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commit 6b02af31e9a721bb15a11380cd22d53b621711f8
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date: Wed Oct 18 17:26:23 2017 +0100
[AARCH64] Rewrite elf_machine_load_address using _DYNAMIC symbol
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 (part of binutils 2.24):
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.
Here is the C version one which should be portable in all cases.
* sysdeps/aarch64/dl-machine.h (elf_machine_load_address): Use
_DYNAMIC symbol to calculate load address.
diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index e86d8b5b63..5a5b8a5de5 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -49,26 +49,11 @@ 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 32bit relocations being resolved at link time
- and that the static address fits in the 32bits. */
-
- ElfW(Addr) static_addr;
- ElfW(Addr) dynamic_addr;
-
- asm (" \n"
-" adrp %1, _dl_start; \n"
-" add %1, %1, #:lo12:_dl_start \n"
-" ldr %w0, 1f \n"
-" b 2f \n"
-"1: \n"
-" .word _dl_start \n"
-"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
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