linux-kernel - Re: [PATCH] fs: inode: Reduce volatile inode wraparound risk when ino

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Message-ID: <20200107173530.GC944@fieldses.org>
Date:   Tue, 7 Jan 2020 12:35:30 -0500
From:   bfields@...ldses.org (J. Bruce Fields)
To:     Chris Down <chris@...isdown.name>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] fs: inode: Reduce volatile inode wraparound risk when
 ino_t is 64 bit

On Sat, Dec 21, 2019 at 10:16:52AM +0000, Chris Down wrote:
> Darrick J. Wong writes:
> >On Fri, Dec 20, 2019 at 02:49:36AM +0000, Chris Down wrote:
> >>In general, userspace applications expect that (device, inodenum) should
> >>be enough to be uniquely point to one inode, which seems fair enough.
> >
> >Except that it's not.  (dev, inum, generation) uniquely points to an
> >instance of an inode from creation to the last unlink.

I thought that (dev, inum) was supposed to be unique from creation to
last unlink (and last close), and (dev, inum, generation) was supposed
to be unique for all time.

> I didn't mention generation because, even though it's set on tmpfs
> (to prandom_u32()), it's not possible to evaluate it from userspace
> since `ioctl` returns ENOTTY. We can't ask userspace applications to
> introspect on an inode attribute that they can't even access :-)

Is there any reason not to add IOC_GETVERSION support to tmpfs?

I wonder if statx should return it too?

--b.