linux-kernel - Re: [PATCH] ptrace: add ability to get/set signal-blocked mask

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Date:	Tue, 23 Apr 2013 14:59:18 +0400
From:	Andrew Vagin <avagin@...allels.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Andrey Vagin <avagin@...nvz.org>, <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: Re: [PATCH] ptrace: add ability to get/set signal-blocked mask

On Mon, Apr 22, 2013 at 05:07:10PM +0200, Oleg Nesterov wrote:
> On 04/22, Oleg Nesterov wrote:
> >
> > On 04/22, Andrey Vagin wrote:
> > >
> > > +	case PTRACE_SETSIGMASK:
> > > +	{
> > > +		sigset_t new_set;
> > > +
> > > +		if (addr != sizeof(sigset_t)) {
> > > +			ret = -EINVAL;
> > > +			break;
> > > +		}
> > > +
> > > +		if (copy_from_user(&new_set, datavp, sizeof(sigset_t))) {
> > > +			ret = -EFAULT;
> > > +			break;
> > > +		}
> > > +
> > > +		sigdelsetmask(&new_set, sigmask(SIGKILL)|sigmask(SIGSTOP));
> > > +
> > > +		spin_lock_irq(&child->sighand->siglock);
> > > +		__set_task_blocked(child, &new_set);
> > > +		spin_unlock_irq(&child->sighand->siglock);
> >
> > No, please don't...
> >
> > set_current_blocked/__set_task_blocked assume that tsk == current.
> > If nothing else, note recalc_sigpending() in __set_task_blocked().

Thank you for the comment. It's my mistake.

> >
> > I don't understand "This method is not suitable for stopped tasks"

For example, a stopped process has a pending signal and this signal is
not blocked. crtools should dump its state, so that the process remains
in a stopped state with the same pending signal.

For dumping state crtools inject a parasite code with help POKE_DATA,
sets %rip on this code (PTRACE_SETREGS) and resumes the task
(PTRACE_CONT).

If signals are not blocked, the kernel starts to handle the signal after
resuming a process. It's out of our plan.

https://lkml.org/lkml/2011/7/20/138 - an example of code for injecting a
parasite code.

I hope the problem is become more clear.

> > from the changelog, but if you really need PTRACE_SETSIGMASK just
> > change ->blocked under siglock and do recalc_sigpending_tsk(child).
>                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> actually this is not necessary, the tracee will do recalc_sigpending()
> after resume. But perhaps a comment make sense.

__set_task_blocked executes retarget_shared_pending. I think it must be
called here too or am I wrong?

> 
> Oleg.
> 
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