Since the UML runs as a normal Linux process, it is possible to debug it with gdb almost like any other process. It is slightly different because the kernel's threads are already being ptraced for system call interception, so gdb can't ptrace them. However, a mechanism has been added to work around that problem.

In order to debug the kernel, you need build it from source. See this page for information on doing that. Make sure that you enable CONFIG_DEBUGSYM and CONFIG_PT_PROXY during the config. These will compile the kernel with -g, and enable the ptrace proxy so that gdb works with UML, respectively.

Starting the kernel under gdb

You can have the kernel running under the control of gdb from the beginning by putting 'debug' on the command line. You will get an xterm with gdb running inside it. The kernel will send some commands to gdb which will leave it stopped at the beginning of start_kernel. At this point, you can get things going with 'next', 'step', or 'cont'.

There is a transcript of a debugging session here, with breakpoints being set in the scheduler and in an interrupt handler.

gdb has support for debugging code which is dynamically loaded into the process. This support is what is needed to debug kernel modules under UML.
Using that support is somewhat complicated. You have to tell gdb what object file you just loaded into UML and where in memory it is. Then, it can read the symbol table, and figure out where all the symbols are from the load address that you provided. It gets more interesting when you load the module again (i.e. after an rmmod). You have to tell gdb to forget about all its symbols, including the main UML ones for some reason, then load then all back in again.
There's an easy way and a hard way to do this. The easy way is to use the umlgdb expect script written by Chandan Kudige. It basically automates the process for you.
First, you must tell it where your modules are. There is a list in the script that looks like this:

                
set MODULE_PATHS {
"fat" "/usr/src/uml/linux-2.4.18/fs/fat/fat.o"
"isofs" "/usr/src/uml/linux-2.4.18/fs/isofs/isofs.o"
"minix" "/usr/src/uml/linux-2.4.18/fs/minix/minix.o"
}

              

You change that to list the names and paths of the modules that you are going to debug. Then you run it from the toplevel directory of your UML pool and it basically tells you what to do:

                

            ******** GDB pid is 21903 ********
Start UML as: ./linux <kernel switches> debug gdb-pid=21903



GNU gdb 5.0rh-5 Red Hat Linux 7.1
Copyright 2001 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 "i386-redhat-linux"...
(gdb) b sys_init_module
Breakpoint 1 at 0xa0011923: file module.c, line 349.
(gdb) att 1

              

After you run UML and it sits there doing nothing, you hit return at the 'att 1' and continue it:

                
Attaching to program: /home/jdike/linux/2.4/um/./linux, process 1
0xa00f4221 in __kill ()
(UML gdb)  c
Continuing.

              

At this point, you debug normally. When you insmod something, the expect magic will kick in and you'll see something like:

                
 *** Module hostfs loaded *** 
Breakpoint 1, sys_init_module (name_user=0x805abb0 "hostfs", 
    mod_user=0x8070e00) at module.c:349
349             char *name, *n_name, *name_tmp = NULL;
(UML gdb)  finish
Run till exit from #0  sys_init_module (name_user=0x805abb0 "hostfs", 
    mod_user=0x8070e00) at module.c:349
0xa00e2e23 in execute_syscall (r=0xa8140284) at syscall_kern.c:411
411             else res = EXECUTE_SYSCALL(syscall, regs);
Value returned is $1 = 0
(UML gdb)  
p/x (int)module_list + module_list->size_of_struct

$2 = 0xa9021054
(UML gdb)  symbol-file ./linux
Load new symbol table from "./linux"? (y or n) y
Reading symbols from ./linux...
done.
(UML gdb)  
add-symbol-file /home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o 0xa9021054

add symbol table from file "/home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o" at
        .text_addr = 0xa9021054
 (y or n) y

Reading symbols from /home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o...
done.
(UML gdb)  p *module_list
$1 = {size_of_struct = 84, next = 0xa0178720, name = 0xa9022de0 "hostfs", 
  size = 9016, uc = {usecount = {counter = 0}, pad = 0}, flags = 1, 
  nsyms = 57, ndeps = 0, syms = 0xa9023170, deps = 0x0, refs = 0x0, 
  init = 0xa90221f0 <init_hostfs>, cleanup = 0xa902222c <exit_hostfs>, 
  ex_table_start = 0x0, ex_table_end = 0x0, persist_start = 0x0, 
  persist_end = 0x0, can_unload = 0, runsize = 0, kallsyms_start = 0x0, 
  kallsyms_end = 0x0, 
  archdata_start = 0x1b855 <Address 0x1b855 out of bounds>, 
  archdata_end = 0xe5890000 <Address 0xe5890000 out of bounds>, 
  kernel_data = 0xf689c35d <Address 0xf689c35d out of bounds>}
>> Finished loading symbols for hostfs ...

              

That's the easy way. It's highly recommended. The umlgdb script is available in the UML utilities tarball in tools/umlgdb/umlgdb. The hard way is described below in case you're interested in what's going on.
Boot the kernel under the debugger and load the module with insmod or modprobe. With gdb, do:

(UML gdb) p module_list

This is a list of modules that have been loaded into the kernel, with the most recently loaded module first. Normally, the module you want is at module_list. If it's not, walk down the next links, looking at the name fields until find the module you want to debug. Take the address of that structure, and add module.size_of_struct (which in 2.4.10 kernels is 96 (0x60)) to it. Gdb can make this hard addition for you :-):

(UML gdb) printf "%#x\n", (int)module_list module_list->size_of_struct

The offset from the module start occasionally changes (before 2.4.0, it was module.size_of_struct + 4), so it's a good idea to check the init and cleanup addresses once in a while, as describe below. Now do:

(UML gdb) add-symbol-file /path/to/module/on/host that_address

Tell gdb you really want to do it, and you're in business.
If there's any doubt that you got the offset right, like breakpoints appear not to work, or they're appearing in the wrong place, you can check it by looking at the module structure. The init and cleanup fields should look like:

                
init = 0x588066b0 <init_hostfs>, cleanup = 0x588066c0 <exit_hostfs>

              

with no offsets on the symbol names. If the names are right, but they are offset, then the offset tells you how much you need to add to the address you gave to add-symbol-file.

When you want to load in a new version of the module, you need to get gdb to forget about the old one. The only way I've found to do that is to tell gdb to forget about all symbols that it knows about:

(UML gdb) symbol-file

Then reload the symbols from the kernel binary:

(UML gdb) symbol-file /path/to/kernel

and repeat the process above. You'll also need to re-enable breakpoints. They were disabled when you dumped all the symbols because gdb couldn't figure out where they should go.

UML has support for attaching to an already running debugger rather than starting gdb itself. This is present in CVS as of 17 Apr 2001. I sent it to Alan for inclusion in the ac tree, and it will be in my 2.4.4 release.

This is useful when gdb is a subprocess of some UI, such as emacs or ddd. It can also be used to run debuggers other than gdb on UML. Below is an example of using strace as an alternate debugger.

To do this, you need to get the pid of the debugger and pass it in with the 'gdb-pid=<pid>' switch along with the 'debug' switch.

If you are using gdb under some UI, then tell it to 'att 1', and you'll find yourself attached to UML.

If you are using something other than gdb as your debugger, then you'll need to get it to do the equivalent of 'att 1' if it doesn't do it automatically.

An example of an alternate debugger is strace. You can strace the actual kernel as follows:

• Run the following in a shell

  host% sh -c 'echo pid=$$; echo -n hit return; read x; exec strace -p 1 -o strace.out'

• Run UML with 'debug' and 'gdb-pid=<pid>' with the pid printed out by the previous command
• Hit return in the shell, and UML will start running, and strace output will start accumulating in the output file.

Note that this is different from running

host% strace ./linux

That will strace only the main UML thread, the tracing thread, which doesn't do any of the actual kernel work. It just oversees the virtual machine. In contrast, using strace as described above will show you the low-level activity of the virtual machine.