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Message-ID: <20230928212656.GC189345@mit.edu>
Date:   Thu, 28 Sep 2023 17:26:56 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Sterba <dsterba@...e.cz>,
        Amir Goldstein <amir73il@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>, Jeremy Kerr <jk@...abs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
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        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
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        Marc Dionne <marc.dionne@...istor.com>,
        Ian Kent <raven@...maw.net>,
        Luis de Bethencourt <luisbg@...nel.org>,
        Salah Triki <salah.triki@...il.com>,
        "Tigran A. Aivazian" <aivazian.tigran@...il.com>,
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Subject: Re: [PATCH 86/87] fs: switch timespec64 fields in inode to discrete
 integers

On Thu, Sep 28, 2023 at 01:40:55PM -0400, Jeff Layton wrote:
> 
> Correct. We'd lose some fidelity in currently stored timestamps, but as
> Linus and Ted pointed out, anything below ~100ns granularity is
> effectively just noise, as that's the floor overhead for calling into
> the kernel. It's hard to argue that any application needs that sort of
> timestamp resolution, at least with contemporary hardware. 
> 
> Doing that would mean that tests that store specific values in the
> atime/mtime and expect to be able to fetch exactly that value back would
> break though, so we'd have to be OK with that if we want to try it. The
> good news is that it's relatively easy to experiment with new ways to
> store timestamps with these wrappers in place.

The reason why we store 1ns granularity in ext4's on-disk format (and
accept that we only support times only a couple of centuries into the
future, as opposed shooting for an on-disk format good for several
millennia :-), was in case there was userspace that might try to store
a very fine-grained timestamp and want to be able to get it back
bit-for-bit identical.

For example, what if someone was trying to implement some kind of
steganographic scheme where they going store a secret message (or more
likely, a 256-bit AES key) in the nanosecond fields of the file's
{c,m,a,cr}time timestamps, "hiding in plain sight".  Not that I think
that we have to support something like that, since the field is for
*timestamps* not cryptographic bits, so if we break someone who is
doing that, do we care?

I don't think anyone will complain about breaking the userspace API
--- especially since if, say, the CIA was using this for their spies'
drop boxes, they probably wouldn't want to admit it.  :-)

       	    	     	      	      	    - Ted

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