Make nacl_helper easily debuggable

chrome/nacl/nacl_helper_bootstrap_linux.x

 /* Copyright (c) 2011 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  *
  * This is a custom linker script used to build nacl_helper_bootstrap.
  * It has a very special layout.  This script will only work with input
  * that is kept extremely minimal.  If there are unexpected input sections
  * not named here, the result will not be correct.
  *
  * We need to use a standalone loader program rather than just using a
@@ skipping 30 matching lines @@
 
 /*
  * We specify the program headers we want explicitly, to get the layout
  * exactly right and to give the "reserve" segment p_flags of zero, so
  * that it gets mapped as PROT_NONE.
  */
 PHDRS {
   text PT_LOAD FILEHDR PHDRS;
   data PT_LOAD;
   reserve PT_LOAD FLAGS(0);
+  r_debug PT_LOAD;

[Mark Seaborn, 2011/11/14 20:15:41]

Should you put FLAGS?  What's the default?  It doesn't seem to be in the docs. 
Does it default to inferring the flags from the sections?

   note PT_NOTE;
   stack PT_GNU_STACK FLAGS(6);  /* RW, no E */
 }
 
 /*
  * Now we lay out the sections across those segments.
  */
 SECTIONS {
   . = TEXT_START + SIZEOF_HEADERS;
 
@@ skipping 44 matching lines @@
    * It just maps it from the file, i.e. way off the end of the file,
    * which is perfect for reserving the address space.
    */
   . = ALIGN(CONSTANT(COMMONPAGESIZE));
   RESERVE_START = .;
   .reserve : {
     . = RESERVE_TOP - RESERVE_START;
   } :reserve
 
   /*
+   * This must be placed above the reserved address space, so it won't
+   * be clobbered by NaCl.  We want this to be visible at its fixed address
+   * in the memory image so the debugger can make sense of things.
+   */
+  .r_debug : {
+    *(.r_debug)
+  } :r_debug
+
+  /*
    * These are empty input sections the linker generates.
    * If we don't discard them, they pollute the flags in the output segment.
    */
   /DISCARD/ : {
     *(.iplt)
     *(.rel*)
     *(.igot.plt)
   }
 }