qemu + gdb debugging...

[Vesa Jääskeläinen]

Hi All,

I was debugging some problem lately and felt that I need to use
debugger. As I like how JTAG debugging works I wanted to have similar
feeling :). Obivious choice is to use QEMU.

QEMU provides a GDB stub that can be used to debug code running on its
virtual session. I also found out that VMware also features this same
feature, though it needs a bit configuration changes. Anyway...

In order for GDB session to be a bit fancier you are going to need debug
symbols. In order to do that we need Lubomir Rintel's patch to support
debug symbol generation. (Adapted version is attached to this email).

After this everything is ready in GRUB 2 building. Next steps is to
improve GDB handling to support our dynamic module loader. For this
Lubomir's scripts for GDB comes in to play. I adapted them a bit for new
naming and after this I could nicely add breakpoint to code that gets
loaded later on by GRUB module loader and it stopped nicely there on
correct spot.

Based on this finding I would say that we integrate following bits of code.

Lubomir also create GDB stub that allows debugging on real hardware over
serial cable. This is fine for me, but at this time I only need to have
QEMU debugging working. And as this step is shared between those two I
propose that this work is sliced to two pieces.

Thanks,
Vesa Jääskeläinen

Index: ChangeLog
===================================================================
--- ChangeLog   (revision 1999)
+++ ChangeLog   (working copy)
@@ -1,3 +1,11 @@
+2009-02-22  Vesa Jääskeläinen  <address@hidden>
+
+       Based on patch by Lubomir Rintel <address@hidden>.
+
+       * genmk.rb: Add new stage to compile first debug symbol version of
+       module and then use objcopy to generate final image to allow easier
+       debugging.
+
 2009-02-22  Robert Millan  <address@hidden>
 
        * include/multiboot.h (MULTIBOOT_INFO_ALIGN): New macro.
Index: genmk.rb
===================================================================
--- genmk.rb    (revision 1996)
+++ genmk.rb    (working copy)
@@ -101,10 +101,11 @@
     mod_obj = mod_src.suffix('o')
     defsym = 'def-' + @name.suffix('lst')
     undsym = 'und-' + @name.suffix('lst')
+    exec = @name.suffix('mod.exec')
     mod_name = File.basename(@name, '.mod')
     symbolic_name = mod_name.sub(/\.[^\.]*$/, '')
     
-    "CLEANFILES += address@hidden #{mod_obj} #{mod_src} #{pre_obj} #{objs_str} 
#{undsym}
+    "CLEANFILES += address@hidden #{mod_obj} #{mod_src} #{pre_obj} #{objs_str} 
#{undsym} #{exec}
 ifneq ($(#{prefix}_EXPORTS),no)
 CLEANFILES += #{defsym}
 DEFSYMFILES += #{defsym}
@@ -112,11 +113,14 @@
 MOSTLYCLEANFILES += #{deps_str}
 UNDSYMFILES += #{undsym}
 
address@hidden: #{pre_obj} #{mod_obj} $(TARGET_OBJ2ELF)
address@hidden: #{exec}
+       -rm -f $@
+       $(OBJCOPY) --strip-unneeded -K grub_mod_init -K grub_mod_fini -K 
_grub_mod_init -K _grub_mod_fini -R .note -R .comment $^ $@
+
+#{exec}: #{pre_obj} #{mod_obj} $(TARGET_OBJ2ELF)
        -rm -f $@
        $(TARGET_CC) $(#{prefix}_LDFLAGS) $(TARGET_LDFLAGS) $(MODULE_LDFLAGS) 
-Wl,-r,-d -o $@ #{pre_obj} #{mod_obj}
        if test ! -z $(TARGET_OBJ2ELF); then ./$(TARGET_OBJ2ELF) $@ || (rm -f 
$@; exit 1); fi
-       $(STRIP) --strip-unneeded -K grub_mod_init -K grub_mod_fini -K 
_grub_mod_init -K _grub_mod_fini -R .note -R .comment $@
 
 #{pre_obj}: $(#{prefix}_DEPENDENCIES) #{objs_str}
        -rm -f $@

# gmodule.pl - Generate GDB commands to load symbols to right addresses

# GRUB  --  GRand Unified Bootloader
# Copyright (C) 2009  Free Software Foundation, Inc.
# 
# GRUB 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.
# 
# GRUB 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 GRUB.  If not, see <http://www.gnu.org/licenses/>.

use strict;
use warnings;

while (<>) {
        # Line we get contains section number - load address pairs
        # prepended by module name
        my ($file, %load_addr) = split;

        my $text = '';          # This one needs not be prepended by -s
        my $sections = '';      # All but .text

        print "add-symbol-file $file";

        open (READELF, "readelf -S $file |")
                or die $!;

        while (<READELF>) {

                /\[\s*(\d+)\]\s+(\.\S+)/ or next;
                my $sec_num = $1;
                my $sec_name = $2;

                # .text section doesn't have to be prepended by -s .text
                if ($sec_name eq '.text') {
                        $text = $load_addr{$sec_num};
                        next;
                }

                $sections .= " -s $sec_name $load_addr{$sec_num}"
                        if ($load_addr{$sec_num} and $load_addr{$sec_num} ne 
'0x0');
        };
        close (READELF);
        print " $text $sections\n";
}

# grub.gdb - Macros to ease debugging of GRUB and its modules with GDB

# GRUB  --  GRand Unified Bootloader
# Copyright (C) 2009  Free Software Foundation, Inc.
# 
# GRUB 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.
# 
# GRUB 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 GRUB.  If not, see <http://www.gnu.org/licenses/>.

#
# Load debuging information about GNU GRUB 2 modules into GDB
# automatically. Needs readelf, Perl and gmodule.pl script
#
# Note: break_load command won't work with GDB up to 6.6 due to
# bug in processing breakpoint command hooks. GDB 6.8 works fine.
#

# This is needed especially on Linux, so that the debugger doesn't
# request special register values which we do not know about
set osabi none

define _cleanup
        shell rm -f .segments.tmp .loadsym.gdb
end

# Add section numbers and addresses to .segments.tmp
define _dump_module_sections
        set $mod = $arg0

        # FIXME: save logging status
        set logging file .segments.tmp
        set logging redirect on
        set logging overwrite off
        set logging on

        printf "%s.mod.exec", $mod->name
        set $segment = $mod->segment
        while ($segment)
                printf " %i 0x%x", $segment->section, $segment->addr
                set $segment = $segment->next
        end
        printf "\n"

        set logging off
        # FIXME: restore logging status
end
document _dump_module_sections
        Gather information about module whose mod structure was
        given for use with match_and_load_symbols
end

# Generate and execute GDB commands and delete temporary files
# afterwards
define _match_and_load_symbols
        shell perl gmodule.pl <.segments.tmp >.loadsym.gdb
        source .loadsym.gdb
        _cleanup
end
document _match_and_load_symbols
        Launch script, that matches section names with information
        generated by dump_module_sections and load debugging info
        apropriately
end

define load_module
        _cleanup
        _dump_module_sections $arg0
        _match_and_load_symbols
end
document load_module
        Load debugging information for module given as argument.
end

define load_modules
        _cleanup
        set $this = grub_dl_head
        while ($this != 0)
                _dump_module_sections $this->mod
                set $this = $this->next
        end
        _match_and_load_symbols
end
document load_modules
        Load debugging information for all loaded modules.
end

define load_kernel
        file kernel.exec
end
document load_kernel
        Load debugging information for kernel.
end

define break_load
        # Load debugging symbols for module when it's loaded
        break grub_dl_ref
        commands
                load_module mod
                cont
        end
end
document break_load
        Make modules load automatically.
end