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From: Arnd Bergmann <arnd@arndb.de>
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Date: Wed, 13 Aug 2014 22:06:19 +0200
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Cc: John Stultz <john.stultz@linaro.org>, lkml <linux-kernel@vger.kernel.org>,
	"Joseph S. Myers" <joseph@codesourcery.com>
Subject: Re: [Ksummit-discuss] 2038 Kernel Summit Discussion Fodder
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On Wednesday 13 August 2014 03:06:53 Ben Hutchings wrote:
> > On the kernel side, it also adds more complexity, where we have to add
> > even more complex compat support for 64bit systems to handle all the
> > various 32bit applications possible.
> [...]
> 
> Didn't we need to do this already to support x32?  Have compat ioctls
> involving time been botched?

AFAICT, every ioctl that involves passing a __kernel_ulong_t or
__kernel_ulong_t is potentially broken on x32, and this includes
everything passing a time_t or timespec.

The problem is that the libc ioctl() function ends up in the kernel's
compat_ioctl handler, which expects the 32-bit ABI, not the 64-bit ABI.
Most other syscalls in x32 however use the 64-bit ABI.

It works only for drivers that use the same function for .ioctl and
.compat_ioctl, and that encode the size of the data structure correctly
in the ioctl command code. I assume this is how we will do it for all
32-bit architectures with 64-bit time_t, but on x32 it also concerns
other types that use __kernel_long_t.

	Arnd