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

[Hans Zuidam]

Hi Thomas,

On 9 okt. 2015, at 11:06, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
On Mon, 5 Oct 2015, Jaccon Bastiaansen wrote:
>> We did some tests with different compilers, kernel versions and kernel
>> configs, with the following results:

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

With kind regards,
Hans Zuidam

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