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

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Date:   Tue, 1 Aug 2023 00:59:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org, mingo@...hat.com,
        dvhart@...radead.org, dave@...olabs.net, andrealmeid@...lia.com,
        Andrew Morton <akpm@...ux-foundation.org>, urezki@...il.com,
        hch@...radead.org, lstoakes@...il.com,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        malteskarupke@....de
Subject: Re: [PATCH v1 02/14] futex: Extend the FUTEX2 flags

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