From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stas Sergeev <stsp-cmBhpYW9OiY@public.gmane.org>
Subject: Re: [PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag
Date: Sat, 5 Mar 2016 10:39:17 +0300
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To: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
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05.03.2016 01:22, Andy Lutomirski =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> On Mon, Feb 29, 2016 at 1:29 PM, Stas Sergeev <stsp-cmBhpYW9OiY@public.gmane.org> wrote:
>> This patch implements the SS_AUTODISARM flag that can be ORed with
>> SS_ONSTACK when forming ss_flags.
>> When this flag is set, sigaltstack will be disabled when entering
>> the signal handler; more precisely, after saving sas to uc_stack.
>> When leaving the signal handler, the sigaltstack is restored by
>> uc_stack.
>> When this flag is used, it is safe to switch from sighandler with
>> swapcontext(). Without this flag, the subsequent signal will corrupt
>> the state of the switched-away sighandler.
>>
> This looks reasonable to me with one exception: how does a user
> program detect the presence of this feature?
Compile-time detection:
#ifdef SS_AUTODISARM
   # I have this feature
   ...
#endif

Run-time detection:
int err =3D sigaltstack(SS_ONSTACK | SS_AUTODISARM);
if (err =3D=3D EINVAL) {
   i_dont_have_this_feature =3D 1;
   err =3D sigaltstack(SS_ONSTACK);
}

Note: if you want to keep such detection for the future
additions, the mask can be enlarged to, say, ((1 << 24) - 1),
and whenever someone adds a new flag, he can lower the
mask by one bit.
But I think this would be an overkill in that particular case.