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Date:	Fri, 9 Oct 2015 11:25:09 +0100 (IST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Hans Zuidam <h.zuidam@...puter.org>
cc:	Jaccon Bastiaansen <jaccon.bastiaansen@...il.com>, x86@...nel.org,
	mingo@...hat.com, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC]: Possible race condition in kernel futex code

Hans,

On Fri, 9 Oct 2015, Hans Zuidam wrote:
> On 9 okt. 2015, at 11:06, Thomas Gleixner <tglx@...utronix.de> wrote:
> > You cannot use an explicit 32bit read. We need an access which
> > handles the fault gracefully.
>
> The reason for the explicit read suggestion is to avoid the
> _builtin_constant_p() in __copy_from_user_nocheck().  The GCC manual
> says that there may be situations where it returns 0 even though the
> argument is a constant.

That's insane at best.

> Although none of the compiler/kernel combinations we have tried
> showed this happening, we think it is probably better to be safe
> than sorry.

So we would need something like:

   futex_copy_from_user()

which can be mapped to __copy_from_user_inatomic() first. Then go
through all architectures and the asm-generic stuff and provide the
specific variants which are guaranteed to use a 32bit access.

I really prefer that we don't have to do that and the compiler people
get their act together and fix that _builtin_constant_p() thingy.

Thanks,

	tglx

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