Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler

[Stas Sergeev]

01.02.2016 21:28, Andy Lutomirski ÐÐÑÐÑ:
> On Mon, Feb 1, 2016 at 10:16 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > 01.02.2016 21:04, Oleg Nesterov ÐÐÑÐÑ:
> > > Yes, and SS_FORCE means "I know what I do", looks very simple.
> > But to me its not because I don't know what to do with
> > uc_stack after SS_FORCE is applied.
> >
> > > I won't argue, but to me it would be better to keep this EPERM if !force.
> > > Just because we should avoid the incompatible changes if possible.
> > Ok then. Lets implement SS_FORCE.
> > What semantic should it have wrt uc_stack?
> >
> > sigaltstack(SS_DISABLE | SS_FORCE);
> > swapcontext();
> > sigaltstack(set up new_sas);
> > rt_sigreturn();
> >
> > What's at the end? Do we want a surprise for the user
> > that he's new_sas got ignored?
> More detail please.  What context are you returning to with
> rt_sigreturn?  What's in uc_stack?
Whatever was saved there by save_altstack_ex() I guess.
Which is the sas params on signal entry.
And I am returning to the interrupted user context.
I am using SA_SIGINFO with sigaction().
This is actually what I was asking you already yeaterday.
I don't think SS_FORCE can play nicely with uc_stack and
you haven't clarified that part, so lets try again.

> Presumably we should continue to honor uc_stack in rt_sigreturn.
In this case, the above sigaltstack(set up new_sas) just
gets ignored. Are we allright with that? If so, I can code
up the patch. Whatever. :)

>    I'm
> less clear on whether we should have an implicit SS_FORCE when
> restoring uc_stack.
This is obscure and is likely outside of the scope of the
problem at hands.

>    I'm also not clear on why uc_stack exists in the
> first place.
>
> If I were designing this from scratch, I'd have signal delivery for an
> SA_ONSTACK signal save away the altstack information and clear it so
> that nested signals work without checking sp during signal delivery.
How would you then evaluate oss->ss_flags?