linux-kernel - Details on the UAPI of implementing notifications on pipes

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Date:   Tue, 05 Nov 2019 16:02:59 +0000
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org, viro@...iv.linux.org.uk
Cc:     dhowells@...hat.com, Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        nicolas.dichtel@...nd.com, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Details on the UAPI of implementing notifications on pipes

So to implement notifications on top of pipes, I've hacked it together a bit
in the following ways:

 (1) I'm passing O_TMPFILE to the pipe2() system call to indicate that you
     want a notifications pipe.  This prohibits splice and co. from being
     called on it as I don't want to have to try to fix iov_iter_revert() to
     handle kernel notifications being intermixed with splices.

     The choice of O_TMPFILE was just for convenience, but it needs to be
     something different.  I could, for instance, add a constant,
     O_NOTIFICATION_PIPE with the same *value* as O_TMPFILE.  I don't think
     it's likely that it will make sense to use O_TMPFILE with a pipe, but I
     also don't want to eat up another O_* constant just for this.

     Unfortunately, pipe2() doesn't have any other arguments into from which I
     can steal a bit.

 (2) I've added a pair of ioctls to configure the notifications bits.  They're
     ioctls as I just reused the ioctl code from my devmisc driver.  Should I
     use fcntl() instead, such as is done for F_SETPIPE_SZ?

     The ioctls do two things: set the ring size to a number of slots (so
     similarish to F_SETPIPE_SZ) and set filters.

Any thoughts on how better to represent these bits?

Thanks,
David