linux-kernel - Re: [PATCH v2 05/14] futex: Add sys_futex

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Date:   Thu, 10 Aug 2023 14:13:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     André Almeida <andrealmeid@...lia.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        dvhart@...radead.org, dave@...olabs.net, tglx@...utronix.de,
        axboe@...nel.dk, 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, Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v2 05/14] futex: Add sys_futex_wake()

On Wed, Aug 09, 2023 at 07:25:19PM -0300, André Almeida wrote:
> Hi Peter,
> 
> Em 07/08/2023 09:18, Peter Zijlstra escreveu:
> > To complement sys_futex_waitv() add sys_futex_wake(). This syscall
> > implements what was previously known as FUTEX_WAKE_BITSET except it
> > uses 'unsigned long' for the bitmask and takes FUTEX2 flags.
> > 
> > The 'unsigned long' allows FUTEX2_SIZE_U64 on 64bit platforms.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Acked-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> > ---
> 
> [...]
> 
> > +/*
> > + * sys_futex_wake - Wake a number of futexes
> > + * @uaddr:	Address of the futex(es) to wake
> > + * @mask:	bitmask
> > + * @nr:		Number of the futexes to wake
> > + * @flags:	FUTEX2 flags
> > + *
> > + * Identical to the traditional FUTEX_WAKE_BITSET op, except it is part of the
> > + * futex2 family of calls.
> > + */
> > +
> > +SYSCALL_DEFINE4(futex_wake,
> > +		void __user *, uaddr,
> > +		unsigned long, mask,
> > +		int, nr,
> > +		unsigned int, flags)
> > +{
> 
> Do you think we could have a
> 
> 	if (!nr)
> 		return 0;
> 
> here? Otherwise, calling futex_wake(&f, 0, flags) will wake 1 futex (if
> available), which is a strange undocumented behavior in my opinion.

Oh 'cute' that.. yeah, but how about I put it ...

> > +	if (flags & ~FUTEX2_VALID_MASK)
> > +		return -EINVAL;
> > +
> > +	flags = futex2_to_flags(flags);
> > +	if (!futex_flags_valid(flags))
> > +		return -EINVAL;
> > +
> > +	if (!futex_validate_input(flags, mask))
> > +		return -EINVAL;

here, because otherwise we get:

	sys_futex_wake(&f, 0xFFFF, 0, FUTEX2_SIZE_U8)

to return 0, even though that is 'obviously' nonsensical and should
return -EINVAL. Or even garbage flags would be 'accepted'.

(because 0xFFFF is larger than U8 can accomodate)

> > +
> > +	return futex_wake(uaddr, flags, nr, mask);
> > +}