On Mon, 15 May 2017, Will Deacon wrote:

Hi Jiri,

On Mon, May 15, 2017 at 03:07:42PM +0200, Jiri Slaby wrote:

quoted

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.

You wish. The constants are not used, but FUTEX_WAKE_OP _IS_ used by
glibc. They only have one argument it seems:

   #define FUTEX_OP_CLEAR_WAKE_IF_GT_ONE      ((4 << 24) | 1)

but I'm pretty sure that there is enough (probably horrible) code (think
java) out there using FUTEX_WAKE_OP for whatever (non)sensical reasons in
any available combination.

Thanks,

	tglx