Re: [PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag

[Stas Sergeev]

05.03.2016 01:22, Andy Lutomirski ÐÐÑÐÑ:
> On Mon, Feb 29, 2016 at 1:29 PM, Stas Sergeev <stsp@xxxxxxx> 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 = sigaltstack(SS_ONSTACK | SS_AUTODISARM);
if (err == EINVAL) {
  i_dont_have_this_feature = 1;
  err = 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.