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aarch64: Make elf_machine_{load_address,dynamic} robust [BZ #28203]

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The AArch64 ABI is largely platform agnostic and does not specify
_GLOBAL_OFFSET_TABLE_[0] ([1]). glibc ld.so turns out to be probably the
only user of _GLOBAL_OFFSET_TABLE_[0] and GNU ld defines the value
to the link-time address _DYNAMIC. [2]

In 2012, __ehdr_start was implemented in GNU ld and gold in binutils
2.23.  Using adrp+add / (-mcmodel=tiny) adr to access
__ehdr_start/_DYNAMIC gives us a robust way to get the load address and
the link-time address of _DYNAMIC.

[1]: From a psABI maintainer, https://bugs.llvm.org/show_bug.cgi?id=49672#c2
[2]: LLD's aarch64 port does not set _GLOBAL_OFFSET_TABLE_[0] to the
link-time address _DYNAMIC.
LLD is widely used on aarch64 Android and ChromeOS devices.  Software
just works without the need for _GLOBAL_OFFSET_TABLE_[0].

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
This commit is contained in:
Fangrui Song 2021-08-11 09:00:37 -07:00
parent 302247c891
commit 43d06ed218
1 changed files with 9 additions and 15 deletions
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24
sysdeps/aarch64/dl-machine.h
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@ -37,28 +37,22 @@ elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
return ehdr->e_machine == EM_AARCH64;
}
/* Return the link-time address of _DYNAMIC. Conveniently, this is the
first element of the GOT. */
static inline ElfW(Addr) __attribute__ ((unused))
elf_machine_dynamic (void)
{
extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
return _GLOBAL_OFFSET_TABLE_[0];
}
/* Return the run-time load address of the shared object. */
static inline ElfW(Addr) __attribute__ ((unused))
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
extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
return (ElfW(Addr)) &__ehdr_start;
}
_DYNAMIC sysmbol is used here as its link-time address stored in
the special unrelocated first GOT entry. */
/* Return the link-time address of _DYNAMIC. */
extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
return (ElfW(Addr)) &_DYNAMIC - elf_machine_dynamic ();
static inline ElfW(Addr) __attribute__ ((unused))
elf_machine_dynamic (void)
{
extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
return (ElfW(Addr)) _DYNAMIC - elf_machine_load_address ();
}
/* Set up the loaded object described by L so its unrelocated PLT