[AARCH64] Rewrite elf_machine_load_address using _DYNAMIC symbol
Commit Message
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.
>
Comments
On 18/10/17 11:32, Renlin Li wrote:
> 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.
>
This is OK to commit.
@@ -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