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

[Stas Sergeev]

01.02.2016 23:41, Oleg Nesterov ÐÐÑÐÑ:
> On 02/01, Stas Sergeev wrote:
> > 01.02.2016 22:29, Oleg Nesterov ÐÐÑÐÑ:
> > > > > 	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.
> > > Hmm. how this connects to SA_SIGINFO ?
> > AFAIK without SA_SIGINFO you get sigreturn instead of
> > rt_sigreturn, which doesn't seem to do restore_altstack().
> > Or am I wrong?
> >
> > Hmm:
> >
> >          /* Set up the stack frame */
> >          if (is_ia32_frame()) {
> >                  if (ksig->ka.sa.sa_flags & SA_SIGINFO)
> >                          return ia32_setup_rt_frame(usig, ksig, cset, regs);
> >                  else
> >                          return ia32_setup_frame(usig, ksig, cset, regs);
> Ah, ia32... So this is even more confusing.
>
> > > > > > 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.
> > > Still can't understand. The 2nd sigaltstack() is no longer EPERMed because
> > > application used SS_FORCED before that and disabled altstack.
> > >
> > > And it is not ignored, it actually changes alt stack. Until we return from
> > > handler.
> > Before we return, the signals are usually blocked.
> > So whatever is after return is most important.
> Yes, but I still can't understand your "silently ignored". At least how does
> this differ from the case when a non-SA_ONSTACK signal handler does
> sigaltstack() and then rt_sigreturn() restores the old stack.
There is quite a difference.
It is very-very unlikely that non-SA_ONSTACK signal handler does 
sigaltstack().
I think only the test-case could exhibit this.
But with SS_FORCE - most of every SS_FORCE user will be
trapped, because, as you mentioned, not many know about
uc_stack. My patch was allowing them to do only what is safe:
just as it was without a patch.
But anyway, I'll be implementing SS_FORCE because 2 people
have voted.