Re: [RFC]: Possible race condition in kernel futex code

[Thomas Gleixner <tglx@linutronix.de>]

Hans,

On Fri, 9 Oct 2015, Hans Zuidam wrote:
> On 9 okt. 2015, at 11:06, Thomas Gleixner <tglx@linutronix.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