[glibc/maskray/lld] aarch64: Make elf_machine_{load_address, dynamic} robust [BZ #28203]

[Fangrui Song <maskray@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=43d06ed218fc8be58987bdfd00e21e5720f0b862

commit 43d06ed218fc8be58987bdfd00e21e5720f0b862
Author: Fangrui Song <maskray@google.com>
Date:   Wed Aug 11 09:00:37 2021 -0700

    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>

Diff:
---
 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 d29d827ab3..3e10cb462f 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