linux-kernel - Re: SIGSYS annoyance

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Date:	Thu, 9 Jun 2016 10:21:01 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: SIGSYS annoyance

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.

--Andy