linux-kernel - Re: SIGSYS annoyance

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Message-ID: <22362.33704.293079.558345@gargle.gargle.HOWL>
Date:	Fri, 10 Jun 2016 11:08:56 +0200
From:	Mikael Pettersson <mikpelinux@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Kees Cook <keescook@...omium.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: SIGSYS annoyance

Andy Lutomirski writes:
 > On Mon, Jun 6, 2016 at 9:03 AM, Kees Cook <keescook@...omium.org> wrote:
 > > On Fri, Jun 3, 2016 at 10:16 PM, Andy Lutomirski <luto@...capital.net> wrote:
 > >> https://bugzilla.mozilla.org/show_bug.cgi?id=1176099
 > >>
 > >> Should SIGSYS be delivered to the handler even if blocked?  What, if
 > >> anything, does POSIX say?  All I can find is in pthread_sigmask(3p):
 > >>
 > >> If any of the SIGFPE, SIGILL, SIGSEGV, or SIGBUS signals are generated
 > >> while they are blocked, the result is undefined, unless the signal was
 > >> generated by the action of another process, or by one of the functions
 > >> kill(), pthread_kill(), raise(), or sigqueue().
 > >>
 > >> It would be easy enough to change our behavior so that we deliver the
 > >> signal even if it's blocked or to at least add a flag so that users
 > >> can request that behavior.
 > >
 > > I had trouble following that bug. It sounded like glib just needed a
 > > way to define its signal mask, and that's what they ended up
 > > implementing?
 > >
 > > I think the current behavior is correct. SIGSYS is being generated by
 > > the running process (i.e. the seccomp filter) and if it has a handler
 > > but the signal is blocked, we should treat it as uncaught and kill. On
 > > the other hand, it could be seen like "raise", in which case the
 > > blocking should be ignored? Is there an active problem somewhere here?
 > > It seems like the referenced bug has been fixed already.
 > 
 > Agreed.
 > 
 > It could make sense to have a new sigaction flag SA_FORCE: when set,
 > if a non-default handler is installed, the signal is blocked, and the
 > signal is triggered synchronously (forced), then the handler will be
 > called.  But that isn't specific to seccomp.

Blocking a signal is a very deliberate act.  If some piece of code wants
to force-deliver it, it can unblock it first.  IOW, I don't see the need
for this SA_FORCE thing.