Re: [klibc] kernel/libc uapi changes for y2038

From: Arnd Bergmann <arnd@arndb.de>
To: y2038@lists.linaro.org
Cc: klibc@zytor.com, libc-alpha@sourceware.org,
	linux-api@vger.kernel.org, musl@lists.openwall.com,
	linux-kernel@vger.kernel.org, Rich Felker <dalias@libc.org>,
	Thorsten Glaser <tg@mirbsd.de>,
	cferris@google.com, enh@google.com,
	"Joseph S. Myers" <joseph@codesourcery.com>
Subject: Re: [klibc] kernel/libc uapi changes for y2038
Date: Mon, 18 May 2015 22:35:40 +0200	[thread overview]
Message-ID: <3622889.d8DplXjbu8@wuerfel> (raw)
In-Reply-To: <Pine.BSM.4.64L.1505181658330.32668@herc.mirbsd.org>

On Monday 18 May 2015 17:03:08 Thorsten Glaser wrote:

> >MIPS on the other hand is no more broken than any of the other 32-bit
> >ABIs, because it does not use 64-bit __kernel_long_t in its n32 ABI.
> 
> I have heard from a MIPS porter that one of the flavours suffers
> from similar problems as x32, just not to that extent. But I
> don’t recall my source…

MIPS n32 has a lot of the same issues as x86 x32, but I'm pretty
sure that the time_t one is not among them.

> >ioctls. My plan at this point is to eliminate all uses of time_t in
> >the kernel and replace them with time64_t or other safe types.
> >This way, we will eventually find all code that passes 32-bit time types
> >to user space and can fix it. This will take care of the time_t
> >related problems on x32 as well.
> 
> Ah, interesting approach. And existing userspace, as well as new
> userspace that does not declare 64-bit time_t readiness, is still
> safe on currently-not-broken architectures? So, there’s enough time
> to fix this before the various libcs turn that on (and it had better
> be fixed by then, because it becomes ABI by then). Nice idea.

Correct. Another aspect of the approach I'm taking is that the
system-call implementation is shared between the native 64-bit
case and the new 32-bit case, while the handling for the existing
syscalls on 32-bit architectures is shared with the 32-bit compat
code on 64-bit architectures. This means if we introduce a bug
in either of them, we will find out very quickly and don't have
to wait until people start using 64-bit time_t on 32-bit user land.

> I am wondering a bit about the ioctls being hard to find. I have
> not much experience with kernel programming, and even less with
> Linux than with MS-DOS and BSD, but should not each driver have
> a central ioctl entry point, from which it should cast the user
> space data into a (possibly locally declared) structure?

Yes, that's how it works, but unfortunately we have a few thousand
drivers that declare an ioctl function, and I hope to do something
better than brute-force search all of them. The other point is that
we really need to fix all y2038-related bug in drivers, not just
the ones in ioctl. This includes things like file systems storing
time in 32-bit units on disk, or drivers trying to measure how much
time has elapsed without communicating that value elsewhere, but
failing when the time_t number goes negative.

	Arnd
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