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

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Date:   Tue, 7 Jan 2020 22:00:15 -0500
From:   "J. Bruce Fields" <bfields@...ldses.org>
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 Tue, Jan 07, 2020 at 05:44:07PM +0000, Chris Down wrote:
> J. Bruce Fields writes:
> >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.
> 
> Sure, but I mean, we don't really protect against even the first case.
> 
> >>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?
> 
> We can, but that seems like a tangential discussion/patch series.
> For the second case especially, that's something we should do
> separately from this patchset,

Oh, of course, I'm not objecting to this patchset at all, it's a "why
not also do this?" question.

> since this demonstrably fixes issues encountered in production, and
> extending a user-facing APIs is likely to be a much more extensive
> discussion.

Though if it's a question of just a new implementation of an existing
ioctl, I doubt it's such a big deal.  (Not that I'm volunteering to
write the patch.)

--b.