Re: [PATCH v3 4/7] linux/signal.h: Ignore SIGINFO by default in new tasks

From: Christian Brauner <christian.brauner@ubuntu.com>
To: Jiri Slaby <jslaby@suse.cz>
Cc: Arseny Maslennikov <ar@cs.msu.ru>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	Rob Landley <rob@landley.net>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Pavel Machek <pavel@ucw.cz>,
	linux-api@vger.kernel.org,
	"Vladimir D. Seleznev" <vseleznv@altlinux.org>
Subject: Re: [PATCH v3 4/7] linux/signal.h: Ignore SIGINFO by default in new tasks
Date: Thu, 30 Apr 2020 09:14:37 +0200	[thread overview]
Message-ID: <20200430071437.x3ilwkh3lyf4iq6u@wittgenstein> (raw)
In-Reply-To: <780cb05e-a749-77a0-dabc-bd09982aa028@suse.cz>

On Thu, Apr 30, 2020 at 08:53:56AM +0200, Jiri Slaby wrote:
> On 30. 04. 20, 8:42, Arseny Maslennikov wrote:
> > This matches the behaviour of other Unix-like systems that have SIGINFO
> > and causes less harm to processes that do not install handlers for this
> > signal, making the keyboard status character non-fatal for them.
> > 
> > This is implemented with the assumption that SIGINFO is defined
> > to be equivalent to SIGPWR; still, there is no reason for PWR to
> > result in termination of the signal recipient anyway — it does not
> > indicate there is a fatal problem with the recipient's execution
> > context (like e.g. FPE/ILL do), and we have TERM/KILL for explicit
> > termination requests.
> > 
> > To put it another way:
> > The only scenario where system behaviour actually changes is when the
> > signal recipient has default disposition for SIGPWR. If a process
> > chose to interpret a SIGPWR as an incentive to cleanly terminate, it
> > would supply its own handler — and this commit does not affect processes
> > with non-default handlers.
> > 
> > Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
> > ---
> >  include/linux/signal.h | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/signal.h b/include/linux/signal.h
> > index 05bacd2ab..dc31da8fc 100644
> > --- a/include/linux/signal.h
> > +++ b/include/linux/signal.h
> > @@ -369,7 +369,7 @@ extern bool unhandled_signal(struct task_struct *tsk, int sig);
> >   *	|  SIGSYS/SIGUNUSED  |	coredump 	|
> >   *	|  SIGSTKFLT         |	terminate	|
> >   *	|  SIGWINCH          |	ignore   	|
> > - *	|  SIGPWR            |	terminate	|
> > + *	|  SIGPWR            |	ignore   	|
> 
> You need to update signal.7 too:
> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/signal.7#n285

(I fail this whole thread via b4 and it appears that a bunch of messages
are missing on lore. Might just be delay though.)

How this is this not going to break userspace?
Just for a start, SIGPWR (for better or worse) was used for a long time
by some sandboxing/container runtimes to shutdown a process and still
is.

Christian