Re: [PATCH] x86: Allow undefined _DYNAMIC in static executable

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Thu, Sep 28, 2017 at 05:22:07AM -0700, H.J. Lu wrote:
> When --enable-static-pie is used to build static PIE, _DYNAMIC is used
> to compute the load address of static PIE.  But _DYNAMIC is undefined
> when creating static executable.  This patch makes _DYNAMIC weak in PIE
> libc.a so that it can be undefined.
> 
> Any comments?
> 

This is the patch I am checking it in.

H.J.
---
>From 94eea3212e5ace3d447a120c6c0b42cf9427aa1e Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 28 Sep 2017 05:04:35 -0700
Subject: [PATCH] x86: Allow undefined _DYNAMIC in static executable

When --enable-static-pie is used to build static PIE, _DYNAMIC is used
to compute the load address of static PIE.  But _DYNAMIC is undefined
when creating static executable.  This patch makes _DYNAMIC weak in PIE
libc.a so that it can be undefined.

	* sysdeps/i386/dl-machine.h (elf_machine_load_address): Allow
	undefined _DYNAMIC in PIE libc.a.
	* sysdeps/x86_64/dl-machine.h (elf_machine_load_address):
	Likewse.
---
 sysdeps/i386/dl-machine.h   | 11 +++++++++--
 sysdeps/x86_64/dl-machine.h | 11 +++++++++--
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/sysdeps/i386/dl-machine.h b/sysdeps/i386/dl-machine.h
index 2e17eba5c0..242c692c17 100644
--- a/sysdeps/i386/dl-machine.h
+++ b/sysdeps/i386/dl-machine.h
@@ -54,8 +54,15 @@ elf_machine_load_address (void)
   /* Compute the difference between the runtime address of _DYNAMIC as seen
      by a GOTOFF reference, and the link-time address found in the special
      unrelocated first GOT entry.  */
-  extern Elf32_Dyn bygotoff[] asm ("_DYNAMIC") attribute_hidden;
-  return (Elf32_Addr) &bygotoff - elf_machine_dynamic ();
+#ifndef SHARED
+  extern Elf32_Dyn _DYNAMIC[] __attribute__((weak, visibility ("hidden")));
+  if (!_DYNAMIC)
+    return 0;
+#endif
+  /* The address of dynamic must be taken as non-weak to avoid dynamic
+     relocation.  */
+  extern Elf32_Dyn dynamic[] asm ("_DYNAMIC") attribute_hidden;
+  return (Elf32_Addr) &dynamic - elf_machine_dynamic ();
 }
 
 /* Set up the loaded object described by L so its unrelocated PLT
diff --git a/sysdeps/x86_64/dl-machine.h b/sysdeps/x86_64/dl-machine.h
index 6a04cbcdc9..a79e5ca2f9 100644
--- a/sysdeps/x86_64/dl-machine.h
+++ b/sysdeps/x86_64/dl-machine.h
@@ -55,8 +55,15 @@ elf_machine_load_address (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 ();
+#ifndef SHARED
+  extern ElfW(Dyn) _DYNAMIC[] __attribute__((weak, visibility ("hidden")));
+  if (!_DYNAMIC)
+    return 0;
+#endif
+  /* The address of dynamic must be taken as non-weak to avoid dynamic
+     relocation.  */
+  extern ElfW(Dyn) dynamic[] asm ("_DYNAMIC") attribute_hidden;
+  return (ElfW(Addr)) &dynamic - elf_machine_dynamic ();
 }
 
 /* Set up the loaded object described by L so its unrelocated PLT
-- 
2.13.5