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

Oleg Nesterov
2010-01-07T17:54:58Z

Martin, sorry for delay,

On 01/07, Martin Schwidefsky wrote:
>
> On Wed, 6 Jan 2010 13:13:29 -0800 (PST)
> Roland McGrath <roland@redhat.com> wrote:
>
> > > However, with or without CONFIG_UTRACE, 6580807da14c423f0d0a708108e6df6ebc8bc83d
> > > is needed on s390 too, otherwise the child gets unnecessary traps.
> >
> > This confuses me. user_disable_single_step on non-current doesn't do
> > anything not already done by the memset in copy_thread. Ooh, except
> > perhaps it does not clear PSW_MASK_PER. Maybe that matters. That's
> > the only thing I can think of. Maybe Martin can make sense of it.

I am confused as well. Yes, I thought about regs->psw.mask change too,
but I don't understand why it helps..

> The additional traps should not happen anymore with this patch:
> --
> Subject: [PATCH] clear TIF_SINGLE_STEP for new process.
>
> From: Martin Schwidefsky <schwidefsky@de.ibm.com>
>
> Clear the TIF_SINGLE_STEP bit in copy_thread. If the new process is
> not auto-attached by the tracer it is wrong to delivere SIGTRAP to
> the new process.
>
> Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> ---
>
> arch/s390/kernel/process.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff -urpN linux-2.6/arch/s390/kernel/process.c linux-2.6-patched/arch/s390/kernel/process.c
> --- linux-2.6/arch/s390/kernel/process.c 2009-12-03 04:51:21.000000000 +0100
> +++ linux-2.6-patched/arch/s390/kernel/process.c 2010-01-07 09:25:53.000000000 +0100
> @@ -217,6 +217,7 @@ int copy_thread(unsigned long clone_flag
> p->thread.mm_segment = get_fs();
> /* Don't copy debug registers */
> memset(&p->thread.per_info, 0, sizeof(p->thread.per_info));
> + clear_tsk_thread_flag(p, TIF_SINGLE_STEP);

Even if I don't understand s390, I think this patch makes sense
anyway. Or, user_disable_single_step() can clear this bit.

But. Acoording to the testing I did (unless I did something wrong
again) this patch doesn't make any difference in this particular
case. 6580807da14c423f0d0a708108e6df6ebc8bc83d does.

And. Please note that the test-case triggers 799 "false step", but
TIF_SINGLE_STEP is surely cleared (by the caller) after the first
invocation of do_single_step().

Oleg.