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Re: [GLIBC][AARCH64]Rewrite elf_machine_load_address using _DYNAMIC symbol
- From: Szabolcs Nagy <szabolcs dot nagy at arm dot com>
- To: Roland McGrath <roland at hack dot frob dot com>, Renlin Li <renlin dot li at foss dot arm dot com>
- Cc: <nd at arm dot com>, <libc-alpha at sourceware dot org>, Marcus Shawcroft <Marcus dot Shawcroft at arm dot com>, Richard Earnshaw <Richard dot Earnshaw at arm dot com>
- Date: Mon, 7 Nov 2016 15:15:23 +0000
- Subject: Re: [GLIBC][AARCH64]Rewrite elf_machine_load_address using _DYNAMIC symbol
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On 04/11/16 21:24, Roland McGrath wrote:
> On many or perhaps all machines, elf_machine_load_address could now be
> implemented purely in C by using a link-time trick.
> In C, just:
> static inline ElfW(Addr) __attribute__ ((unused))
> elf_machine_load_address (void)
> extern const char _BASE __attribute__ ((visibility ("hidden")));
> return (ElfW(Addr)) _BASE;
> Then add a trivial input linker script to the ld.so link:
> PROVIDE_HIDDEN(_BASE = 0);
> I know this works for x86_64 and aarch64, and does not require a load.
> (On x86_64 it's a single lea; on aarch64 it's a single adr+add pair.)
this is less maintenance work, because code can be shared,
but it is not a portable solution: it relies on linker
scripts and on the compiler not doing anything silly.
i think asm is preferable, unless we know that all
supported linkers handle this on all targets.