linux-kernel - Re: [PATCH 1/2] sigaltstack: implement SS

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Message-ID: <56DA8D25.20600@list.ru>
Date:	Sat, 5 Mar 2016 10:39:17 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Richard Weinberger <richard@....at>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Heinrich Schuchardt <xypron.glpk@....de>,
	Jason Low <jason.low2@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Josh Triplett <josh@...htriplett.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Aleksa Sarai <cyphar@...har.com>,
	Amanieu d'Antras <amanieu@...il.com>,
	Paul Moore <pmoore@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Stas Sergeev <stsp@...rs.sourceforge.net>
Subject: Re: [PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag

05.03.2016 01:22, Andy Lutomirski пишет:
> On Mon, Feb 29, 2016 at 1:29 PM, Stas Sergeev <stsp@...t.ru> 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.