Re: [PATCH v1 02/14] futex: Extend the FUTEX2 flags

On Tue, Aug 01, 2023 at 12:43:24AM +0200, Thomas Gleixner wrote:

quoted

Also, sys_futex_{wake,wait}() have this thing as a syscall argument,
surely you don't want to put this union there as well?

Why not? The anon union does not break the ABI unless I'm missing
something. Existing user space can still use 'flags' and people who care
about readability can use the bitfield, no?

Its inside struct futex_waitv and not an explicit syscall argument, right?

Nope, see patches 5 and 6, they introduce:

sys_futex_wake(void __user *uaddr, unsigned long mask, int nr, unsigned int flags);
sys_futex_wait(void __user *uaddr, unsigned long val,
               unsigned long mask, unsigned int flags,
	       struct __kernel_timespec __user *timeout, clockid_t clockid);

Using a union, would turn that into:

sys_futex_wake(void __user *uaddr, unsigned long mask, int nr, union futex_flags flags);
sys_futex_wait(void __user *uaddr, unsigned long val,
               unsigned long mask, union futex_flags flags,
	       struct __kernel_timespec __user *timeout, clockid_t clockid);

Which then gets people to write garbage like:

	futex_wake(add, 0xFFFF, 1, (union futex_flags){ .flags = FUTEX2_SIZE_U16 | FUTEX2_PRIVATE));
or
	futex_wake(add, 0xFFFF, 1, (union futex_flags){ .size = FUTEX2_SIZE_U16, private = true, ));

You really want that ?