linux-kernel - Re: [PATCH v2 2/4] exit: support non-blocking pidfds

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200903235659.GC210207@localhost>
Date:   Thu, 3 Sep 2020 16:56:59 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org,
        Christian Brauner <christian@...uner.io>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Aleksa Sarai <cyphar@...har.com>,
        linux-kselftest@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-api@...r.kernel.org, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 2/4] exit: support non-blocking pidfds

On Wed, Sep 02, 2020 at 12:21:28PM +0200, Christian Brauner wrote:
> Passing a non-blocking pidfd to waitid() currently has no effect, i.e.  is not
> supported. There are users which would like to use waitid() on pidfds that are
> O_NONBLOCK and mix it with pidfds that are blocking and both pass them to
> waitid().
> The expected behavior is to have waitid() return -EAGAIN for non-blocking
> pidfds and to block for blocking pidfds without needing to perform any
> additional checks for flags set on the pidfd before passing it to waitid().
> Non-blocking pidfds will return EAGAIN from waitid() when no child process is
> ready yet. Returning -EAGAIN for non-blocking pidfds makes it easier for event
> loops that handle EAGAIN specially.
> 
> It also makes the API more consistent and uniform. In essence, waitid() is
> treated like a read on a non-blocking pidfd or a recvmsg() on a non-blocking
> socket.
> With the addition of support for non-blocking pidfds we support the same
> functionality that sockets do. For sockets() recvmsg() supports MSG_DONTWAIT
> for pidfds waitid() supports WNOHANG. Both flags are per-call options. In
> contrast non-blocking pidfds and non-blocking sockets are a setting on an open
> file description affecting all threads in the calling process as well as other
> processes that hold file descriptors referring to the same open file
> description. Both behaviors, per call and per open file description, have
> genuine use-cases.
> 
> The implementation should be straightforward, we simply raise the WNOHANG flag
> when a non-blocking pidfd is passed and when do_wait() returns without finding
> an eligible task and the pidfd is non-blocking we set EAGAIN.  If no child
> process exists non-blocking pidfd users will continue to see ECHILD but if
> child processes exist but have not yet exited users will see EAGAIN.
> 
> A concrete use-case that was brought on-list was Josh's async pidfd library.
> Ever since the introduction of pidfds and more advanced async io various
> programming languages such as Rust have grown support for async event
> libraries. These libraries are created to help build epoll-based event loops
> around file descriptors. A common pattern is to automatically make all file
> descriptors they manage to O_NONBLOCK.
> 
> For such libraries the EAGAIN error code is treated specially. When a function
> is called that returns EAGAIN the function isn't called again until the event
> loop indicates the the file descriptor is ready.  Supporting EAGAIN when
> waiting on pidfds makes such libraries just work with little effort.
> 
> Link: https://lore.kernel.org/lkml/20200811181236.GA18763@localhost/
> Link: https://github.com/joshtriplett/async-pidfd
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Sargun Dhillon <sargun@...gun.me>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Suggested-by: Josh Triplett <josh@...htriplett.org>
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>

With or without the discussed change to WNOHANG behavior for
compatibility:
Reviewed-by: Josh Triplett <josh@...htriplett.org>

Also, I think you should flip the order of patches 1 and 2, so that
there isn't a one-patch window in kernel history where you can create an
O_NONBLOCK pidfd with pidfd_open but it has no effect. I'd expect
userspace to use pidfd_open accepting or EINVAL-ing the flag as an
indication of whether it'll work.