Re: [GLIBC][AARCH64]Rewrite elf_machine_load_address using _DYNAMIC symbol

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

On 04/11/16 09:42, Renlin Li wrote:
> Hi all,
> 
> This patch rewrites aarch64 elf_machine_load_address to use special _DYNAMIC
> symbol instead of _dl_start.
> 
> The static address of _DYNAMIC symbol is stored in the first GOT entry.
> Here is the change which makes this solution work.
> https://sourceware.org/ml/binutils/2013-06/msg00248.html
> 
> i386, x86_64 targets use the same method to do this as well.
> 
> The original implementation relies on a trick that R_AARCH64_ABS32 relocation
> being resolved at link time and the static address fits in the 32bits.
> However, in LP64, normally, the address is defined to be 64 bit.
> 
> Additionally, the original inline assembly is not optimized. It uses 4
> instructions including a jump.
> 
> Optimally, the new implementation here is just two instructions:
> ldr %1, _GLOBAL_OFFSET_TABLE_
> adr %2, _DYNAMIC
> 
> The size of ld.so is around 130K, so it's save to use ldr, adr to get the address.
> The address range for those two instruction is +/-1MB.
> 
> And by the way, this method is ILP32 safe as well.
> aarch64 linux toolchain regression test OK. OK to commit?
> 
> Regards,
> Renlin Li
> 
> 
> ChangeLog:
> 
> 2016-11-04  Renlin Li  <renlin.li@arm.com>
> 
>     * sysdeps/aarch64/dl-machine.h (elf_machine_load_address): Use
>     _DYNAMIC symbol to calculate load address.

This is OK.

(Roland notes that introducing a BASE symbol with a
linker script would even avoid loading GOT[0], but
that can be done separately across targets)