linux-kernel - Re: [PATCH] signal: remove the wrong signal_pending() check in restore_user

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Date:   Tue, 4 Jun 2019 22:24:41 +0000
From:   Eric Wong <e@...24.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Davidlohr Bueso <dbueso@...e.de>, Jens Axboe <axboe@...nel.dk>,
        Davidlohr Bueso <dave@...olabs.net>,
        Jason Baron <jbaron@...mai.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-aio@...ck.org, omar.kilani@...il.com,
        Thomas Gleixner <tglx@...utronix.de>,
        stable <stable@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH] signal: remove the wrong signal_pending() check in
 restore_user_sigmask()

Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Tue, Jun 4, 2019 at 6:41 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > This is the minimal fix for stable, I'll send cleanups later.
> 
> Ugh. I htink this is correct, but I wish we had a better and more
> intuitive interface.

I had the same thoughts, but am not a regular kernel hacker,
so I didn't say anything earlier.

> In particular, since restore_user_sigmask() basically wants to check
> for "signal_pending()" anyway (to decide if the mask should be
> restored by signal handling or by that function), I really get the
> feeling that a lot of these patterns like
> 
> > -       restore_user_sigmask(ksig.sigmask, &sigsaved);
> > -       if (signal_pending(current) && !ret)
> > +
> > +       interrupted = signal_pending(current);
> > +       restore_user_sigmask(ksig.sigmask, &sigsaved, interrupted);
> > +       if (interrupted && !ret)
> >                 ret = -ERESTARTNOHAND;
> 
> are wrong to begin with, and we really should aim for an interface
> which says "tell me whether you completed the system call, and I'll
> give you an error return if not".
> 
> How about we make restore_user_sigmask() take two return codes: the
> 'ret' we already have, and the return we would get if there is a
> signal pending and w're currently returning zero.
> 
> IOW, I think the above could become
> 
>         ret = restore_user_sigmask(ksig.sigmask, &sigsaved, ret, -ERESTARTHAND);
> 
> instead if we just made the right interface decision.

But that falls down if ret were ever expected to match several
similar error codes (not sure if it happens)

When I was considering fixing this on my own a few weeks ago, I
was looking for an inline that could quickly tell if `ret' was
any of the EINTR-like error codes; but couldn't find one...

It'd probably end up being switch/case statement so I'm not sure
if it'd be too big and slow or not...

The caller would just do:

	ret = restore_user_sigmask(ksig.sigmask, &sigsaved, ret);

And restore_user_sigmask would call some "was_interrupted(ret)"
inline which could return true if `ret' matched any of the
too-many-to-keep-track-of EINTR-like codes.  But I figured
there's probably a good reason it did not exist, already *shrug*

/me goes back to the wonderful world of userspace...