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<TITLE>User Mode Linux HOWTO : Kernel debugging</TITLE>
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<H2><A NAME="debugging"></A> <A NAME="s11">11.</A> <A HREF="UserModeLinux-HOWTO.html#toc11">Kernel debugging</A></H2>
<P>
<B>Note:</B> The interface that makes debugging, as described here,
possible is present in 2.4.0-test6 kernels and later.</P>
<P>
Since the user-mode kernel 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.</P>
<P>
In order to debug the kernel, you need build it from source. See
<A HREF="UserModeLinux-HOWTO-2.html#compile">Compiling the kernel and modules</A> 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.</P>
<P> </P>
<H2><A NAME="ss11.1">11.1</A> <A HREF="UserModeLinux-HOWTO.html#toc11.1">Starting the kernel under gdb</A>
</H2>
<P>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'.</P>
<P>
There is a transcript of a debugging session
<A HREF="debug-session.html">here</A> , with breakpoints being set in the
scheduler and in an interrupt handler.</P>
<H2><A NAME="ss11.2">11.2</A> <A HREF="UserModeLinux-HOWTO.html#toc11.2">Attaching gdb to the kernel</A>
</H2>
<P>If you don't have the kernel running under gdb, you can attach gdb to
it later by sending the tracing thread a SIGUSR1. The first line of
the console output identifies its pid:
<BLOCKQUOTE><CODE>
<PRE>
tracing thread pid = 20093
</PRE>
</CODE></BLOCKQUOTE>
When you send it the signal:
<BLOCKQUOTE><CODE>
<PRE>
kill -USR1 20093
</PRE>
</CODE></BLOCKQUOTE>
you will get an xterm with gdb running in it.</P>
<P>
If you have the mconsole compiled into UML, then the mconsole client
can be used to start gdb:
<BLOCKQUOTE><CODE>
<PRE>
(mconsole) config gdb=xterm
</PRE>
</CODE></BLOCKQUOTE>
will fire up an xterm with gdb running in it.</P>
<H2><A NAME="ss11.3">11.3</A> <A HREF="UserModeLinux-HOWTO.html#toc11.3">Debugging modules</A>
</H2>
<P>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.</P>
<P>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.</P>
<P>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. It is currently in the utilities
tarball, but not the RPM. It will be in the RPM following 2.4.18-15.</P>
<P>First, you must tell it where your modules are. There is a list in
the script that looks like this:
<BLOCKQUOTE><CODE>
<PRE>
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"
}
</PRE>
</CODE></BLOCKQUOTE>
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:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
******** 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
</PRE>
</CODE></BLOCKQUOTE>
After you run UML and it sits there doing nothing, you hit return at
the 'att 1' and continue it:
<BLOCKQUOTE><CODE>
<PRE>
Attaching to program: /home/jdike/linux/2.4/um/./linux, process 1
0xa00f4221 in __kill ()
(gdb) c
Continuing.
</PRE>
</CODE></BLOCKQUOTE>
At this point, you debug normally. When you insmod something, the
expect magic will kick in and you'll see something like:
<BLOCKQUOTE><CODE>
<PRE>
*** 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;
(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
(gdb) p/x (int)module_list + module_list->size_of_struct
$2 = 0xa9021054
(gdb) symbol-file ./linux
Load new symbol table from "./linux"? (y or n) y
Reading symbols from ./linux...
done.
(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.
(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 ...
</PRE>
</CODE></BLOCKQUOTE>
That's the easy way. It's highly recommended. The hard way is
described below in case you're interested in what's going on.</P>
<P>Boot the kernel under the debugger and load the module with insmod or
modprobe. With gdb, do:
<BLOCKQUOTE><CODE>
<PRE>
(gdb) p module_list
</PRE>
</CODE></BLOCKQUOTE>
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 :-):
<BLOCKQUOTE><CODE>
<PRE>
printf "%#x\n", (int)module_list module_list->size_of_struct
</PRE>
</CODE></BLOCKQUOTE>
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:
<BLOCKQUOTE><CODE>
<PRE>
(gdb) add-symbol-file /path/to/module/on/host that_address
</PRE>
</CODE></BLOCKQUOTE>
Tell gdb you really want to do it, and you're in business.</P>
<P>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:
<BLOCKQUOTE><CODE>
<PRE>
init = 0x588066b0 <init_hostfs>, cleanup = 0x588066c0 <exit_hostfs>
</PRE>
</CODE></BLOCKQUOTE>
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.</P>
<P>
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:
<BLOCKQUOTE><CODE>
<PRE>
(gdb) symbol-file
</PRE>
</CODE></BLOCKQUOTE>
Then reload the symbols from the kernel binary:
<BLOCKQUOTE><CODE>
<PRE>
(gdb) symbol-file /path/to/kernel
</PRE>
</CODE></BLOCKQUOTE>
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.</P>
<H2><A NAME="ss11.4">11.4</A> <A HREF="UserModeLinux-HOWTO.html#toc11.4">Using alternate debuggers</A>
</H2>
<P>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.</P>
<P>
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.</P>
<P>
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.</P>
<P>
If you are using gdb under some UI, then tell it to 'att 1', and you'll find
yourself attached to UML.</P>
<P>
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.</P>
<P>
An example of an alternate debugger is strace. You can strace the actual
kernel as follows:
<UL>
<LI>Run the following in a shell
<BLOCKQUOTE><CODE>
<PRE>
sh -c 'echo pid=$$; echo -n hit return; read x; exec strace -p 1 -o
strace.out'
</PRE>
</CODE></BLOCKQUOTE>
</LI>
<LI>Run UML with 'debug' and 'gdb-pid=<pid>' with the pid printed out by the
previous command
</LI>
<LI>Hit return in the shell, and UML will start running, and strace output will
start accumulating in the output file.
</LI>
</UL>
Note that this is different from running
<BLOCKQUOTE><CODE>
<PRE>
strace ./linux
</PRE>
</CODE></BLOCKQUOTE>
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.</P>
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