[glibc] x86_64: Simplify elf_machine_{load_address,dynamic}

[Fangrui Song <maskray@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=b37b75d269883a2c553bb7019a813094eb4e2dd1

commit b37b75d269883a2c553bb7019a813094eb4e2dd1
Author: Fangrui Song <maskray@google.com>
Date:   Tue Aug 17 10:45:57 2021 -0700

    x86_64: Simplify elf_machine_{load_address,dynamic}
    
    and drop reliance on _GLOBAL_OFFSET_TABLE_[0] being the link-time
    address of _DYNAMIC. &__ehdr_start is a better way to get the load address.
    
    Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Diff:
---
 sysdeps/x86_64/dl-machine.h | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/sysdeps/x86_64/dl-machine.h b/sysdeps/x86_64/dl-machine.h
index a8596aa3fa..ceee50734e 100644
--- a/sysdeps/x86_64/dl-machine.h
+++ b/sysdeps/x86_64/dl-machine.h
@@ -35,27 +35,20 @@ elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
 }
 
 
-/* Return the link-time address of _DYNAMIC.  Conveniently, this is the
-   first element of the GOT.  This must be inlined in a function which
-   uses global data.  */
+/* Return the run-time load address of the shared object.  */
 static inline ElfW(Addr) __attribute__ ((unused))
-elf_machine_dynamic (void)
+elf_machine_load_address (void)
 {
-  /* This produces an IP-relative reloc which is resolved at link time. */
-  extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
-  return _GLOBAL_OFFSET_TABLE_[0];
+  extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
+  return (ElfW(Addr)) &__ehdr_start;
 }
 
-
-/* Return the run-time load address of the shared object.  */
+/* Return the link-time address of _DYNAMIC.  */
 static inline ElfW(Addr) __attribute__ ((unused))
-elf_machine_load_address (void)
+elf_machine_dynamic (void)
 {
-  /* Compute the difference between the runtime address of _DYNAMIC as seen
-     by an IP-relative reference, and the link-time address found in the
-     special unrelocated first GOT entry.  */
   extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
-  return (ElfW(Addr)) &_DYNAMIC - elf_machine_dynamic ();
+  return (ElfW(Addr)) _DYNAMIC - elf_machine_load_address ();
 }
 
 /* Set up the loaded object described by L so its unrelocated PLT