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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 31, 2023 at 11:14:11PM +0200, Thomas Gleixner wrote:
> On Mon, Jul 31 2023 at 21:20, Peter Zijlstra wrote:
> > -#define FUTEX2_MASK (FUTEX2_64 | FUTEX2_PRIVATE)
> > +#define FUTEX2_MASK (FUTEX2_SIZE_MASK | FUTEX2_PRIVATE)
> 
> Along with some comment which documents that the size "flags" constitute
> a number field and not flags in the sense of binary flags.
> 
> And please name these size constants so it really becomes obvious:
> 
> #define FUTEX2_SIZE_U32		2

So you want them named:

#define FUTEX2_SIZE_U8		0x00
#define FUTEX2_SIZE_U16		0x01
#define FUTEX2_SIZE_U32		0x02
#define FUTEX2_SIZE_U64		0x03

#define FUTEX2_SIZE_MASK	0x03

Sure, can do.

> >  /**
> >   * futex_parse_waitv - Parse a waitv array from userspace
> > @@ -208,11 +208,11 @@ static int futex_parse_waitv(struct fute
> >  			return -EINVAL;
> >  
> >  		if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall()) {
> > -			if ((aux.flags & FUTEX2_64) == FUTEX2_64)
> > +			if ((aux.flags & FUTEX2_SIZE_MASK) == FUTEX2_64)
> >  				return -EINVAL;
> >  		}
> 
> That should be part of the actual 64bit futex enablement, no?

The 'unsigned long' thing is part of the syscalls, which is why I had it
now.

>   
> > -		if ((aux.flags & FUTEX2_64) != FUTEX2_32)
> > +		if ((aux.flags & FUTEX2_SIZE_MASK) != FUTEX2_32)
> >  			return -EINVAL;
> 
> In hindsight I think it was as mistake just to have this __u32 flags
> field in the new interface. Soemthing like the incomplete below might be
> retrofittable, no?
> 
> --- a/include/uapi/linux/futex.h
> +++ b/include/uapi/linux/futex.h
> @@ -74,7 +74,12 @@
>  struct futex_waitv {
>  	__u64 val;
>  	__u64 uaddr;
> -	__u32 flags;
> +	union {
> +		__u32	flags;
> +		__u32	size	: 2,
> +				: 5,
> +			private	: 1;
> +	};
>  	__u32 __reserved;
>  };

Durr, I'm not sure I remember if that does the right thing across
architectures -- might just work. But I'm fairly sure this isn't the
only case of a field in a flags thing in our APIs. Although obviously I
can't find another case in a hurry :/

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

I'd much prefer to just keep the 'unsigned int flags' thing and perhaps
put a comment on-top of the '#define FUTEX2_*' thingies. Note that
having it a field instead of a bunch of flags makes sense, since you can
only have a single size, not a combination of sizes.