gold patch committed: Reverse .ctors in .init_array

[Ian Lance Taylor <iant at google dot com>]

Alan pointed out that when a .ctors sections which has multiple words is
mapped to a .init_array section, we need to reverse the order of the
entries in the .ctors section.  This is because .ctors and .init_array
are executed in reverse order, and we don't want the linker to change
the order in which the constructors are executed.  The same applies to
.dtors mapped to .fini_array.  This patch implements that in gold.
Committed to mainline.

Ian


2011-06-24  Ian Lance Taylor  <iant@google.com>

	* layout.cc: Include "object.h".
	(ctors_sections_in_init_array): New static variable.
	(Layout::is_ctors_in_init_array): New function.
	(Layout::layout): Add entry to ctors_sections_in_init_array if
	appropriate.
	* layout.h (class Layout): Declare is_ctors_in_init_array.
	* reloc.cc (Sized_relobj_file::do_relocate): Call reverse_words if
	is_ctors_reverse_view is set.
	(Sized_relobj_file::write_sections): Add layout parameter.  Change
	all callers.  Set is_ctors_reverse_view field of View_size.
	(Sized_relobj_file::reverse_words): New function.
	* object.h (Sized_relobj_file::View_size): Add
	is_ctors_reverse_view field.
	(class Sized_relobj_file): Update declarations.
	* testsuite/initpri3.c: New test.
	* testsuite/Makefile.am: (check_PROGRAMS): Add initpri3a and
	initpri3b.
	(initpri3a_SOURCES, initpri3a_DEPENDENCIES): New variables.
	(initpri3a_LDFLAGS, initpri3a_LDADD): New variables.
	(initpri3b_SOURCES, initpri3b_DEPENDENCIES): New variables.
	(initpri3b_LDFLAGS, initpri3b_LDADD): New variables.
	* testsuite/Makefile.in: Rebuild.


Index: layout.cc
===================================================================
RCS file: /cvs/src/src/gold/layout.cc,v
retrieving revision 1.205
diff -p -u -r1.205 layout.cc
--- layout.cc	23 Jun 2011 00:46:13 -0000	1.205
+++ layout.cc	25 Jun 2011 00:35:35 -0000
@@ -46,6 +46,7 @@
 #include "ehframe.h"
 #include "compressed_output.h"
 #include "reduced_debug_output.h"
+#include "object.h"
 #include "reloc.h"
 #include "descriptors.h"
 #include "plugin.h"
@@ -617,6 +618,30 @@ Layout::find_output_segment(elfcpp::PT t
   return NULL;
 }
 
+// When we put a .ctors or .dtors section with more than one word into
+// a .init_array or .fini_array section, we need to reverse the words
+// in the .ctors/.dtors section.  This is because .init_array executes
+// constructors front to back, where .ctors executes them back to
+// front, and vice-versa for .fini_array/.dtors.  Although we do want
+// to remap .ctors/.dtors into .init_array/.fini_array because it can
+// be more efficient, we don't want to change the order in which
+// constructors/destructors are run.  This set just keeps track of
+// these sections which need to be reversed.  It is only changed by
+// Layout::layout.  It should be a private member of Layout, but that
+// would require layout.h to #include object.h to get the definition
+// of Section_id.
+static Unordered_set<Section_id, Section_id_hash> ctors_sections_in_init_array;
+
+// Return whether OBJECT/SHNDX is a .ctors/.dtors section mapped to a
+// .init_array/.fini_array section.
+
+bool
+Layout::is_ctors_in_init_array(Relobj* relobj, unsigned int shndx) const
+{
+  return (ctors_sections_in_init_array.find(Section_id(relobj, shndx))
+	  != ctors_sections_in_init_array.end());
+}
+
 // Return the output section to use for section NAME with type TYPE
 // and section flags FLAGS.  NAME must be canonicalized in the string
 // pool, and NAME_KEY is the key.  ORDER is where this should appear
@@ -922,11 +947,11 @@ Layout::layout(Sized_relobj_file<size, b
     }
 
   // By default the GNU linker sorts input sections whose names match
-  // .ctor.*, .dtor.*, .init_array.*, or .fini_array.*.  The sections
-  // are sorted by name.  This is used to implement constructor
-  // priority ordering.  We are compatible.  When we put .ctor
-  // sections in .init_array and .dtor sections in .fini_array, we
-  // must also sort plain .ctor and .dtor sections.
+  // .ctors.*, .dtors.*, .init_array.*, or .fini_array.*.  The
+  // sections are sorted by name.  This is used to implement
+  // constructor priority ordering.  We are compatible.  When we put
+  // .ctor sections in .init_array and .dtor sections in .fini_array,
+  // we must also sort plain .ctor and .dtor sections.
   if (!this->script_options_->saw_sections_clause()
       && !parameters->options().relocatable()
       && (is_prefix_of(".ctors.", name)
@@ -938,6 +963,22 @@ Layout::layout(Sized_relobj_file<size, b
 		  || strcmp(name, ".dtors") == 0))))
     os->set_must_sort_attached_input_sections();
 
+  // If this is a .ctors or .ctors.* section being mapped to a
+  // .init_array section, or a .dtors or .dtors.* section being mapped
+  // to a .fini_array section, we will need to reverse the words if
+  // there is more than one.  Record this section for later.  See
+  // ctors_sections_in_init_array above.
+  if (!this->script_options_->saw_sections_clause()
+      && !parameters->options().relocatable()
+      && shdr.get_sh_size() > size / 8
+      && (((strcmp(name, ".ctors") == 0
+	    || is_prefix_of(".ctors.", name))
+	   && strcmp(os->name(), ".init_array") == 0)
+	  || ((strcmp(name, ".dtors") == 0
+	       || is_prefix_of(".dtors.", name))
+	      && strcmp(os->name(), ".fini_array") == 0)))
+    ctors_sections_in_init_array.insert(Section_id(object, shndx));
+
   // FIXME: Handle SHF_LINK_ORDER somewhere.
 
   elfcpp::Elf_Xword orig_flags = os->flags();
Index: layout.h
===================================================================
RCS file: /cvs/src/src/gold/layout.h,v
retrieving revision 1.93
diff -p -u -r1.93 layout.h
--- layout.h	23 Jun 2011 00:46:13 -0000	1.93
+++ layout.h	25 Jun 2011 00:35:35 -0000
@@ -651,6 +651,12 @@ class Layout
   static bool
   match_file_name(const Relobj* relobj, const char* file_name);
 
+  // Return whether section SHNDX in RELOBJ is a .ctors/.dtors section
+  // with more than one word being mapped to a .init_array/.fini_array
+  // section.
+  bool
+  is_ctors_in_init_array(Relobj* relobj, unsigned int shndx) const;
+
   // Check if a comdat group or .gnu.linkonce section with the given
   // NAME is selected for the link.  If there is already a section,
   // *KEPT_SECTION is set to point to the signature and the function
Index: reloc.cc
===================================================================
RCS file: /cvs/src/src/gold/reloc.cc,v
retrieving revision 1.67
diff -p -u -r1.67 reloc.cc
--- reloc.cc	31 May 2011 04:26:49 -0000	1.67
+++ reloc.cc	25 Jun 2011 00:35:35 -0000
@@ -659,7 +659,7 @@ Sized_relobj_file<size, big_endian>::do_
   // section data to the output file.  The second one applies
   // relocations.
 
-  this->write_sections(pshdrs, of, &views);
+  this->write_sections(layout, pshdrs, of, &views);
 
   // To speed up relocations, we set up hash tables for fast lookup of
   // input offsets to output addresses.
@@ -678,6 +678,8 @@ Sized_relobj_file<size, big_endian>::do_
     {
       if (views[i].view != NULL)
 	{
+	  if (views[i].is_ctors_reverse_view)
+	    this->reverse_words(views[i].view, views[i].view_size);
 	  if (!views[i].is_postprocessing_view)
 	    {
 	      if (views[i].is_input_output_view)
@@ -712,7 +714,8 @@ struct Read_multiple_compare
 
 template<int size, bool big_endian>
 void
-Sized_relobj_file<size, big_endian>::write_sections(const unsigned char* pshdrs,
+Sized_relobj_file<size, big_endian>::write_sections(const Layout* layout,
+						    const unsigned char* pshdrs,
 						    Output_file* of,
 						    Views* pviews)
 {
@@ -761,6 +764,7 @@ Sized_relobj_file<size, big_endian>::wri
 	  pvs->address = posd->address();
 	  pvs->is_input_output_view = false;
 	  pvs->is_postprocessing_view = false;
+	  pvs->is_ctors_reverse_view = false;
 
 	  continue;
 	}
@@ -875,6 +879,12 @@ Sized_relobj_file<size, big_endian>::wri
       pvs->view_size = view_size;
       pvs->is_input_output_view = output_offset == invalid_address;
       pvs->is_postprocessing_view = os->requires_postprocessing();
+      pvs->is_ctors_reverse_view =
+	(!parameters->options().relocatable()
+	 && view_size > size / 8
+	 && (strcmp(os->name(), ".init_array") == 0
+	     || strcmp(os->name(), ".fini_array") == 0)
+	 && layout->is_ctors_in_init_array(this, i));
     }
 
   // Actually read the data.
@@ -1483,6 +1493,26 @@ Sized_relobj_file<size, big_endian>::fin
     }
 }
 
+// Reverse the words in a section.  Used for .ctors sections mapped to
+// .init_array sections.  See ctors_sections_in_init_array in
+// layout.cc.
+
+template<int size, bool big_endian>
+void
+Sized_relobj_file<size, big_endian>::reverse_words(unsigned char* view,
+						   section_size_type view_size)
+{
+  typedef typename elfcpp::Swap<size, big_endian>::Valtype Valtype;
+  Valtype* vview = reinterpret_cast<Valtype*>(view);
+  section_size_type vview_size = view_size / (size / 8);
+  for (section_size_type i = 0; i < vview_size / 2; ++i)
+    {
+      Valtype tmp = vview[i];
+      vview[i] = vview[vview_size - 1 - i];
+      vview[vview_size - 1 - i] = tmp;
+    }
+}
+
 // Class Merged_symbol_value.
 
 template<int size>
Index: object.h
===================================================================
RCS file: /cvs/src/src/gold/object.h,v
retrieving revision 1.110
diff -p -u -r1.110 object.h
--- object.h	24 May 2011 21:57:28 -0000	1.110
+++ object.h	25 Jun 2011 00:35:35 -0000
@@ -2220,6 +2220,7 @@ class Sized_relobj_file : public Sized_r
     section_size_type view_size;
     bool is_input_output_view;
     bool is_postprocessing_view;
+    bool is_ctors_reverse_view;
   };
 
   typedef std::vector<View_size> Views;
@@ -2311,7 +2312,8 @@ class Sized_relobj_file : public Sized_r
   // Write section data to the output file.  Record the views and
   // sizes in VIEWS for use when relocating.
   void
-  write_sections(const unsigned char* pshdrs, Output_file*, Views*);
+  write_sections(const Layout*, const unsigned char* pshdrs, Output_file*,
+		 Views*);
 
   // Relocate the sections in the output file.
   void
@@ -2320,6 +2322,11 @@ class Sized_relobj_file : public Sized_r
 		    Views* pviews)
   { this->do_relocate_sections(symtab, layout, pshdrs, of, pviews); }
 
+  // Reverse the words in a section.  Used for .ctors sections mapped
+  // to .init_array sections.
+  void
+  reverse_words(unsigned char*, section_size_type);
+
   // Scan the input relocations for --emit-relocs.
   void
   emit_relocs_scan(Symbol_table*, Layout*, const unsigned char* plocal_syms,
Index: testsuite/initpri3.c
===================================================================
RCS file: testsuite/initpri3.c
diff -N testsuite/initpri3.c
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ testsuite/initpri3.c	25 Jun 2011 00:35:35 -0000
@@ -0,0 +1,80 @@
+/* initpri3.c -- test ctor odering when using init_array.
+
+   Copyright 2011 Free Software Foundation, Inc.
+   Written by Ian Lance Taylor <iant@google.com>.
+
+   This file is part of gold.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
+   MA 02110-1301, USA.  */
+
+/* This tests that the linker correctly orders .ctor entries when
+   putting them into .init_array, as is the default.  */
+
+#include <assert.h>
+
+int i = 1;
+
+static void
+ctor1 (void)
+{
+  assert (i == 1);
+  i = 2;
+}
+
+static void
+ctor2 (void)
+{
+  assert (i == 2);
+  i = 3;
+}
+
+static void
+dtor1 (void)
+{
+  assert (i == 3);
+  i = 2;
+}
+
+static void
+dtor2 (void)
+{
+  assert (i == 2);
+  i = 1;
+}
+
+/* The .ctors section is run in reverse order, the .dtors section in
+   run in forward order.  We give these arrays the "aligned" attribute
+   because the x86_64 ABI would otherwise give them a 16-byte
+   alignment, which may leave a hole in the section.  */
+
+void (*ctors[]) (void)
+  __attribute__ ((aligned (4), section (".ctors"))) = {
+  ctor2,
+  ctor1
+};
+
+void (*dtors[]) (void)
+  __attribute__ ((aligned (4), section (".dtors"))) = {
+  dtor1,
+  dtor2
+};
+
+int
+main (void)
+{
+  assert (i == 3);
+  return 0;
+}
Index: testsuite/Makefile.am
===================================================================
RCS file: /cvs/src/src/gold/testsuite/Makefile.am,v
retrieving revision 1.165
diff -p -u -r1.165 Makefile.am
--- testsuite/Makefile.am	24 Jun 2011 18:08:49 -0000	1.165
+++ testsuite/Makefile.am	25 Jun 2011 00:35:36 -0000
@@ -846,6 +846,18 @@ initpri2_DEPENDENCIES = gcctestdir/ld
 initpri2_LDFLAGS = -Bgcctestdir/
 initpri2_LDADD =
 
+check_PROGRAMS += initpri3a
+initpri3a_SOURCES = initpri3.c
+initpri3a_DEPENDENCIES = gcctestdir/ld
+initpri3a_LDFLAGS = -Bgcctestdir/
+initpri3a_LDADD =
+
+check_PROGRAMS += initpri3b
+initpri3b_SOURCES = initpri3.c
+initpri3b_DEPENDENCIES = gcctestdir/ld
+initpri3b_LDFLAGS = -Bgcctestdir/ -Wl,--no-ctors-in-init-array
+initpri3b_LDADD =
+
 # Test --detect-odr-violations
 check_SCRIPTS += debug_msg.sh