aarch64 prelink issue

[Mark Salter <msalter@redhat.com>]

I'm trying to get prelink working for aarch64 and ran into a problem
in the aarch64 elf_machine_dynamic() function which is simply:

/* 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)
{
  ElfW(Addr) addr = (ElfW(Addr)) &_DYNAMIC;
  return addr;
}

This routine is only used early before rtld has relocated itself and is
expected to return the static link address of the .dynamic section. The
problem is that the above code generates a got entry + reloc for the
reference to &_DYNAMIC. When the rtld is prelinked, the early startup
code in rtld adds the load address to an already relocated value and
then segfaults as soon as it uses the bogus result.

I worked around this with:

diff --git a/ports/sysdeps/aarch64/dl-machine.h b/ports/sysdeps/aarch64/dl-machine.h
index 94f1108..f69c618 100644
--- a/ports/sysdeps/aarch64/dl-machine.h
+++ b/ports/sysdeps/aarch64/dl-machine.h
@@ -36,7 +36,14 @@ elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
 static inline ElfW(Addr) __attribute__ ((unused))
 elf_machine_dynamic (void)
 {
-  ElfW(Addr) addr = (ElfW(Addr)) &_DYNAMIC;
+  ElfW(Addr) addr;
+
+  asm ("					\n\
+        ldr	%w0, 1f				\n\
+        b	2f                              \n\
+1:	 .word	_DYNAMIC			\n\
+2:						\n\
+       " : "=r" (addr));
   return addr;
 }
 
I suppose the prelink tool could work around this itself which would
allow it to work on all versions of glibc. Even in that case, the
above patch still saves an unnecessary got+reloc.

--Mark