Re: [PATCH] Make .eh_frame and .gcc_except_table read-only if possible

[Jakub Jelinek <jakub at redhat dot com>]

On Sat, Nov 23, 2002 at 08:42:40PM -0800, Richard Henderson wrote:
> On Sat, Nov 23, 2002 at 09:46:07PM +0100, Jakub Jelinek wrote:
> > 	* crtstuff.c (__EH_FRAME_BEGIN__, __FRAME_END__): Add const.
> 
> Will break solaris.

Argh, just tested it. Don't see any sense in the apparent rule
"if any input section is read-only, output is read-only,
otherwise read-write" which Solaris ld uses. If I want to write
to at least one of the input sections, the output section must be
read-write.
Below is updated patch which checks for this in configure.

2002-11-25  Jakub Jelinek  <jakub@redhat.com>

	* varasm.c (default_exception_section): Move to...
	* except.c (default_exception_section): ... here. Make
	.gcc_except_table read-only if it is not expected to have any
	dynamic relocations.
	* dwarf2out.c (default_eh_frame_section): Make .eh_frame read-only
	if it is not expected to have any dynamic relocations.
	* configure.in (HAVE_LD_RO_RW_SECTION_MIXING): Check what ld does
	when linking read-only and read-write sections together.
	* configure, config.in: Rebuilt.
	* crtstuff.c (EH_FRAME_SECTION_CONST): Define.
	(__EH_FRAME_BEGIN__, __FRAME_END__): Add it.

--- gcc/except.c.jj	2002-09-27 14:21:54.000000000 +0200
+++ gcc/except.c	2002-11-25 15:28:11.000000000 +0100
@@ -3589,6 +3590,29 @@ sjlj_output_call_site_table ()
   call_site_base += n;
 }
 
+/* Tell assembler to switch to the section for the exception handling
+   table.  */
+
+void
+default_exception_section ()
+{
+  if (targetm.have_named_sections)
+    {
+      int tt_format = ASM_PREFERRED_EH_DATA_FORMAT (/*code=*/0, /*global=*/1);
+      int flags;
+
+      flags = (! flag_pic
+	       || ((tt_format & 0x70) != DW_EH_PE_absptr
+		   && (tt_format & 0x70) != DW_EH_PE_aligned))
+	      ? 0 : SECTION_WRITE;
+      named_section_flags (".gcc_except_table", flags);
+    }
+  else if (flag_pic)
+    data_section ();
+  else
+    readonly_data_section ();
+}
+
 void
 output_function_exception_table ()
 {
--- gcc/varasm.c.jj	2002-11-14 01:12:10.000000000 +0100
+++ gcc/varasm.c	2002-11-23 19:04:35.000000000 +0100
@@ -582,20 +582,6 @@ variable_section (decl, reloc)
     (*targetm.asm_out.select_section) (decl, reloc, DECL_ALIGN (decl));
 }
 
-/* Tell assembler to switch to the section for the exception handling
-   table.  */
-
-void
-default_exception_section ()
-{
-  if (targetm.have_named_sections)
-    named_section (NULL_TREE, ".gcc_except_table", 0);
-  else if (flag_pic)
-    data_section ();
-  else
-    readonly_data_section ();
-}
-
 /* Tell assembler to switch to the section for string merging.  */
 
 void
--- gcc/crtstuff.c.jj	2002-11-14 00:54:14.000000000 +0100
+++ gcc/crtstuff.c	2002-11-25 17:42:13.000000000 +0100
@@ -90,6 +90,11 @@ call_ ## FUNC (void)					\
 #if defined(EH_FRAME_SECTION_NAME) && !defined(USE_PT_GNU_EH_FRAME)
 # define USE_EH_FRAME_REGISTRY
 #endif
+#if defined(EH_FRAME_SECTION_NAME) && defined(HAVE_LD_RO_RW_SECTION_MIXING)
+# define EH_FRAME_SECTION_CONST const
+#else
+# define EH_FRAME_SECTION_CONST
+#endif
 
 /* We do not want to add the weak attribute to the declarations of these
    routines in unwind-dw2-fde.h because that will cause the definition of
@@ -188,7 +193,7 @@ STATIC func_ptr __DTOR_LIST__[1]
 #ifdef USE_EH_FRAME_REGISTRY
 /* Stick a label at the beginning of the frame unwind info so we can register
    and deregister it with the exception handling library code.  */
-STATIC char __EH_FRAME_BEGIN__[]
+STATIC EH_FRAME_SECTION_CONST char __EH_FRAME_BEGIN__[]
      __attribute__((section(EH_FRAME_SECTION_NAME), aligned(4)))
      = { };
 #endif /* USE_EH_FRAME_REGISTRY */
@@ -453,7 +458,7 @@ STATIC func_ptr __DTOR_END__[1]
 #ifdef EH_FRAME_SECTION_NAME
 /* Terminate the frame unwind info section with a 4byte 0 as a sentinel;
    this would be the 'length' field in a real FDE.  */
-STATIC int __FRAME_END__[]
+STATIC EH_FRAME_SECTION_CONST int __FRAME_END__[]
      __attribute__ ((unused, mode(SI), section(EH_FRAME_SECTION_NAME),
 		     aligned(4)))
      = { 0 };
--- gcc/dwarf2out.c.jj	2002-10-29 16:30:44.000000000 +0100
+++ gcc/dwarf2out.c	2002-11-23 19:02:13.000000000 +0100
@@ -117,7 +117,20 @@ void
 default_eh_frame_section ()
 {
 #ifdef EH_FRAME_SECTION_NAME
-  named_section_flags (EH_FRAME_SECTION_NAME, SECTION_WRITE);
+  int fde_encoding = ASM_PREFERRED_EH_DATA_FORMAT (/*code=*/1, /*global=*/0);
+  int per_encoding = ASM_PREFERRED_EH_DATA_FORMAT (/*code=*/2, /*global=*/1);
+  int lsda_encoding = ASM_PREFERRED_EH_DATA_FORMAT (/*code=*/0, /*global=*/0);
+  int flags;
+
+  flags = (! flag_pic
+	   || ((fde_encoding & 0x70) != DW_EH_PE_absptr
+	       && (fde_encoding & 0x70) != DW_EH_PE_aligned
+	       && (per_encoding & 0x70) != DW_EH_PE_absptr
+	       && (per_encoding & 0x70) != DW_EH_PE_aligned
+	       && (lsda_encoding & 0x70) != DW_EH_PE_absptr
+	       && (lsda_encoding & 0x70) != DW_EH_PE_aligned))
+	  ? 0 : SECTION_WRITE;
+  named_section_flags (EH_FRAME_SECTION_NAME, flags);
 #else
   tree label = get_file_function_name ('F');
 
--- gcc/configure.in.jj	2002-11-24 00:43:11.000000000 +0100
+++ gcc/configure.in	2002-11-25 17:35:25.000000000 +0100
@@ -2209,6 +2209,44 @@ if test x"$gcc_cv_as_gstabs_flag" = xyes
 fi
 AC_MSG_RESULT($gcc_cv_as_gstabs_flag)
 
+AC_MSG_CHECKING(linker read-only and read-write section mixing)
+gcc_cv_ld_ro_rw_mix=unknown
+if test x$gcc_cv_gld_major_version != x -a x$gcc_cv_gld_minor_version != x; then
+  if test "$gcc_cv_gld_major_version" -eq 2 -a "$gcc_cv_gld_minor_version" -ge 10 -o "$gcc_cv_gld_major_version" -gt 2 && grep 'EMUL = elf' ../ld/Makefile > /dev/null; then
+    gcc_cv_ld_ro_rw_mix=read-write
+  fi
+elif test x$gcc_cv_as != x -a x$gcc_cv_ld != x -a x$gcc_cv_objdump != x ; then
+  echo '.section "myfoosect", "a"' > conftest1.s
+  echo '.section "myfoosect", "aw"' > conftest2.s
+  echo '.byte 1' >> conftest2.s
+  echo '.section "myfoosect", "a"' > conftest3.s
+  echo '.byte 0' >> conftest3.s
+  if $gcc_cv_as -o conftest1.o conftest1.s \
+     && $gcc_cv_as -o conftest2.o conftest2.s \
+     && $gcc_cv_as -o conftest3.o conftest3.s \
+     && $gcc_cv_ld -shared -o conftest1.so conftest1.o \
+	conftest2.o conftest3.o; then
+    gcc_cv_ld_ro_rw_mix=`$gcc_cv_objdump -h conftest1.so \
+			 | grep -A2 myfoosect`
+    if echo "$gcc_cv_ld_ro_rw_mix" | grep CONTENTS > /dev/null; then
+      if echo "$gcc_cv_ld_ro_rw_mix" | grep READONLY > /dev/null; then
+	gcc_cv_ld_ro_rw_mix=read-only
+      else
+	gcc_cv_ld_ro_rw_mix=read-write
+      fi
+    fi
+  fi
+changequote(,)dnl
+  rm -f conftest.* conftest[123].*
+changequote([,])dnl
+fi
+if test x$gcc_cv_ld_ro_rw_mix = xread-write; then
+	AC_DEFINE(HAVE_LD_RO_RW_SECTION_MIXING, 1,
+  [Define if your linker links a mix of read-only
+   and read-write sections into a read-write section.])
+fi
+AC_MSG_RESULT($gcc_cv_ld_ro_rw_mix)
+
 AC_MSG_CHECKING(linker PT_GNU_EH_FRAME support)
 gcc_cv_ld_eh_frame_hdr=no
 if test x$gcc_cv_gld_major_version != x -a x$gcc_cv_gld_minor_version != x; then
--- gcc/configure.jj	2002-11-24 00:43:11.000000000 +0100
+++ gcc/configure	2002-11-25 17:37:51.000000000 +0100
@@ -8113,6 +8113,44 @@ EOF
 fi
 echo "$ac_t""$gcc_cv_as_gstabs_flag" 1>&6
 
+echo $ac_n "checking linker read-only and read-write section mixing""... $ac_c" 1>&6
+echo "configure:8118: checking linker read-only and read-write section mixing" >&5
+gcc_cv_ld_ro_rw_mix=unknown
+if test x$gcc_cv_gld_major_version != x -a x$gcc_cv_gld_minor_version != x; then
+  if test "$gcc_cv_gld_major_version" -eq 2 -a "$gcc_cv_gld_minor_version" -ge 10 -o "$gcc_cv_gld_major_version" -gt 2 && grep 'EMUL = elf' ../ld/Makefile > /dev/null; then
+    gcc_cv_ld_ro_rw_mix=read-write
+  fi
+elif test x$gcc_cv_as != x -a x$gcc_cv_ld != x -a x$gcc_cv_objdump != x ; then
+  echo '.section "myfoosect", "a"' > conftest1.s
+  echo '.section "myfoosect", "aw"' > conftest2.s
+  echo '.byte 1' >> conftest2.s
+  echo '.section "myfoosect", "a"' > conftest3.s
+  echo '.byte 0' >> conftest3.s
+  if $gcc_cv_as -o conftest1.o conftest1.s \
+     && $gcc_cv_as -o conftest2.o conftest2.s \
+     && $gcc_cv_as -o conftest3.o conftest3.s \
+     && $gcc_cv_ld -shared -o conftest1.so conftest1.o \
+	conftest2.o conftest3.o; then
+    gcc_cv_ld_ro_rw_mix=`$gcc_cv_objdump -h conftest1.so \
+			 | grep -A2 myfoosect`
+    if echo "$gcc_cv_ld_ro_rw_mix" | grep CONTENTS > /dev/null; then
+      if echo "$gcc_cv_ld_ro_rw_mix" | grep READONLY > /dev/null; then
+	gcc_cv_ld_ro_rw_mix=read-only
+      else
+	gcc_cv_ld_ro_rw_mix=read-write
+      fi
+    fi
+  fi
+  rm -f conftest.* conftest[123].*
+fi
+if test x$gcc_cv_ld_ro_rw_mix = xread-write; then
+	cat >> confdefs.h <<\EOF
+#define HAVE_LD_RO_RW_SECTION_MIXING 1
+EOF
+
+fi
+echo "$ac_t""$gcc_cv_ld_ro_rw_mix" 1>&6
+
 echo $ac_n "checking linker PT_GNU_EH_FRAME support""... $ac_c" 1>&6
 echo "configure:8118: checking linker PT_GNU_EH_FRAME support" >&5
 gcc_cv_ld_eh_frame_hdr=no
--- gcc/config.in.jj	2002-11-24 00:43:10.000000000 +0100
+++ gcc/config.in	2002-11-25 17:21:11.000000000 +0100
@@ -586,6 +586,10 @@
 /* Define if your assembler supports the --gstabs option. */
 #undef HAVE_AS_GSTABS_DEBUG_FLAG
 
+/* Define if your linker links a mix of read-only
+   and read-write sections into a read-write section. */
+#undef HAVE_LD_RO_RW_SECTION_MIXING
+
 /* Define if your linker supports --eh-frame-hdr option. */
 #undef HAVE_LD_EH_FRAME_HDR
 


	Jakub