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.
