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From 43d06ed218fc8be58987bdfd00e21e5720f0b862 Mon Sep 17 00:00:00 2001
From: Fangrui Song <maskray@google.com>
Date: Wed, 11 Aug 2021 09:00:37 -0700
Subject: [PATCH] aarch64: Make elf_machine_{load_address,dynamic} robust [BZ
#28203]
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>
---
sysdeps/aarch64/dl-machine.h | 24 +++++++++---------------
1 file changed, 9 insertions(+), 15 deletions(-)
diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index d29d827..3e10cb4 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -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
--
1.8.3.1
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