Re: [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:
>  
> 
>> -----Original Message-----
>> From: 
>> address@hidden
>>  
>> [mailto:address@hidden
> ngnu.org] On Behalf Of Jan Kiszka
>> Sent: Donnerstag, 8. Mai 2008 10:29
>> To: address@hidden
>> Subject: [Qemu-devel] Re: Debugging vmlinux with qemu and 
>> gdb. Unable to step, next, print or to get any information..
>>
>> 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
>>
> 
> Hello Jan,
> thanks for your reply!
> I've tried your suggestion with "set arch i386:x86-64"
> But it seems that it has no effect on this problem. But it was a good
> idea.
> 
> Additionally I've tried all architectures just to see what happens.
> 
> This is what ive done:
> 
> (gdb) set architecture i386:x86-64
> The target architecture is assumed to be i386:x86-64
> (gdb) c
> Continuing.
> 
> Program received signal SIGTRAP, Trace/breakpoint trap.
> 0x0000000000000000 in ?? ()
> (gdb) s
> Cannot find bounds of current function
> (gdb) n
> Cannot find bounds of current function
> (gdb) info locals
> No symbol table info available.
> (gdb) set architecture i386
> The target architecture is assumed to be i386
> (gdb) c
> Continuing.
> 
> // <manual break here because gdb diddn't automatically>
> 
> Program received signal SIGINT, Interrupt.
> 0x8020aed9 in ?? ()
> (gdb) p this
> No symbol "this" in current context.
> (gdb) info locals
> No symbol table info available.
> (gdb) l *0x8020aed9
> No source file for address 0x8020aed9.
> (gdb) p *0x8020aed9
> Cannot access memory at address 0x8020aed9
> (gdb) set architecture i386:intel
> The target architecture is assumed to be i386:intel
> (gdb) c
> Continuing.
> 
> // <manual break here because gdb diddn't automatically>
> 
> Program received signal SIGINT, Interrupt.
> 0x8020aed9 in ?? ()
> (gdb) p this
> No symbol "this" in current context.
> (gdb) info locals
> No symbol table info available.
> (gdb) s
> Cannot find bounds of current function
> (gdb) n
> Cannot find bounds of current function
> (gdb) l *0x8020aed9
> No source file for address 0x8020aed9.
> (gdb) set architecture i386:x86-64:intel
> The target architecture is assumed to be i386:x86-64:intel
> (gdb) c
> Continuing.
> 
> Program received signal SIGTRAP, Trace/breakpoint trap.
> 0x0000000000000000 in ?? ()
> (gdb) info locals
> No symbol table info available.
> (gdb) n
> Cannot find bounds of current function
> (gdb) s
> Cannot find bounds of current function
> (gdb) p this
> No symbol "this" in current context.
> (gdb) set architecture i8086
> The target architecture is assumed to be i8086
> (gdb) c
> Continuing.
> 
> // <manual break here because gdb diddn't automatically>
> 
> Program received signal SIGINT, Interrupt.
> 0x8020aed9 in ?? ()
> (gdb) s
> Cannot find bounds of current function
> (gdb) n
> Cannot find bounds of current function
> (gdb) info locals
> No symbol table info available.
> (gdb) set architecture
> auto               i386:intel         i386:x86-64:intel
> i386               i386:x86-64        i8086
> (gdb) set architecture auto
> The target architecture is set automatically (currently i386:x86-64)
> (gdb)  
> 
> Any further ideas what is going / I'm doing wrong ?

Missed breakpoints most often mean that the executed image and the one
loaded by gdb do not match. Try comparing offline and runtime
disassemblies of the same locations.

Jan

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