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

[Stas Sergeev <stsp-cmBhpYW9OiY@public.gmane.org>]

01.02.2016 23:41, Oleg Nesterov пишет:
> On 02/01, Stas Sergeev wrote:
>> 01.02.2016 22:29, Oleg Nesterov пишет:
>>>>> 	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.