linux-kernel - Re: [RFC PATCH] vfs: shutdown lease notifications on file close

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Message-ID: <20171013170105.GF21978@ZenIV.linux.org.uk>
Date:   Fri, 13 Oct 2017 18:01:06 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        linux-fsdevel@...r.kernel.org,
        Jeff Layton <jlayton@...chiereds.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] vfs: shutdown lease notifications on file close

On Fri, Oct 13, 2017 at 08:56:10AM -0700, Dan Williams wrote:
> While implementing MAP_DIRECT, an mmap flag that arranges for an
> FL_LAYOUT lease to be established, Al noted:
> 
>     You are not even guaranteed that descriptor will remain be still
>     open by the time you pass it down to your helper, nevermind the
>     moment when event actually happens...
> 
> The first problem can be solved with an fd{get,put} at mmap
> {entry,exit}.

Huh?  fdget() does *NOT* guarantee that descriptor won't get closed.  What
it does is guarantee that struct file won't get closed under you, which
is nowhere near the same thing.  And while we are at it, it certainly
_is_ called by mmap()...

> The second problem appears to be a general issue.
> 
> Leases follow the lifetime of the inode, so it is possible for a lease
> to be broken after the file is closed. When that happens userspace may
> get a notification on a stale fd. Of course it is not recommended that a
> process close a file descriptor with an active lease, but if it does we
> should assume that the notification is not needed either. Walk leases at
> close time and invalidate any pending fasync instances.

What the hell is special about close(2) and not, e.g. dup2(2)?  Or execve(2)
triggering close-on-exec, etc...  Besides, you are changing a user-visible
behaviour here.  Suppose your process forks and the child closes all
descriptors; should that stop SIGIO delivery to the parent?

Let's step back for a minute; could you describe how the userland is supposed
to use that thing?