[gold commit] Fix handling of __ehdr_start

[ccoutant at google dot com (Cary Coutant)]

This patch fixes gold's treatment of the special __ehdr_start
symbol to better match Gnu ld. The description of the problem
and subsequent discussion can be found here:

  https://sourceware.org/ml/binutils/2014-03/msg00309.html

and continuing into April:

  https://sourceware.org/ml/binutils/2014-04/msg00005.html

-cary


commit bd7babc4f78060d51442b5e2da82786a80f98ee5
Author: Cary Coutant <ccoutant@google.com>
Date:   Wed Apr 2 14:21:14 2014 -0700

    Fix handling of __ehdr_start when it cannot be defined.
    
    2014-05-02  Cary Coutant  <ccoutant@google.com>
    
    	* defstd.cc (in_segment): Define __ehdr_start here...
    	* layout.cc (Layout::finalize): ...Instead of here.  Set the
    	output segment when known.
    	* resolve.cc (Symbol::override_base_with_special): Remember
    	the original binding.
    	* symtab.cc (Symbol::set_output_segment): New function.
    	(Symbol::set_undefined): New function.
    	* symtab.h (Symbol::is_weak_undefined): Check original undef
    	binding.
    	(Symbol::is_strong_undefined): New function.
    	(Symbol::set_output_segment): New function.
    	(Symbol::set_undefined): New function.
    	* target-reloc.h (is_strong_undefined): Remove.
    	(issue_undefined_symbol_error): Call Symbol::is_weak_undefined.
    	Check for hidden undefs.
    	(relocate_section): Call Symbol::is_strong_undefined.
    
    	* testsuite/Makefile.am (ehdr_start_test_1)
    	(ehdr_start_test_2, ehdr_start_test_3)
    	(ehdr_start_test_4, ehdr_start_test_5): New test cases.
    	* testsuite/Makefile.in: Regenerate.
    	* testsuite/ehdr_start_def.cc: New source file.
    	* testsuite/ehdr_start_test.cc: New source file.
    	* testsuite/ehdr_start_test.t: New linker script.
    	* testsuite/ehdr_start_test_4.sh: New shell script.

diff --git a/gold/defstd.cc b/gold/defstd.cc
index a50e75d..cee68a0 100644
--- a/gold/defstd.cc
+++ b/gold/defstd.cc
@@ -141,6 +141,20 @@ const Define_symbol_in_segment in_segment[] =
     true			// only_if_ref
   },
   {
+    "__ehdr_start",		// name
+    elfcpp::PT_LOAD,		// segment_type
+    elfcpp::PF(0),		// segment_flags_set
+    elfcpp::PF(0),		// segment_flags_clear
+    0,				// value
+    0,				// size
+    elfcpp::STT_NOTYPE,		// type
+    elfcpp::STB_GLOBAL,		// binding
+    elfcpp::STV_HIDDEN,		// visibility
+    0,				// nonvis
+    Symbol::SEGMENT_START,	// offset_from_base
+    true			// only_if_ref
+  },
+  {
     "etext",			// name
     elfcpp::PT_LOAD,		// segment_type
     elfcpp::PF_X,		// segment_flags_set
diff --git a/gold/layout.cc b/gold/layout.cc
index 02f691e..147f740 100644
--- a/gold/layout.cc
+++ b/gold/layout.cc
@@ -2749,12 +2749,14 @@ Layout::finalize(const Input_objects* input_objects, Symbol_table* symtab,
   // If there is a load segment that contains the file and program headers,
   // provide a symbol __ehdr_start pointing there.
   // A program can use this to examine itself robustly.
-  if (load_seg != NULL)
-    symtab->define_in_output_segment("__ehdr_start", NULL,
-				     Symbol_table::PREDEFINED, load_seg, 0, 0,
-				     elfcpp::STT_NOTYPE, elfcpp::STB_GLOBAL,
-				     elfcpp::STV_HIDDEN, 0,
-				     Symbol::SEGMENT_START, true);
+  Symbol *ehdr_start = symtab->lookup("__ehdr_start");
+  if (ehdr_start != NULL && ehdr_start->is_predefined())
+    {
+      if (load_seg != NULL)
+	ehdr_start->set_output_segment(load_seg, Symbol::SEGMENT_START);
+      else
+        ehdr_start->set_undefined();
+    }
 
   // Set the file offsets of all the non-data sections we've seen so
   // far which don't have to wait for the input sections.  We need
diff --git a/gold/resolve.cc b/gold/resolve.cc
index 9b442e2..8cc637a 100644
--- a/gold/resolve.cc
+++ b/gold/resolve.cc
@@ -915,6 +915,10 @@ Symbol::override_base_with_special(const Symbol* from)
   bool same_name = this->name_ == from->name_;
   gold_assert(same_name || this->has_alias());
 
+  // If we are overriding an undef, remember the original binding.
+  if (this->is_undefined())
+    this->set_undef_binding(this->binding_);
+
   this->source_ = from->source_;
   switch (from->source_)
     {
diff --git a/gold/symtab.cc b/gold/symtab.cc
index 1a69f5b..4e8afb1 100644
--- a/gold/symtab.cc
+++ b/gold/symtab.cc
@@ -527,6 +527,31 @@ Symbol::set_output_section(Output_section* os)
     }
 }
 
+// Set the symbol's output segment.  This is used for pre-defined
+// symbols whose segments aren't known until after layout is done
+// (e.g., __ehdr_start).
+
+void
+Symbol::set_output_segment(Output_segment* os, Segment_offset_base base)
+{
+  gold_assert(this->is_predefined_);
+  this->source_ = IN_OUTPUT_SEGMENT;
+  this->u_.in_output_segment.output_segment = os;
+  this->u_.in_output_segment.offset_base = base;
+}
+
+// Set the symbol to undefined.  This is used for pre-defined
+// symbols whose segments aren't known until after layout is done
+// (e.g., __ehdr_start).
+
+void
+Symbol::set_undefined()
+{
+  gold_assert(this->is_predefined_);
+  this->source_ = IS_UNDEFINED;
+  this->is_predefined_ = false;
+}
+
 // Class Symbol_table.
 
 Symbol_table::Symbol_table(unsigned int count,
diff --git a/gold/symtab.h b/gold/symtab.h
index b06c7b4..b6366d4 100644
--- a/gold/symtab.h
+++ b/gold/symtab.h
@@ -238,7 +238,7 @@ class Symbol
   override_visibility(elfcpp::STV);
 
   // Set whether the symbol was originally a weak undef or a regular undef
-  // when resolved by a dynamic def.
+  // when resolved by a dynamic def or by a special symbol.
   inline void
   set_undef_binding(elfcpp::STB bind)
   {
@@ -249,7 +249,8 @@ class Symbol
       }
   }
 
-  // Return TRUE if a weak undef was resolved by a dynamic def.
+  // Return TRUE if a weak undef was resolved by a dynamic def or
+  // by a special symbol.
   inline bool
   is_undef_binding_weak() const
   { return this->undef_binding_weak_; }
@@ -517,7 +518,20 @@ class Symbol
   // Return whether this is a weak undefined symbol.
   bool
   is_weak_undefined() const
-  { return this->is_undefined() && this->binding() == elfcpp::STB_WEAK; }
+  {
+    return (this->is_undefined()
+	    && (this->binding() == elfcpp::STB_WEAK
+		|| this->is_undef_binding_weak()));
+  }
+
+  // Return whether this is a strong undefined symbol.
+  bool
+  is_strong_undefined() const
+  {
+    return (this->is_undefined()
+	    && this->binding() != elfcpp::STB_WEAK
+	    && !this->is_undef_binding_weak());
+  }
 
   // Return whether this is an absolute symbol.
   bool
@@ -782,6 +796,18 @@ class Symbol
   void
   set_output_section(Output_section*);
 
+  // Set the symbol's output segment.  This is used for pre-defined
+  // symbols whose segments aren't known until after layout is done
+  // (e.g., __ehdr_start).
+  void
+  set_output_segment(Output_segment*, Segment_offset_base);
+
+  // Set the symbol to undefined.  This is used for pre-defined
+  // symbols whose segments aren't known until after layout is done
+  // (e.g., __ehdr_start).
+  void
+  set_undefined();
+
   // Return whether there should be a warning for references to this
   // symbol.
   bool
@@ -1030,7 +1056,7 @@ class Symbol
   // True if UNDEF_BINDING_WEAK_ has been set (bit 32).
   bool undef_binding_set_ : 1;
   // True if this symbol was a weak undef resolved by a dynamic def
-  // (bit 33).
+  // or by a special symbol (bit 33).
   bool undef_binding_weak_ : 1;
   // True if this symbol is a predefined linker symbol (bit 34).
   bool is_predefined_ : 1;
diff --git a/gold/target-reloc.h b/gold/target-reloc.h
index f49020a..e44519b 100644
--- a/gold/target-reloc.h
+++ b/gold/target-reloc.h
@@ -143,12 +143,6 @@ class Default_comdat_behavior
   }
 };
 
-inline bool
-is_strong_undefined(const Symbol* sym)
-{
-  return sym->is_undefined() && sym->binding() != elfcpp::STB_WEAK;
-}
-
 // Give an error for a symbol with non-default visibility which is not
 // defined locally.
 
@@ -190,7 +184,7 @@ issue_undefined_symbol_error(const Symbol* sym)
     return false;
 
   // We don't report weak symbols.
-  if (sym->binding() == elfcpp::STB_WEAK)
+  if (sym->is_weak_undefined())
     return false;
 
   // We don't report symbols defined in discarded sections.
@@ -216,6 +210,10 @@ issue_undefined_symbol_error(const Symbol* sym)
 	return false;
     }
 
+  // If the symbol is hidden, report it.
+  if (sym->visibility() == elfcpp::STV_HIDDEN)
+    return true;
+
   // When creating a shared library, only report unresolved symbols if
   // -z defs was used.
   if (parameters->options().shared() && !parameters->options().defs())
@@ -419,7 +417,7 @@ relocate_section(
 	gold_undefined_symbol_at_location(sym, relinfo, i, offset);
       else if (sym != NULL
 	       && sym->visibility() != elfcpp::STV_DEFAULT
-	       && (is_strong_undefined(sym) || sym->is_from_dynobj()))
+	       && (sym->is_strong_undefined() || sym->is_from_dynobj()))
 	visibility_error(sym);
 
       if (sym != NULL && sym->has_warning())
diff --git a/gold/testsuite/Makefile.am b/gold/testsuite/Makefile.am
index 52cc05e..1f275b0 100644
--- a/gold/testsuite/Makefile.am
+++ b/gold/testsuite/Makefile.am
@@ -2199,6 +2199,50 @@ gdb_index_test_4.stdout: gdb_index_test_4
 
 endif HAVE_PUBNAMES
 
+# Test that __ehdr_start is defined correctly.
+check_PROGRAMS += ehdr_start_test_1
+ehdr_start_test_1_SOURCES = ehdr_start_test.cc
+ehdr_start_test_1_DEPENDENCIES = gcctestdir/ld
+ehdr_start_test_1_CXXFLAGS =
+ehdr_start_test_1_LDFLAGS = -Bgcctestdir/
+ehdr_start_test_1_LDADD =
+
+# Test that __ehdr_start is defined correctly with a weak reference.
+check_PROGRAMS += ehdr_start_test_2
+ehdr_start_test_2_SOURCES = ehdr_start_test.cc
+ehdr_start_test_2_DEPENDENCIES = gcctestdir/ld
+ehdr_start_test_2_CXXFLAGS = -DEHDR_START_WEAK
+ehdr_start_test_2_LDFLAGS = -Bgcctestdir/
+ehdr_start_test_2_LDADD =
+
+# Test that __ehdr_start is defined correctly when used with a linker script.
+check_PROGRAMS += ehdr_start_test_3
+ehdr_start_test_3_SOURCES = ehdr_start_test.cc
+ehdr_start_test_3_DEPENDENCIES = gcctestdir/ld $(srcdir)/ehdr_start_test.t
+ehdr_start_test_3_CXXFLAGS = -DEHDR_START_WEAK
+ehdr_start_test_3_LDFLAGS = -Bgcctestdir/ -Wl,-T,$(srcdir)/ehdr_start_test.t
+ehdr_start_test_3_LDADD =
+
+# Test that __ehdr_start is left undefined when the text segment is not
+# appropriately aligned.
+check_SCRIPTS += ehdr_start_test_4.sh
+check_DATA += ehdr_start_test_4.syms
+MOSTLYCLEANFILES += ehdr_start_test_4
+ehdr_start_test_4.syms: ehdr_start_test_4
+	$(TEST_NM) ehdr_start_test_4 > $@
+ehdr_start_test_4: ehdr_start_test_4.o gcctestdir/ld
+	$(CXXLINK) -Bgcctestdir/ld/ -Wl,-Ttext=0x100100 $<
+ehdr_start_test_4.o: ehdr_start_test.cc
+	$(CXXCOMPILE) -c -DEHDR_START_WEAK -o $@ $<
+
+# Test that __ehdr_start is not overridden when supplied by the user.
+check_PROGRAMS += ehdr_start_test_5
+ehdr_start_test_5_SOURCES = ehdr_start_test.cc ehdr_start_def.cc
+ehdr_start_test_5_DEPENDENCIES = gcctestdir/ld
+ehdr_start_test_5_CXXFLAGS = -DEHDR_START_USER_DEF
+ehdr_start_test_5_LDFLAGS = -Bgcctestdir/
+ehdr_start_test_5_LDADD =
+
 # End-to-end incremental linking tests.
 # Incremental linking is currently supported only on the x86_64 target.
 
diff --git a/gold/testsuite/ehdr_start_def.cc b/gold/testsuite/ehdr_start_def.cc
new file mode 100644
index 0000000..f102a78
--- /dev/null
+++ b/gold/testsuite/ehdr_start_def.cc
@@ -0,0 +1,26 @@
+// ehdr_start_def.cc -- test for __ehdr_start linker-defined symbol.
+
+// Copyright (C) 2014 Free Software Foundation, Inc.
+// Written by Cary Coutant <ccoutant@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.
+
+// We provide a user-defined __ehdr_start, to make sure that the
+// linker does not override this with the linker-defined symbol.
+
+char __ehdr_start[] = { 'a', 'b', 'c', 'd' };
diff --git a/gold/testsuite/ehdr_start_test.cc b/gold/testsuite/ehdr_start_test.cc
new file mode 100644
index 0000000..a119b5e
--- /dev/null
+++ b/gold/testsuite/ehdr_start_test.cc
@@ -0,0 +1,67 @@
+// ehdr_start_test.cc -- test for __ehdr_start linker-defined symbol.
+
+// Copyright (C) 2014 Free Software Foundation, Inc.
+// Written by Cary Coutant <ccoutant@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.
+
+// The goal of this program is to produce as many different types of
+// relocations as we can in a stand-alone program that does not use
+// TLS.  This program is compiled without optimization.
+
+#include "config.h"
+
+#include <cassert>
+#include <cstdio>
+
+#include "elfcpp.h"
+
+#ifdef EHDR_START_WEAK
+#define WEAK_ATTR __attribute__ ((weak))
+#else
+#define WEAK_ATTR
+#endif
+
+extern char __ehdr_start[] WEAK_ATTR;
+
+int
+main() {
+  printf("&__ehdr_start = %p\n", &__ehdr_start);
+
+#ifdef EHDR_START_UNDEF
+  assert(&__ehdr_start == 0);
+#else
+  assert(&__ehdr_start != NULL);
+
+  printf("ELF header: \\x%02x%c%c%c\n", __ehdr_start[0], __ehdr_start[1],
+	 __ehdr_start[2], __ehdr_start[3]);
+#ifdef EHDR_START_USER_DEF
+  assert(__ehdr_start[0] == 'a'
+	 && __ehdr_start[1] == 'b'
+	 && __ehdr_start[2] == 'c'
+	 && __ehdr_start[3] == 'd');
+#else
+  assert(__ehdr_start[elfcpp::EI_MAG0] == elfcpp::ELFMAG0
+	 && __ehdr_start[elfcpp::EI_MAG1] == elfcpp::ELFMAG1
+	 && __ehdr_start[elfcpp::EI_MAG2] == elfcpp::ELFMAG2
+	 && __ehdr_start[elfcpp::EI_MAG3] == elfcpp::ELFMAG3);
+#endif
+#endif
+
+  return 0;
+}
diff --git a/gold/testsuite/ehdr_start_test.t b/gold/testsuite/ehdr_start_test.t
new file mode 100644
index 0000000..50daa64
--- /dev/null
+++ b/gold/testsuite/ehdr_start_test.t
@@ -0,0 +1,42 @@
+/* ehdr_start_test.t -- __ehdr_start test for gold
+
+   Copyright (C) 2008-2014 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.  */
+
+/* With luck this will work on all platforms.  */
+
+SECTIONS
+{
+  /* Set the text segment to start on a non-page boundary.  */
+  . = 0x10000040;
+
+  .text : { *(.text) }
+  . += 0x100000;
+  . = ALIGN(0x100);
+
+  .tdata : { *(.tdata .tdata.* .gnu.linkonce.td.*) }
+  .tbss : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }
+  .data.rel.ro : { *(.data.rel.ro.local* .gnu.linkonce.d.rel.ro.local.*)
+		   *(.data.rel.ro* .gnu.linkonce.d.rel.ro.*) }
+  .dynamic : { *(.dynamic) }
+  .got : { *(.got) }
+  .got.plt : { *(.got.plt) }
+  .data : { *(.data .data.* .gnu.linkonce.d.*) }
+}
diff --git a/gold/testsuite/ehdr_start_test_4.sh b/gold/testsuite/ehdr_start_test_4.sh
new file mode 100755
index 0000000..ad0a5a7
--- /dev/null
+++ b/gold/testsuite/ehdr_start_test_4.sh
@@ -0,0 +1,40 @@
+#!/bin/sh
+
+# ehdr_start_test_4.sh -- test that __ehdr_start symbol is undefined.
+
+# Copyright (C) 2014 Free Software Foundation, Inc.
+# Written by Cary Coutant <ccoutant@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.
+
+check()
+{
+    if ! grep -q "$2" "$1"
+    then
+	echo "Did not find expected symbol in $1:"
+	echo "   $2"
+	echo ""
+	echo "Actual output below:"
+	cat "$1"
+	exit 1
+    fi
+}
+
+check ehdr_start_test_4.syms "w __ehdr_start"
+
+exit 0