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

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Date:   Thu, 30 Apr 2020 09:19:52 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Jiri Slaby <jslaby@...e.cz>, Arseny Maslennikov <ar@...msu.ru>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rob Landley <rob@...dley.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Pavel Machek <pavel@....cz>, linux-api@...r.kernel.org,
        "Vladimir D. Seleznev" <vseleznv@...linux.org>
Subject: Re: [PATCH v3 4/7] linux/signal.h: Ignore SIGINFO by default in new
 tasks

On Thu, Apr 30, 2020 at 09:14:37AM +0200, Christian Brauner wrote:
> 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@...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?

That's my main hesitation for taking this patchset.

> 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.

That's a good reason to not do this :(

greg k-h