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

[Stas Sergeev]

01.02.2016 22:29, Oleg Nesterov ÐÐÑÐÑ:
> On 02/01, Stas Sergeev wrote:
> > 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.
> Unlikely. Suppose you do sigalstack() and then a non SA_ONSTACK signal handler
> runs and calls sigaltstack() again. This won't fail, but restore_sigaltstack()
> will restore the old alt stack after return.
OK.

> I too do not know why uc_stack exists, in fact I do not know about it until
> today when I read your patch ;) But it is here, and I do not think SS_FORCE
> can add more confusion than we already have.
OK.

> > > 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.
> 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);
        } else if (is_x32_frame()) {
                return x32_setup_rt_frame(ksig, cset, regs);
        } else {
                return __setup_rt_frame(ksig->sig, ksig, set, regs);
        }



> > > > 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.