[Qemu-devel] Re: Debugging vmlinux with qemu and gdb. Unable to step, next, print or to get any information..

[Jan Kiszka]

Keilhau Timo ( Student ) wrote:
> Hello List!
> 
> I am trying to debug linux 2.6.25 kernel with qemu -s and gdb on 64 bit
> amd system.
> But I am experiencing strange behaviour with qemu and gdb..
> Gdb stops at a given breakpoint but I cant step, next, print etc..
> 
> Software:
> Host OS used:                 opensuse 10.3
> Host kernelversion:           2.6.22.5-31-default
> guest:                                Debian Etch 4.0r3 amd64 with 2.6.25
> The kernel used to debug:     linux-2.6.25.tar.bz2
> Virtualization Software:      qemu pc emulator version 0.9.0
> Host make utillity            GNU Make 3.81
> Host debugger:                        GNU gdb 6.6.50.20070726-cvs
> (Also tried gdb 6.6, gdb 6.8 compiled from source)
> 
> Look here:
> 
> // Starting qemu on host:
> 
> $ qemu-system-x86_64 -s -kernel bzImage -hda
> qemu_mini_debian_root_fs.img -append "root=/dev/hda1" -initrd
> debian_boot/initrd.img-2.6.25-customtk-i -no-kqemu -redir
> tcp:10022:10.0.2.15:22
> 
> // Boots fine.
> // vmlinux is compiled with CFLAGS=-g3 -ggdb, I have also tried only
> with -g
> // On host:
> 
> $ nm vmlinux | grep sys_sendmsg
> ffffffff803e9ac5 T sys_sendmsg
> 
> 
> // Starting gdb on host and setting a breakpoint:
> 
> $ gdb vmlinux
> 
> GNU gdb 6.6.50.20070726-cvs
> Copyright (C) 2007 Free Software Foundation, Inc.
> GDB is free software, covered by the GNU General Public License, and you
> are
> welcome to change it and/or distribute copies of it under certain
> conditions.
> Type "show copying" to see the conditions.
> There is absolutely no warranty for GDB.  Type "show warranty" for
> details.
> This GDB was configured as "x86_64-suse-linux"...
> Using host libthread_db library "/lib64/libthread_db.so.1".
> (gdb) l sys_sendmsg
> 1783    /*
> 1784     *      BSD sendmsg interface
> 1785     */
> 1786
> 1787    asmlinkage long sys_sendmsg(int fd, struct msghdr __user *msg,
> unsigned flags)
> 1788    {
> 1789            struct compat_msghdr __user *msg_compat =
> 1790                (struct compat_msghdr __user *)msg;
> 1791            struct socket *sock;
> 1792            char address[MAX_SOCK_ADDR];
> (gdb) b 1787
> Breakpoint 1 at 0xffffffff803e9ac5: file net/socket.c, line 1787.
> (gdb)
> 
> // Now connect to qemu's gdb-stub:
> 
> (gdb) target remote :1234
> Remote debugging using :1234
> 0x0000000000000000 in ?? ()
> (gdb) c
> Continuing.
> 
> // On guest launching a ping for example, to trigger the breakpoint:
> 
> $ ping 212.76.144.43
> 
> // On Host gdb stops, but it looks strange no address info etc is
> shown?!?!
> 
> Program received signal SIGTRAP, Trace/breakpoint trap.
> 0x0000000000000000 in ?? ()

Make sure gdb is assuming the right arch at this point (=> set arch
i386:x86-64). If you initially break into the guest when it is still in
real mode, gdb stays in i386 mode even if the guest's mode changes.

Jan