qemu + gdb debugging...

qemu + gdb debugging...

[Vesa Jääskeläinen]

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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

[-- Attachment #2: grub2-debug-symbols.diff --]
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Index: ChangeLog
===================================================================
--- ChangeLog	(revision 1999)
+++ ChangeLog	(working copy)
@@ -1,3 +1,11 @@
+2009-02-22  Vesa Jääskeläinen  <chaac@nic.fi>
+
+	Based on patch by Lubomir Rintel <lkundrak@fedoraproject.org>.
+
+	* 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  <rmh@aybabtu.com>
 
 	* 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 += #{@name} #{mod_obj} #{mod_src} #{pre_obj} #{objs_str} #{undsym}
+    "CLEANFILES += #{@name} #{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}
 
-#{@name}: #{pre_obj} #{mod_obj} $(TARGET_OBJ2ELF)
+#{@name}: #{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 $@

[-- Attachment #3: gmodule.pl --]
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# 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";
}

[-- Attachment #4: grub.gdb --]
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# 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

Re: qemu + gdb debugging...

[Vesa Jääskeläinen]

Oh... The usage information was left behind :)

You might need to have most recent QEMU and GDB versions for most
challenging bugs :)...

1. Copy grub.gdb and gmodule.pl to your build directory.

2. Create image for QEMU (grub2.iso in my case).

3. Launch QEMU with GDB stub:

qemu -s -S -cdrom grub2.iso

4. Launch GDB

gdb -x grub.gdb

5. Load connect to QEMU

target remote localhost:1234

6. Load debug symbols for grub 2 kernel

load_kernel

7. Setup breakpoint in grub_main

break grub_main

8. Continue execution until there

c

9. Next time you are at grub_main

10. Delete grub_main breakpoint if it haunts you

del 1

; where 1 is being breakpoint number use 'info break' to see numbers if
you lost that.

11. Setup automatic symbol loading

break_load

12. Happy debugging!

If you want to debug boot code that can be done also... but its a bit
more tricky... and I leave that to another time :).

Tips for RM code debugging: 'set arch i8086' and 'x/10i ($cs<<4)+$eip'.

Thanks,
Vesa Jääskeläinen