From: Arnd Bergmann <arnd@arndb.de>
To: "Theodore Ts'o" <tytso@mit.edu>
Cc: David Howells <dhowells@redhat.com>, linux-afs@vger.kernel.org,
        linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org,
        samba-technical@lists.samba.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
        Deepa Dinamani <deepa.kernel@gmail.com>
Subject: Re: [PATCH 01/12] Ext4: Fix extended timestamp encoding and decoding
Date: Mon, 30 Nov 2015 15:37:43 +0100
Message-ID: <4966628.zR0sijQ42t@wuerfel>
In-Reply-To: <20151130141605.GA4316@thunk.org>
References: <20151120145422.18930.72662.stgit@warthog.procyon.org.uk> <2872067.shHdUXoF07@wuerfel> <20151130141605.GA4316@thunk.org>
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Sender: linux-nfs-owner@vger.kernel.org

On Monday 30 November 2015 09:16:05 Theodore Ts'o wrote:
> On Sun, Nov 29, 2015 at 10:30:39PM +0100, Arnd Bergmann wrote:
> > The other large missing piece is the system call implementation. I have
> > posted a series earlier this year before my parental leave, and it's
> > currently lacking review from libc folks, and blocked on me to update
> > the series and post it again.
> 
> I assume that this also means there hasn't been much thought about
> userspace support above libc?  i.e., how to take a 64-bit time64_t (or
> changing the size of time_t) and translating that to a string using
> some kind of version of ctime() and asctime(), and how to parse a
> post-2038 date string and turning it into a 64-bit time_t on a 32-bit
> platform?
> 
> The reason why I'm asking is because I'm thinking about how to add the
> appropriate regression test support to e2fsprogs for 32-bit platforms.
> I'm probably going to just skip the tests on architectures where
> sizeof(time_t) == 4 for now, since with a 32-bit time_t adding support
> for post-2038 in a e2fsprogs-specific way is (a) something I don't
> have time for, and (b) probably a waste of time since presumably we
> will either need to have a more general solution, or simply decide to
> give up on 32-bit platforms by 2038....

We are definitely going to be using 32-bit embedded platforms in 2038,
but we won't be using a 32-bit time_t then, so basing the check on
sizeof(time_t) sounds reasonable. I assume most generic distros will
stay with 32-bit time_t for compatibility reasons and just not give
long term support for 32-bit architectures, while the embedded
distros will move over to 64-bit time_t, but on those you recompile
all user space for each product anyway.

The glibc functions should all work with a 64-bit time_t as they do
today on 64-bit architectures. There is an open discussion on how
you move to 64-bit time_t. With the current
glibc plan at https://sourceware.org/glibc/wiki/Y2038ProofnessDesign,
you will have to set -D_TIME_BITS=64 to enable it explicitly, but
I'd also like to see a way to build a glibc that defaults to that
and does not allow backwards compatibility, which is important for
folks that want to ship a system that has they can guarantee to
survive 2038.

	Arnd