http://sourceware.org/ml/gdb-patches/2010-07/msg00237.html
Subject: [patch] Fix regression on prelinked executables

Hi,

there is a regression since gdb-7.0 for a combination of:
 * prelinked
 * main executable
 * using separate debug info
 * using copy relocations

It is since a patch for both PIE and (AFAIK) OSX support:
	[commit] syms_from_objfile: Relativize also MAINLINE
	http://sourceware.org/ml/gdb-patches/2010-01/msg00080.html

which started to use problematic addr_info_make_relative even for main
executables.  prelink<->gdb discussion at:
	https://bugzilla.redhat.com/show_bug.cgi?id=614659

Currently in the unfortunately executables GDB has invalid displcement for
symbols in .bss:
	int bssvar, *bssvarp = &bssvar;
	(gdb) p &bssvar
	$1 = (int *) 0x600b54
	(gdb) p bssvarp
	$2 = (int *) 0x600b50

<abstract-higher-point-of-view>
addr_info_make_relative could just simply subtract entry point address and
provide single CORE_ADDR objfile->offset (instead of the current
section_offsets array with offsets specific for each section).  Linux systems
use always single offset for the whole objfile.  AFAIK these per-section
offsets are there for some embedded targets.  Curiously GDB already uses at
many places
	baseaddr = ANOFFSET (objfile->section_offsets, SECT_OFF_TEXT (objfile));
instead of using offset for the appropriate section at that place and nobody
complains.
</abstract-higher-point-of-view>

No regressions on {x86_64,x86_64-m32,i686}-fedora13-linux-gnu.

Proposing for the gdb-7.2 branch.  I had problems fixing up my crashing X.


Thanks,
Jan


gdb/
2010-07-15  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* symfile.c (addr_section_name): New function.
	(addrs_section_compar): Use it.
	(addr_info_make_relative): Use it.  Move variable sect_name into a more
	inner block.  Make ".dynbss" and ".sdynbss" checks more strict.

gdb/testsuite/
2010-07-15  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.base/prelink-lib.c (copyreloc): New initialized variable.
	* gdb.base/prelink.c (copyreloc, bssvar, bssvarp): New variables.
	(main): Use copyreloc.
	* gdb.base/prelink.exp (split debug of executable)
	(.dynbss vs. .bss address shift): New tests.

Index: gdb-7.1/gdb/symfile.c
===================================================================
--- gdb-7.1.orig/gdb/symfile.c	2010-07-19 23:11:10.000000000 +0200
+++ gdb-7.1/gdb/symfile.c	2010-07-20 18:34:50.000000000 +0200
@@ -557,6 +557,23 @@ relative_addr_info_to_section_offsets (s
     }
 }
 
+/* Transform section name S for a name comparison.  prelink can split section
+   `.bss' into two sections `.dynbss' and `.bss' (in this order).  Similarly
+   prelink can split `.sbss' into `.sdynbss' and `.sbss'.  Use virtual address
+   of the new `.dynbss' (`.sdynbss') section as the adjacent new `.bss'
+   (`.sbss') section has invalid (increased) virtual address.  */
+
+static const char *
+addr_section_name (const char *s)
+{
+  if (strcmp (s, ".dynbss") == 0)
+    return ".bss";
+  if (strcmp (s, ".sdynbss") == 0)
+    return ".sbss";
+
+  return s;
+}
+
 /* Relativize absolute addresses in ADDRS into offsets based on ABFD.  Fill-in
    also SECTINDEXes specific to ABFD there.  This function can be used to
    rebase ADDRS to start referencing different BFD than before.  */
@@ -607,8 +624,17 @@ addr_info_make_relative (struct section_
       if (sect && strcmp (sect_name, bfd_get_section_name (abfd, sect)) != 0)
 	sect = NULL;
 
-      if (sect == NULL)
-	sect = bfd_get_section_by_name (abfd, sect_name);
+      /* Prevent the search by name if `.bss' has the address already set from
+	 `.dynbss'.  */
+      if (sect == NULL
+          && !(0
+	       || (strcmp (sect_name, ".bss") == 0
+		   && i > 0
+		   && strcmp (addrs->other[i - 1].name, ".dynbss") == 0)
+	       || (strcmp (sect_name, ".sbss") == 0
+		   && i > 0
+		   && strcmp (addrs->other[i - 1].name, ".sdynbss") == 0)))
+	sect = bfd_get_section_by_name (abfd, addr_section_name (sect_name));
       if (sect)
 	{
 	  /* This is the index used by BFD. */
@@ -634,12 +660,18 @@ addr_info_make_relative (struct section_
 	     a warning.  Shared libraries contain just the section
 	     ".gnu.liblist" but it is not marked as loadable there.  There is
 	     no other way to identify them than by their name as the sections
-	     created by prelink have no special flags.  */
+	     created by prelink have no special flags.
+
+	     For the sections `.bss' and `.sbss' see addr_section_name.  */
 
 	  if (!(strcmp (sect_name, ".gnu.liblist") == 0
 		|| strcmp (sect_name, ".gnu.conflict") == 0
-		|| strcmp (sect_name, ".dynbss") == 0
-		|| strcmp (sect_name, ".sdynbss") == 0))
+		|| (strcmp (sect_name, ".bss") == 0
+		    && i > 0
+		    && strcmp (addrs->other[i - 1].name, ".dynbss") == 0)
+		|| (strcmp (sect_name, ".sbss") == 0
+		    && i > 0
+		    && strcmp (addrs->other[i - 1].name, ".sdynbss") == 0)))
 	    warning (_("section %s not found in %s"), sect_name,
 		     bfd_get_filename (abfd));
 
Index: gdb-7.1/gdb/testsuite/gdb.base/prelink-lib.c
===================================================================
--- gdb-7.1.orig/gdb/testsuite/gdb.base/prelink-lib.c	2010-01-01 08:32:01.000000000 +0100
+++ gdb-7.1/gdb/testsuite/gdb.base/prelink-lib.c	2010-07-19 23:11:56.000000000 +0200
@@ -16,6 +16,8 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+int copyreloc = 1;
+
 int
 g (void (*p)(void))
 {
Index: gdb-7.1/gdb/testsuite/gdb.base/prelink.c
===================================================================
--- gdb-7.1.orig/gdb/testsuite/gdb.base/prelink.c	2010-01-01 08:32:01.000000000 +0100
+++ gdb-7.1/gdb/testsuite/gdb.base/prelink.c	2010-07-19 23:11:56.000000000 +0200
@@ -18,6 +18,11 @@
 
 #include <stdio.h>
 
+extern int copyreloc;
+
+/* Test GDB itself finds `&bssvar' right.   */
+static int bssvar, *bssvarp = &bssvar;
+
 extern void (*h (void)) (void (*)(void));
 
 int
@@ -25,5 +30,6 @@ main (void)
 {
   void (*f) (void (*)(void)) = h ();
   printf ("%p\n", f);
+  printf ("%d\n", copyreloc);
   f (0);
 }
Index: gdb-7.1/gdb/testsuite/gdb.base/prelink.exp
===================================================================
--- gdb-7.1.orig/gdb/testsuite/gdb.base/prelink.exp	2010-07-19 23:11:12.000000000 +0200
+++ gdb-7.1/gdb/testsuite/gdb.base/prelink.exp	2010-07-20 00:06:18.000000000 +0200
@@ -84,6 +84,13 @@ if { [gdb_compile "${srcdir}/${subdir}/$
     return -1;
 }
 
+set test "split debug of executable"
+if [gdb_gnu_strip_debug $binfile] {
+    fail $test
+} else {
+    pass $test
+}
+
 set found 0
 set coredir "${objdir}/${subdir}/coredir.[getpid]"
 file mkdir $coredir
@@ -118,7 +125,7 @@ if {[catch "system \"/usr/sbin/prelink -
     untested "${testfile}.so was not prelinked, maybe system libraries are not prelinked?"
     return 0
 }
-catch "system \"/usr/sbin/prelink -qNR --no-exec-shield ${libfile}\""
+catch "system \"/usr/sbin/prelink -qNR --no-exec-shield ${libfile} ${binfile}\""
 
 # Start with a fresh gdb
 
@@ -131,3 +138,5 @@ gdb_load ${binfile}
 gdb_test "set verbose on"
 
 gdb_test "core-file $objdir/$subdir/prelink.core" {Using PIC \(Position Independent Code\) prelink displacement.*} "prelink"
+
+gdb_test "p &bssvar == bssvarp" " = 1" ".dynbss vs. .bss address shift"