> just works without the need for _GLOBAL_OFFSET_TABLE_[0].

This is OK to commit.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

> 
> ---
> Changes from v1
> * Use C instead of asm. -mcmodel=tiny adr is possible.
> * Adjust commit message
> ---
>  sysdeps/aarch64/dl-machine.h | 24 +++++++++---------------
>  1 file changed, 9 insertions(+), 15 deletions(-)
> 
> diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
> index d29d827ab3..3e10cb462f 100644
> --- a/sysdeps/aarch64/dl-machine.h
> +++ b/sysdeps/aarch64/dl-machine.h
> @@ -37,28 +37,22 @@ elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
>    return ehdr->e_machine == EM_AARCH64;
>  }
>  
> -/* Return the link-time address of _DYNAMIC.  Conveniently, this is the
> -   first element of the GOT. */
> -static inline ElfW(Addr) __attribute__ ((unused))
> -elf_machine_dynamic (void)
> -{
> -  extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
> -  return _GLOBAL_OFFSET_TABLE_[0];
> -}
> -
>  /* Return the run-time load address of the shared object.  */
>  
>  static inline ElfW(Addr) __attribute__ ((unused))
>  elf_machine_load_address (void)
>  {
> -  /* To figure out the load address we use the definition that for any symbol:
> -     dynamic_addr(symbol) = static_addr(symbol) + load_addr
> +  extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
> +  return (ElfW(Addr)) &__ehdr_start;
> +}
>  
> -    _DYNAMIC sysmbol is used here as its link-time address stored in
> -    the special unrelocated first GOT entry.  */
> +/* Return the link-time address of _DYNAMIC.  */
>  
> -    extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
> -    return (ElfW(Addr)) &_DYNAMIC - elf_machine_dynamic ();
> +static inline ElfW(Addr) __attribute__ ((unused))
> +elf_machine_dynamic (void)
> +{
> +  extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
> +  return (ElfW(Addr)) _DYNAMIC - elf_machine_load_address ();
>  }
>  
>  /* Set up the loaded object described by L so its unrelocated PLT
> -- 
> 2.32.0.605.g8dce9f2422-goog
>