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

[Renlin Li <renlin.li@foss.arm.com>]

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Hi Szabolcs,

Here is the C version one which should be portable in all cases.
aarch64 native glibc regression test checked Okay.

Regards,
Renlin

ChangeLog:

2017-10-18  Renlin Li  <renlin.li@arm.com>

	* sysdeps/aarch64/dl-machine.h (elf_machine_load_address): Use
	_DYNAMIC symbol to calculate load address.


On 17/10/17 17:28, Szabolcs Nagy wrote:
> On 17/10/17 16:41, Szabolcs Nagy wrote:
>> 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)
>>
>
> please wait with this.
>
> looking at the static pie patches, it seems that also needs
> to compute the base address and that cannot assume -mcmodel=tiny,
> i don't remember if there was a particular reason -mcmodel=large
> would be problematic, if inline asm was only used to save a
> few instructions then please resend the patch but using c code
> (like what x86_64 is doing), that's less fragile.
>

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diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index b124547..e765612 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -51,40 +51,11 @@ 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
 
-     The choice of symbol is arbitrary. The static address we obtain
-     by constructing a non GOT reference to the symbol, the dynamic
-     address of the symbol we compute using adrp/add to compute the
-     symbol's address relative to the PC.
-     This depends on 32/16bit relocations being resolved at link time
-     and that the static address fits in the 32/16 bits.  */
-
-  ElfW(Addr) static_addr;
-  ElfW(Addr) dynamic_addr;
-
-  asm ("					\n"
-"	adrp	%1, _dl_start;			\n"
-#ifdef __LP64__
-"	add	%1, %1, #:lo12:_dl_start	\n"
-#else
-"	add	%w1, %w1, #:lo12:_dl_start	\n"
-#endif
-"	ldr	%w0, 1f				\n"
-"	b	2f				\n"
-"1:						\n"
-#ifdef __LP64__
-"	.word	_dl_start			\n"
-#else
-# ifdef __AARCH64EB__
-"	.short  0                               \n"
-# endif
-"	.short  _dl_start                       \n"
-# ifndef __AARCH64EB__
-"	.short  0                               \n"
-# endif
-#endif
-"2:						\n"
-    : "=r" (static_addr),  "=r" (dynamic_addr));
-  return dynamic_addr - static_addr;
+    _DYNAMIC sysmbol is used here as its link-time address stored in
+    the special unrelocated first GOT entry.  */
+
+    extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
+    return (ElfW(Addr)) &_DYNAMIC - elf_machine_dynamic ();
 }
 
 /* Set up the loaded object described by L so its unrelocated PLT