linux-kernel - Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

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Message-ID: <ZWjaSAhG9KI2i9NK@tycho.pizza>
Date:   Thu, 30 Nov 2023 11:54:00 -0700
From:   Tycho Andersen <tycho@...ho.pizza>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Tycho Andersen <tandersen@...flix.com>,
        mathieu.desnoyers@...icios.com
Subject: Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

On Thu, Nov 30, 2023 at 07:37:02PM +0100, Florian Weimer wrote:
> * Tycho Andersen:
> 
> > From: Tycho Andersen <tandersen@...flix.com>
> >
> > We are using the pidfd family of syscalls with the seccomp userspace
> > notifier. When some thread triggers a seccomp notification, we want to do
> > some things to its context (munge fd tables via pidfd_getfd(), maybe write
> > to its memory, etc.). However, threads created with ~CLONE_FILES or
> > ~CLONE_VM mean that we can't use the pidfd family of syscalls for this
> > purpose, since their fd table or mm are distinct from the thread group
> > leader's. In this patch, we relax this restriction for pidfd_open().
> 
> Does this mean that pidfd_getfd cannot currently be used to get
> descriptors for a TID if that TID doesn't happen to share its descriptor
> set with the thread group leader?

Correct, that's what I'm trying to solve.

> I'd like to offer a userspace API which allows safe stashing of
> unreachable file descriptors on a service thread.

By "safe" here do you mean not accessible via pidfd_getfd()?

Tycho