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

Oleg Nesterov
2010-01-05T15:36:47Z

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.

Oh, I am not sure. But I don't understand TIF_SINGLE_STEP on s390,
absolutely.

For example, why do_signal() sets TIF_SINGLE_STEP? Why can't we do

--- a/arch/s390/kernel/signal.c
+++ b/arch/s390/kernel/signal.c
@@ -500,18 +500,10 @@ void do_signal(struct pt_regs *regs)
clear_thread_flag(TIF_RESTORE_SIGMASK);

/*
- * If we would have taken a single-step trap
- * for a normal instruction, act like we took
- * one for the handler setup.
- */
- if (current->thread.per_info.single_step)
- set_thread_flag(TIF_SINGLE_STEP);
-
- /*
* Let tracing know that we've done the handler setup.
*/
tracehook_signal_handler(signr, &info, &ka, regs,
- test_thread_flag(TIF_SINGLE_STEP));
+ current->thread.per_info.single_step);
}
return;
}

?

Apart from arch/s390/signal.c, TIF_SINGLE_STEP is used by entry.S
but I don't understand this asm at all.

Anyway. I modified the debugging patch a bit:

--- K/arch/s390/kernel/traps.c~ 2009-12-22 10:41:52.909174198 -0500
+++ K/arch/s390/kernel/traps.c 2010-01-05 09:49:19.541792379 -0500
@@ -384,6 +384,8 @@ void __kprobes do_single_step(struct pt_
}
if (tracehook_consider_fatal_signal(current, SIGTRAP))
force_sig(SIGTRAP, current);
+ else
+ printk("XXX: %d %d\n", current->pid, test_thread_flag(TIF_SINGLE_STEP));
}

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

Now, when I run this test-case

#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;
}

dmesg shows 799 lines of

XXX: 2389 0

The kernel is 2.6.32.2 + utrace, but CONFIG_UTRACE is not set.

Oleg.