Re: s390 && user_enable_single_step() (Was: odd utrace testing results on s390x)

Oleg Nesterov
2010-01-06T20:23:58Z

On 01/05, Martin Schwidefsky wrote:
>
> On Mon, 4 Jan 2010 13:11:47 -0800 (PST)
> Roland McGrath <roland@redhat.com> wrote:
>
> > > This probably means that copy_process()->user_disable_single_step()
> > > is not enough to clear the "this task wants single-stepping" copied
> > > from parent.
> >
> > I would suspect s390's TIF_SINGLE_STEP flag here. That flag means "a
> > single-step trap occurred". This is what causes do_single_step to be
> > called before returning to user mode, rather than the machine trap doing it
> > directly as is done in the other arch implementations.
>
> Just my thinking as well.
>
> > If I'm right, then "this task wants single-stepping" is not the problem,
> > and that really is fully cleared. In fact, looking at s390's copy_thread
> > (arch/s390/kernel/process.c) it clears out all the state that is actually
> > touched by user_enable_single_step and user_disable_single_step. So for
> > s390 the new fork.c call is actually superfluous AFAICT.
>
> /* Don't copy debug registers */
> memset(&p->thread.per_info, 0, sizeof(p->thread.per_info));
>
> Yep, the call from fork.c is indeed superfluous.

I can't explain this, but if I remove copy_process()->user_disable_single_step()
the test-case below triggers "XXX" printk's from do_single_step() with or
without CONFIG_UTRACE. the patch is

--- arch/s390/kernel/traps.c~ 2009-12-22 10:41:52.909174198 -0500
+++ arch/s390/kernel/traps.c 2010-01-05 11:03:55.006487697 -0500
@@ -384,6 +384,9 @@ void __kprobes do_single_step(struct pt_
}
if (tracehook_consider_fatal_signal(current, SIGTRAP))
force_sig(SIGTRAP, current);
+ else
+ printk("XXX: %s/%d %d\n", current->comm, current->pid,
+ test_thread_flag(TIF_SINGLE_STEP));
}

static void default_trap_handler(struct pt_regs * regs, long interruption_code)

Oleg.

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <assert.h>

int main(void)
{
int pid, status;

if (!(pid = fork())) {
assert(ptrace(PTRACE_TRACEME) == 0);
kill(getpid(), SIGSTOP);

if (!fork())
return 43;

wait(&status);
return WEXITSTATUS(status);
}

for (;;) {
assert(pid == wait(&status));
if (WIFEXITED(status))
break;
assert(ptrace(PTRACE_SINGLESTEP, pid, 0,0) == 0);
}

assert(WEXITSTATUS(status) == 43);
return 0;
}