Subject: [PATCH 1/1] futex: remove duplicated code

From: [email-masked] (Will Deacon)
Date: Mon, 15 May 2017 14:16:45 +0100

Hi Jiri,

On Mon, May 15, 2017 at 03:07:42PM +0200, Jiri Slaby wrote:
> There is code duplicated over all architecture's headers for
> futex_atomic_op_inuser. Namely op decoding, access_ok check for uaddr,
> and comparison of the result.
> 
> Remove this duplication and leave up to the arches only the needed
> assembly which is now in arch_futex_atomic_op_inuser.
> 
> Note that s390 removed access_ok check in d12a29703 ("s390/uaccess:
> remove pointless access_ok() checks") as access_ok there returns true.
> We introduce it back to the helper for the sake of simplicity (it gets
> optimized away anyway).

Whilst I think this is a good idea, the code in question actually results
in undefined behaviour per the C spec and is reported by UBSAN. See my
patch fixing arm64 here (which I'd forgotten about):

https://www.spinics.net/lists/linux-arch/msg38564.html

But, as stated in the thread above, I think we should go a step further
and remove FUTEX_OP_{OR,ANDN,XOR,OPARG_SHIFT} altogether. They don't
appear to be used by userspace, and this whole thing is a total mess.

Any thoughts?

Will