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

[Stas Sergeev]

01.02.2016 21:52, Oleg Nesterov ÐÐÑÐÑ:
> Stas, I probably missed something, but I don't understand your concerns,
>
> On 02/01, Stas Sergeev 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.
> Nothing? restore_sigaltstack() should work as expected?
That's likely the reason for EPERM: restore_sigaltstack()
does the job, so manual modifications are disallowed.
Allowing them will bring in the surprises where the changes
done by the user are ignored.

> > > 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();
> Yes, or
>
> 	sigaltstack({ DISABLE | FORCE}, &old_ss);
> 	swapcontext();
> 	sigaltstack(&old_ss, NULL);
> 	rt_sigreturn();
>
> and if you are going to return from sighandler you do not even need the 2nd
> sigaltstack(), you can rely on sigreturn.
Yes, that's what I do in my app already.
But its only there when SA_SIGINFO is used.

> > What's at the end? Do we want a surprise for the user
> > that he's new_sas got ignored?
> Can't understand.... do you mean "set up new_sas" will be ignored because
> rt_sigreturn() does restore_sigaltstack() ? I see no problem here...
Allowing the modifications that were previously EPERMed
but will now be silently ignored, may be seen as a problem.
But if it isn't - fine, lets code that.