linux-kernel - Re: linux interprets an fcntl int arg as long

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Date:   Tue, 1 Nov 2022 13:12:39 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Szabolcs Nagy' <szabolcs.nagy@....com>,
        Theodore Ts'o <tytso@....edu>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: linux interprets an fcntl int arg as long

On Tue, Nov 01, 2022 at 12:19:51PM +0000, David Laight wrote:
> From: 'Szabolcs Nagy' <szabolcs.nagy@....com>
> > Sent: 01 November 2022 11:45
> > 
> > The 11/01/2022 10:02, David Laight wrote:
> > > From: Szabolcs Nagy

> > kernel code:
> > ------------
> > SYSCALL_DEFINE3(fcntl, unsigned int, fd, unsigned int, cmd, unsigned long, arg)
> > {
> 
> That is just a wrapper and calls do_fcntl().
> which needs changing to be add:
> 	arg &= ~0U;
> before the switch(cmd) {

Just to check, do you mean the switch in do_fcntl(), or the switch within memfd_fcntl() ?

The former handles other APIs which do expect arg to be a long (e.g.
F_SET_RW_HINT and F_GET_RW_HINT expect it to hold a full 64-bit pointer), so
that'd break things.

The latter would work (as would casting arg to int when calling
memfd_add_seals()).

Mark.