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Gold patch to create separate output sections for certain text section prefixes

Sriraman Tallam via binutils binutils@sourceware.org
Tue Feb 20 17:28:00 GMT 2018 
Ping.

* layout.cc (Layout::default_section_order): Check for text section
prefixes.
(Layout::text_section_name_mapping): New static member.
(Layout::text_section_name_mapping_count): New static member.
(Layout::match_section_name): New static function.
(Layout::output_section_name): Check for text section prefixes.
* layout.h (Output_section_order::ORDER_TEXT_HOT): New enum value.
(Output_section_order::ORDER_TEXT_STARTUP): New enum value.
(Output_section_order::ORDER_TEXT_EXIT): New enum value.
(Output_section_order::ORDER_TEXT_UNLIKELY): New enum value.
(Layout::text_section_name_mapping): New static member.
(Layout::text_section_name_mapping_count): New static member.
(Layout::match_section_name): New static function.
* options.h (keep_text_section_prefix): New -z option.
* testsuite/Makefile.am (keep_text_section_prefix): New test.
* testsuite/Makefile.in: Regenerate.
* testsuite/keep_text_section_prefix.cc: New test source.
* testsuite/keep_text_section_prefix.sh: New test script.



On Thu, Feb 15, 2018 at 5:20 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> Hi,
>
> This patch creates separate output sections for .text.hot,
> .text.startup, .text.exit and .text.unlikely and is controlled by
> -z,keep-text-section-prefix.  With this feature, the PT_LOAD segment
> would look like this:
>
>  Section to Segment mapping:
>   Segment Sections...
>    00
>    01     .interp
>    02     .interp  ... .rela.dyn .rela.plt .init .plt .text.hot .text
> .text.startup .text.exit .text.unlikely .fini .rodata .eh_frame
> .eh_frame_hdr ...
> ....
>
> This feature would be useful to:
>
> a) Map only a subset of  the text sections to huge pages.
> b) m(un)lock a subset  the text sections.
> c) Code layout verification.
>
> I added -z,text-unlikely-segment here:
> https://sourceware.org/ml/binutils/2017-10/msg00023.html for similar
> reasons.
>
> However, we found out that creating multiple executable ELF segments
> has other issues as some other tools like symbolizers make assumptions
> about ELF binaries having just one executable segment and this breaks
> them.  We are looking at fixing these but an alternate approach gives
> us flexibility in deploying this.
>
> Thoughts?  Patch attached.
>
> * layout.cc (Layout::default_section_order): Check for text section
> prefixes.
> (Layout::text_section_name_mapping): New static member.
> (Layout::text_section_name_mapping_count): New static member.
> (Layout::match_section_name): New static function.
> (Layout::output_section_name): Check for text section prefixes.
> * layout.h (Output_section_order::ORDER_TEXT_HOT): New enum value.
> (Output_section_order::ORDER_TEXT_STARTUP): New enum value.
> (Output_section_order::ORDER_TEXT_EXIT): New enum value.
> (Output_section_order::ORDER_TEXT_UNLIKELY): New enum value.
> (Layout::text_section_name_mapping): New static member.
> (Layout::text_section_name_mapping_count): New static member.
> (Layout::match_section_name): New static function.
> * options.h (keep_text_section_prefix): New -z option.
> * testsuite/Makefile.am (keep_text_section_prefix): New test.
> * testsuite/Makefile.in: Regenerate.
> * testsuite/keep_text_section_prefix.cc: New test source.
> * testsuite/keep_text_section_prefix.sh: New test script.
>
> Thanks
> Sri
-------------- next part --------------
	* layout.cc (Layout::default_section_order): Check for text section
	prefixes.
	(Layout::text_section_name_mapping): New static member.
	(Layout::text_section_name_mapping_count): New static member.
	(Layout::match_section_name): New static function.
	(Layout::output_section_name): Check for text section prefixes.
	* layout.h (Output_section_order::ORDER_TEXT_HOT): New enum value.
	(Output_section_order::ORDER_TEXT_STARTUP): New enum value.
	(Output_section_order::ORDER_TEXT_EXIT): New enum value.
	(Output_section_order::ORDER_TEXT_UNLIKELY): New enum value.
	(Layout::text_section_name_mapping): New static member.
	(Layout::text_section_name_mapping_count): New static member.
	(Layout::match_section_name): New static function.
	* options.h (keep_text_section_prefix): New -z option.
	* testsuite/Makefile.am (keep_text_section_prefix): New test.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/keep_text_section_prefix.cc: New test source.
	* testsuite/keep_text_section_prefix.sh: New test script. 
	


diff --git a/gold/layout.cc b/gold/layout.cc
index 0ec7278a4a..36fd6fde20 100644
--- a/gold/layout.cc
+++ b/gold/layout.cc
@@ -1905,6 +1905,19 @@ Layout::default_section_order(Output_section* os, bool is_relro_local)
 	    return ORDER_INIT;
 	  else if (strcmp(os->name(), ".fini") == 0)
 	    return ORDER_FINI;
+	  else if (parameters->options().keep_text_section_prefix())
+	    {
+	      // -z,keep-text-section-prefix introduces additional
+	      // output sections.
+	      if (strcmp(os->name(), ".text.hot") == 0)
+		return ORDER_TEXT_HOT;
+	      else if (strcmp(os->name(), ".text.startup") == 0)
+		return ORDER_TEXT_STARTUP;
+	      else if (strcmp(os->name(), ".text.exit") == 0)
+		return ORDER_TEXT_EXIT;
+	      else if (strcmp(os->name(), ".text.unlikely") == 0)
+		return ORDER_TEXT_UNLIKELY;
+	    }
 	}
       return is_execinstr ? ORDER_TEXT : ORDER_READONLY;
     }
@@ -5134,6 +5147,20 @@ const Layout::Section_name_mapping Layout::section_name_mapping[] =
   MAPPING_INIT(".ARM.exidx", ".ARM.exidx"),
   MAPPING_INIT(".gnu.linkonce.armexidx.", ".ARM.exidx"),
 };
+
+// Mapping for ".text" section prefixes with -z,keep-text-section-prefix.
+const Layout::Section_name_mapping Layout::text_section_name_mapping[] =
+{
+  MAPPING_INIT(".text.hot.", ".text.hot"),
+  MAPPING_INIT_EXACT(".text.hot", ".text.hot"),
+  MAPPING_INIT(".text.unlikely.", ".text.unlikely"),
+  MAPPING_INIT_EXACT(".text.unlikely", ".text.unlikely"),
+  MAPPING_INIT(".text.startup.", ".text.startup"),
+  MAPPING_INIT_EXACT(".text.startup", ".text.startup"),
+  MAPPING_INIT(".text.exit.", ".text.exit"),
+  MAPPING_INIT_EXACT(".text.exit", ".text.exit"),
+  MAPPING_INIT(".text.", ".text"),
+};
 #undef MAPPING_INIT
 #undef MAPPING_INIT_EXACT
 
@@ -5141,6 +5168,39 @@ const int Layout::section_name_mapping_count =
   (sizeof(Layout::section_name_mapping)
    / sizeof(Layout::section_name_mapping[0]));
 
+const int Layout::text_section_name_mapping_count =
+  (sizeof(Layout::text_section_name_mapping)
+   / sizeof(Layout::text_section_name_mapping[0]));
+
+// Find section name NAME in PSNM and return the mapped name if found
+// with the length set in PLEN.
+const char *
+Layout::match_section_name(const Layout::Section_name_mapping* psnm,
+                   const int count,
+                   const char* name, size_t* plen)
+{
+  for (int i = 0; i < count; ++i, ++psnm)
+    {
+      if (psnm->fromlen > 0)
+	{
+	  if (strncmp(name, psnm->from, psnm->fromlen) == 0)
+	    {
+	      *plen = psnm->tolen;
+	      return psnm->to;
+	    }
+	}
+      else
+	{
+	  if (strcmp(name, psnm->from) == 0)
+	    {
+	      *plen = psnm->tolen;
+	      return psnm->to;
+	    }
+	}
+    }
+  return NULL;
+}
+
 // Choose the output section name to use given an input section name.
 // Set *PLEN to the length of the name.  *PLEN is initialized to the
 // length of NAME.
@@ -5184,27 +5244,21 @@ Layout::output_section_name(const Relobj* relobj, const char* name,
   // not found in the table, we simply use it as the output section
   // name.
 
-  const Section_name_mapping* psnm = section_name_mapping;
-  for (int i = 0; i < section_name_mapping_count; ++i, ++psnm)
+  if (parameters->options().keep_text_section_prefix()
+      && is_prefix_of(".text", name))
     {
-      if (psnm->fromlen > 0)
-	{
-	  if (strncmp(name, psnm->from, psnm->fromlen) == 0)
-	    {
-	      *plen = psnm->tolen;
-	      return psnm->to;
-	    }
-	}
-      else
-	{
-	  if (strcmp(name, psnm->from) == 0)
-	    {
-	      *plen = psnm->tolen;
-	      return psnm->to;
-	    }
-	}
+      const char* match = match_section_name(text_section_name_mapping,
+					     text_section_name_mapping_count,
+					     name, plen);
+      if (match != NULL)
+	return match;
     }
 
+  const char* match = match_section_name(section_name_mapping,
+					 section_name_mapping_count, name, plen);
+  if (match != NULL)
+    return match;
+
   // As an additional complication, .ctors sections are output in
   // either .ctors or .init_array sections, and .dtors sections are
   // output in either .dtors or .fini_array sections.
diff --git a/gold/layout.h b/gold/layout.h
index bbdc1622f6..0c34d3db19 100644
--- a/gold/layout.h
+++ b/gold/layout.h
@@ -422,9 +422,21 @@ enum Output_section_order
   // The PLT.
   ORDER_PLT,
 
+  // The hot text sections, prefixed by .text.hot.
+  ORDER_TEXT_HOT,
+
   // The regular text sections.
   ORDER_TEXT,
 
+  // The startup text sections, prefixed by .text.startup.
+  ORDER_TEXT_STARTUP,
+
+  // The startup text sections, prefixed by .text.startup.
+  ORDER_TEXT_EXIT,
+
+  // The unlikely text sections, prefixed by .text.unlikely.
+  ORDER_TEXT_UNLIKELY,
+
   // The .fini section.
   ORDER_FINI,
 
@@ -1015,6 +1027,14 @@ class Layout
   };
   static const Section_name_mapping section_name_mapping[];
   static const int section_name_mapping_count;
+  static const Section_name_mapping text_section_name_mapping[];
+  static const int text_section_name_mapping_count;
+
+  // Find section name NAME in map and return the mapped name if found
+  // with the length set in PLEN.
+  static const char* match_section_name(const Section_name_mapping* map,
+					const int count, const char* name,
+					size_t* plen);
 
   // During a relocatable link, a list of group sections and
   // signatures.
diff --git a/gold/options.h b/gold/options.h
index b39d5ff7ba..1853f2e65f 100644
--- a/gold/options.h
+++ b/gold/options.h
@@ -1496,6 +1496,10 @@ class General_options
 	      N_("Move .text.unlikely sections to a separate segment."),
 	      N_("Do not move .text.unlikely sections to a separate "
 		 "segment."));
+  DEFINE_bool(keep_text_section_prefix, options::DASH_Z, '\0', false,
+	      N_("Keep .text.hot, .text.startup, .text.exit and .text.unlikely "
+		 "as separate sections in the final binary."),
+	      N_("Merge all .text.* prefix sections (default)."));
 
 
  public:
diff --git a/gold/testsuite/Makefile.am b/gold/testsuite/Makefile.am
index 16cae8004c..34fe4bd698 100644
--- a/gold/testsuite/Makefile.am
+++ b/gold/testsuite/Makefile.am
@@ -354,6 +354,18 @@ text_unlikely_segment: text_unlikely_segment.o gcctestdir/ld
 text_unlikely_segment_readelf.stdout: text_unlikely_segment
 	$(TEST_READELF) -Wl $< >$@
 
+check_SCRIPTS += keep_text_section_prefix.sh
+check_DATA += keep_text_section_prefix_readelf.stdout keep_text_section_prefix_nm.stdout
+MOSTLYCLEANFILES += keep_text_section_prefix
+keep_text_section_prefix.o: keep_text_section_prefix.cc
+	$(CXXCOMPILE) -O0 -c -ffunction-sections -g -o $@ $<
+keep_text_section_prefix: keep_text_section_prefix.o gcctestdir/ld
+	$(CXXLINK)  -Bgcctestdir/ -Wl,-z,keep-text-section-prefix keep_text_section_prefix.o
+keep_text_section_prefix_readelf.stdout: keep_text_section_prefix
+	$(TEST_READELF) -Wl $< >$@
+keep_text_section_prefix_nm.stdout: keep_text_section_prefix
+	$(TEST_NM) -n $< >$@
+
 check_PROGRAMS += icf_virtual_function_folding_test
 MOSTLYCLEANFILES += icf_virtual_function_folding_test icf_virtual_function_folding_test.map
 icf_virtual_function_folding_test.o: icf_virtual_function_folding_test.cc
diff --git a/gold/testsuite/keep_text_section_prefix.cc b/gold/testsuite/keep_text_section_prefix.cc
index e69de29bb2..1395d4c20b 100644
--- a/gold/testsuite/keep_text_section_prefix.cc
+++ b/gold/testsuite/keep_text_section_prefix.cc
@@ -0,0 +1,52 @@
+/* keep_text_section_prefix.cc -- a test case for gold
+
+   Copyright (C) 2018 onwards Free Software Foundation, Inc.
+   Written by Sriraman Tallam <tmsriram@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.  */
+
+extern "C" {
+__attribute__((section(".text.unlikely.foo")))
+int unlikely_foo(void) {
+  return 0;
+}
+
+__attribute__((section(".text.hot.foo")))
+int hot_foo(void) {
+  return 0;
+}
+
+__attribute__((section(".text.startup.foo")))
+int startup_foo(void) {
+  return 0;
+}
+
+__attribute__((section(".text.exit.foo")))
+int exit_foo(void) {
+  return 0;
+}
+
+__attribute__((section(".text.foo")))
+int regular_foo(void) {
+  return 0;
+}
+
+int main(void) {
+  return 0;
+}
+}
diff --git a/gold/testsuite/keep_text_section_prefix.sh b/gold/testsuite/keep_text_section_prefix.sh
index e69de29bb2..44a2b47494 100755
--- a/gold/testsuite/keep_text_section_prefix.sh
+++ b/gold/testsuite/keep_text_section_prefix.sh
@@ -0,0 +1,77 @@
+#!/bin/sh
+
+# keep_text_section_prefix.sh -- test
+
+# Copyright (C) 2018 Free Software Foundation, Inc.
+# Written by Sriraman Tallam <tmsriram@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 verify if .text sections are separated
+# according to their prefix. .text.hot, .text.unlikely, .text.startup and
+# .text.exit must be separated in the final binary.
+
+set -e
+
+check()
+{
+    awk "
+BEGIN { saw1 = 0; saw2 = 0; err = 0; }
+/.*$2\$/ { saw1 = 1; }
+/.*$3\$/ {
+     saw2 = 1;
+     if (!saw1)
+       {
+	  printf \"layout of $2 and $3 is not right\\n\";
+	  err = 1;
+	  exit 1;
+       }
+    }
+END {
+      if (!saw1 && !err)
+        {
+	  printf \"did not see $2\\n\";
+	  exit 1;
+	}
+      if (!saw2 && !err)
+	{
+	  printf \"did not see $3\\n\";
+	  exit 1;
+	}
+    }" $1
+}
+
+check_str()
+{
+    if ! grep -q "$2" "$1"
+    then
+	echo "Did not find expected output in $1:"
+	echo "   $2"
+	echo ""
+	echo "Actual output below:"
+	cat "$1"
+	exit 1
+    fi
+}
+
+check_str keep_text_section_prefix_readelf.stdout ".text.hot .text .text.startup .text.exit .text.unlikely"
+
+check keep_text_section_prefix_nm.stdout "hot_foo" "regular_foo"
+check keep_text_section_prefix_nm.stdout "regular_foo" "startup_foo"
+check keep_text_section_prefix_nm.stdout "startup_foo" "exit_foo"
+check keep_text_section_prefix_nm.stdout "exit_foo" "unlikely_foo"
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