linux-kernel - Re: [PATCH 3/4] signal: support pidctl() with pidfd_send

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Date:   Mon, 25 Mar 2019 20:41:59 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Jann Horn <jannh@...gle.com>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        bl0pbl33p@...il.com, "Dmitry V. Levin" <ldv@...linux.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>,
        cyphar@...har.com, Al Viro <viro@...iv.linux.org.uk>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Daniel Colascione <dancol@...gle.com>
Subject: Re: [PATCH 3/4] signal: support pidctl() with pidfd_send_signal()

On Mon, Mar 25, 2019 at 07:39:25PM +0100, Jann Horn wrote:
> On Mon, Mar 25, 2019 at 5:21 PM Christian Brauner <christian@...uner.io> wrote:
> > Let pidfd_send_signal() use pidfds retrieved via pidctl(). With this patch
> > pidfd_send_signal() becomes independent of procfs. This fullfils the
> > request made when we merged the pidfd_send_signal() patchset. The
> > pidfd_send_signal() syscall is now always available allowing for it to be
> > used by users without procfs mounted or even users without procfs support
> > compiled into the kernel.
> [...]
> >  static bool access_pidfd_pidns(struct pid *pid)
> >  {
> > +       int ret;
> >         struct pid_namespace *active = task_active_pid_ns(current);
> >         struct pid_namespace *p = ns_of_pid(pid);
> >
> > -       for (;;) {
> > -               if (!p)
> > -                       return false;
> > -               if (p == active)
> > -                       break;
> > -               p = p->parent;
> > -       }
> > +       ret = pidnscmp(active, p);
> > +       if (ret < 0)
> > +               return false;
> >
> >         return true;
> >  }
> 
> Nit, if we keep this function: "if (...) return false; return true;"
> seems like an antipattern to me. How about "return ret >= 0", or even
> "return pidnscmp(active, p) >= 0"?

Yip, sounds good.