linux-kernel - Re: is avoiding compat ioctls possible?

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Date:	Wed, 28 Oct 2009 03:53:17 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Dave Airlie <airlied@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	DRI Development Mailing List 
	<dri-devel@...ts.sourceforge.net>, Arnd Bergmann <arnd@...db.de>,
	David Miller <davem@...emloft.net>
Subject: Re: is avoiding compat ioctls possible?

Dave Airlie <airlied@...il.com> writes:

> They used uint64_t to represent userspace pointers and userspace
> casted into those and the kernel casts back out and passes it to copy_*_user

uint64_t is actually dangerous due to different alignment on x86-32 vs 64,
better use compat_u64/s64

> Now I thought cool I don't need to worry about compat ioctl hackery I can
> run 32 on 64 bit apps fine and it'll all just work.
>
> Now Dave Miller points out that I'm obivously deluded and we really need
> to add compat ioctls so that the kernel can truncate correctly 32-bit address
> in case userspace shoves garbage into the top 32bits of the u64.

When the user space sees a u64 field it should never shove garbage here.
You just have to cast on 32bit for this, which is a bit ugly.

However some architectures need special operations on compat pointers
(s390 iirc), but if you don't support those it might be reasonable
to not support that.

> Is there really no way to avoid compat ioctls? was I delusional in
> thinking there was?

Experience shows that people make mistakes and you sooner or
later need them anyways to work around them.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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