linux-kernel - Re: [PATCH 00/25] Change time_t and clock

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Message-ID: <20140515220717.1ced0f1a@alan.etchedpixels.co.uk>
Date:	Thu, 15 May 2014 22:07:17 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	James Bottomley <James.Bottomley@...senpartnership.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Christoph Hellwig <hch@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ley Foon Tan <lftan@...era.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LeyFoon Tan <lftan.linux@...il.com>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

> > could do intelligent interpolation about what the epoch is, so in 2039
> > if we get a low 32 bit time_t value, we assume epoch == 1, conversely if
> > we see a high one, we assume epoch == 0.  We could add epoch on the end
> > of the syscalls and detect if it's not present and fill in an
> > interpolated value.  Interfaces which truly want 64 bit time_t would get
> > it from epoch + 32 bit time_t.

True sort of for the kernel (except you've got problems with file system
compatibility and I believe stuff like NFS and that fact some file
systems just break).

> The main advantage that I see with 64-bit time_t is that a lot of user
> space already works with it, because NetBSD and OpenBSD use this, and
> all 64-bit Linux systems already work with this without changing the ABI.
> If we want a POSIX extension to cover a new ABI, this would be the most
> likely candidate.

The main advantage IMHO of just using 64bit on 32bit boxes is getting it
right. I'd go so far as to consider changing userspace time_t to be a
struct. Yes it'll break lots of code but at least you'll know about it.

(and for some other systems like 2BSD it's probably the least of evils
because there isn't a 64bit type in the compiler!)

> As mentioned earlier, between kernel and user space it's probably best
> to avoid time_t and timeval completely and just use timespec64 or some
> other safe type, but there has to be a way to port user space that relies
> on time_t or timespec.

This I think is bang on. We've already got lots of cases where we can't
shoehorn useful info into syscalls due to time_t, including stuff like
exposing accurate stamps in stat(). On an 8MHz 68000 not much happened in
the same second, on a 32 core x86 it's a bit different.

Alan
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