[PATCH v2] aarch64: Make elf_machine_{load_address,dynamic} robust [BZ #28203]

[Fangrui Song via Libc-alpha <libc-alpha@sourceware.org>]

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.

With https://sourceware.org/pipermail/libc-alpha/2021-August/129864.html,
this patch, and disabling traditional TLSGD tests (neither Clang nor
LLD's aarch64 port supports), LLD linked glibc has the same number of
`make check` failures.

[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].

---
Changes from v1
* Use C instead of asm. -mcmodel=tiny adr is possible.
* Adjust commit message
---
 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
-- 
2.32.0.605.g8dce9f2422-goog