Re: [PATCH] unshare: Use rcu_assign_pointer when setting sighand

[Janak Desai <janak@us.ibm.com>]

Oleg Nesterov wrote:

> Andrew Morton wrote:
>  
>
> > Oleg Nesterov <oleg@tv-sign.ru> wrote:
> >    
> >
> > > Isn't it better to just replace this code with
> > > 'BUG_ON(new_sigh != NULL)' ?
> > >
> > > It is never executed, but totally broken, afaics.
> > > task_lock() has nothing to do with ->sighand changing.
> > >
> > >      
> > /*
> > * Unsharing of sighand for tasks created with CLONE_SIGHAND is not
> > * supported yet
> > */
> > static int unshare_sighand(unsigned long unshare_flags, struct sighand_struct **new_sighp)
> >
> > It's all just a place-holder at present.
> >
> > If we don't plan on ever supporting unshare(CLONE_SIGHAND) we should take
> > that code out and make it return EINVAL.  Right now.
> >
> > And because we don't presently support CLONE_SIGHAND we should return
> > EINVAL if it's set.  Right now.
> >
> > And we should change sys_unshare() to reject not-understood flags.  Right
> > now.
> >
> > If we don't do these things we'll silently break 2.6.16-back-compatibility
> > of applications which are coded for future kernels.
> >    
>
> unshare_sighand() is ok, it never populates *new_sighp, it just returns
> errror code: 0 when ->sighand is not shared, EINVAL otherwise.
>
> I argued about 'if (new_sigh)' code in sys_unshare() because it lies about
> locking rules.
>
> Btw, copy_process() forbids CLONE_SIGHAND without CLONE_VM (is there a
> good reason for that?), but one can do unshare(CLONE_VM). This is odd.
>  
Yes, copy_process forbids cloning of signal handlers without cloning of vm.
However, it does allow cloning of vm without cloning of signal handlers. For
those processes, that are sharing vm but not signal handlers, unsharing 
of vm
is allowed.

> Oleg.
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