Re: [CRIU] [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2)

[Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 02/10, Andrew Vagin wrote:
>
> On Sat, Feb 09, 2013 at 11:53:04PM +0100, Michael Kerrisk (man-pages) wrote:
> >
> > This looks promising, but I am not sure I understand the user-space
> > API. Could you explain how it would look to (say) pull all per-thread
> > signals from user space?
> >
>
> 	siginfo_t *signals = malloc(...);
> 	...
>
> 	ret = ptrace(PTRACE_ATTACH, pid, NULL, NULL);

Yes. I assume this should not be a problem since c/r needs to ptrace
the task anyway?

> For shared (per-process) signals one line should be changed:
>  ret = ptrace(PTRACE_PEEKSIGNAL, pid, PTRACE_PEEK_SHARED + i, signals + i);

Yes. Except I'd say "PTRACE_PEEK_SHARED | i" to clarify that
PTRACE_PEEK_SHARED is "flag". Note that we can more flags, say,
PTRACE_PEEK_DEQUEUE which obviously means "dequeue signal" (but in
this case we should filter out SIGKILL/SIGSTOP).

And note that either way (I mean ptrace or signalfd) this can not
actually peek a signal which doesn't have the sigqueue (say,
SEND_SIG_FORCED). I guess this is not the problem, c/r can look
at /proc/pid/status#*Pnd aditionally.

OK, you seem to agree with this approach, I'll try to make the
working patch tomorrow. At least ptrace_peek_signal() needs another
is_compat argument for compat_ptrace_request().

Oleg.

> > > --- x/kernel/ptrace.c
> > > +++ x/kernel/ptrace.c
> > > @@ -618,6 +618,35 @@ static int ptrace_setsiginfo(struct task
> > >         return error;
> > >  }
> > >
> > > +static int ptrace_peek_signal(struct task_struct *child,
> > > +                               unsigned long addr, siginfo_t __user *uinfo)
> > > +{
> > > +       siginfo_t info;
> > > +       struct sigpending *pending;
> > > +       int ret = -ESOMETHING;
> > > +
> > > +       pending = &child->pending;
> > > +       if (addr & PTRACE_PEEK_SHARED) {
> > > +               addr &= ~PTRACE_PEEK_SHARED;
> > > +               pending = &child->signal->shared_pending;
> > > +       }
> > > +
> > > +       spin_lock_irq(&child->sighand->siglock);
> > > +       list_for_each_entry(q, &pending->list, list) {
> > > +               if (!addr--) {
> > > +                       copy_siginfo(info, &q->info);
> > > +                       ret = 0;
> > > +                       break;
> > > +               }
> > > +       }
> > > +       spin_lock_irq(&child->sighand->siglock);
> > > +
> > > +       if (!ret)
> > > +               ret = copy_siginfo_to_user(uinfo, info);
> > > +       if (!ret)
> > > +               ret = __put_user(info, si_code);
> > > +       return ret;
> > > +}
> > >
> > >  #ifdef PTRACE_SINGLESTEP
> > >  #define is_singlestep(request)         ((request) == PTRACE_SINGLESTEP)
> > > @@ -742,6 +771,10 @@ int ptrace_request(struct task_struct *c
> > >                 ret = put_user(child->ptrace_message, datalp);
> > >                 break;
> > >
> > > +       case PTRACE_PEEKSIGNAL:
> > > +               ret = ptrace_peek_signal(child, addr, datavp);
> > > +               break;
> > > +
> > >         case PTRACE_GETSIGINFO:
> > >                 ret = ptrace_getsiginfo(child, &siginfo);
> > >                 if (!ret)
> > >
> >
> >
> >
> > --
> > Michael Kerrisk
> > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> > Author of "The Linux Programming Interface"; http://man7.org/tlpi/
> > _______________________________________________
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