linux-kernel - Re: [PATCH v6 4/5] seccomp: add support for passing fds via USER

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Date:   Thu, 6 Sep 2018 10:22:46 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Jann Horn <jannh@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        containers@...ts.linux-foundation.org,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>, suda.akihiro@....ntt.co.jp
Subject: Re: [PATCH v6 4/5] seccomp: add support for passing fds via
 USER_NOTIF

On Thu, Sep 06, 2018 at 06:15:18PM +0200, Jann Horn wrote:
> On Thu, Sep 6, 2018 at 5:29 PM Tycho Andersen <tycho@...ho.ws> wrote:
> > The idea here is that the userspace handler should be able to pass an fd
> > back to the trapped task, for example so it can be returned from socket().
> [...]
> > diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
> > index d1498885c1c7..1c0aab306426 100644
> > --- a/Documentation/userspace-api/seccomp_filter.rst
> > +++ b/Documentation/userspace-api/seccomp_filter.rst
> > @@ -235,6 +235,9 @@ The interface for a seccomp notification fd consists of two structures:
> >          __u64 id;
> >          __s32 error;
> >          __s64 val;
> > +        __u8 return_fd;
> > +        __u32 fd;
> > +        __u32 fd_flags;
> 
> Normally,  syscalls that take an optional file descriptor accept a
> signed 32-bit number, with -1 standing for "no file descriptor". Is
> there a reason why this uses a separate variable to signal whether an
> fd was provided?

No real reason other than I looked at the bpf code and they were using
__u32 for bpf (but I think in their case the fd args are not
optional). I'll switch it to __s32/-1 for the next version.

> Apart from that, this patch looks good to me.

Thanks,

Tycho