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[uml-user] gdb HOWTO

From: Mark Huang <mlhuang@CS...> - 2004-05-17 20:53

Is there an updated gdb HOWTO on any UML websites or Wikis, that someone could 
point me at? I've googled for days looking for instructions on how to debug 
UML with current tools. The last time I played around with UML a year or two 
ago, you just added "debug" to the command line and it all just worked. Or 
you killed it with -USR1, or you ran the umlgdb script.

Now I'm running host kernel 2.6.3-9mdk patched for skas3, gdb 6.0-2mdk, 
gcc-3.3.2-6mdk, and I'm at a loss as to what broke debugging. I suppose I 
could just downgrade my host back to Mandrake 8.2 and uml-patch-2.4.20-2, 
which is what I was using the last time I got this to work, but I'd like to 
know if anyone's had any success debugging UML under newer host 
distributions.

When I try to run the umlgdb script as either root or myself, I get:

(gdb) att 1b sys_init_module
Attaching to program: /home/mlhuang/uml/linux, process 1
ptrace: Operation not permitted.

If I run UML from gdb directly, and set a breakpoint at "start_kernel", I get 
a few of these messages:

Detaching after fork from child process 5153.

and UML seems to merrily continue beyond the breakpoint I set. If I set 
follow-fork-mode to 'child', I again get:

Attaching after fork to child process 5255.
ptrace: Operation not permitted

Program terminated with signal SIGKILL, Killed.
The program no longer exists.

I'm currently trying this with uml-patch-2.4.25-1.bz2 and the 
following .config variables:

CONFIG_MODE_SKAS=y
# CONFIG_MODE_TT is not set
CONFIG_DEBUGSYM=y
CONFIG_PT_PROXY=y

Thanks in advance,

--Mark

Re: [uml-user] gdb HOWTO

From: Mark Huang <mlhuang@CS...> - 2004-05-17 21:06

> CONFIG_PT_PROXY=y

I can answer my own question now, after reading through the e-mail I just sent 
and having the obvious spelled out for me (by myself). Disable the ptrace 
proxy in skas mode, since gdb wants to trace the binary, too. The 
Configure.help for CONFIG_PT_PROXY ("If you want to do kernel debugging, say 
Y here..."), and the -EPERM error from ptrace(), are both misleading:

/usr/src/linux/kernel/ptrace.c:ptrace_attach:

        /* the same process cannot be attached many times */
        if (task->ptrace & PT_PTRACED)
                goto bad;

Re: [uml-user] gdb HOWTO

From: Mark Huang <mlhuang@CS...> - 2004-05-17 21:33

I take it back, disabling CONFIG_PT_PROXY didn't help and may have hurt. I 
thought it fixed the problem, but in fact, all I did was accidentally set my 
breakpoint (at 'start_kernel') after the first SIGUSR1 fired. I re-enabled 
CONFIG_PT_PROXY and nothing changed. If I set the breakpoint before execution 
(rather than after the first SIGUSR1 fires), UML continues past the 
breakpoint. I'm puzzled again and would appreciate any suggestions (again).

--Mark

Re: [uml-user] gdb HOWTO

From: BlaisorBlade <blaisorblade_spam@ya...> - 2004-06-17 12:13

Alle 22:52, luned=EC 17 maggio 2004, Mark Huang ha scritto:
> Is there an updated gdb HOWTO on any UML websites or Wikis, that someone
> could point me at? I've googled for days looking for instructions on how =
to
> debug UML with current tools. The last time I played around with UML a ye=
ar
> or two ago, you just added "debug" to the command line and it all just
> worked. Or you killed it with -USR1, or you ran the umlgdb script.

Well, that was for TT mode, now you are in SKAS mode, which is actually mor=
e=20
normal to debug:

http://user-mode-linux.sourceforge.net/debugging-skas.html

By the way, you should really disable PT_PROXY...

> Now I'm running host kernel 2.6.3-9mdk patched for skas3, gdb 6.0-2mdk,
> gcc-3.3.2-6mdk, and I'm at a loss as to what broke debugging.

Well, what is your guest? If it is a 2.6 one, I must warn you that umlgdb h=
as=20
not been updated to the module changes in 2.6. For the rest, everything wil=
l=20
still work. If you use a 2.4 guest, umlgdb should still work.
=2D-=20
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729