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Date:   Thu, 13 Dec 2018 12:34:44 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Goldwyn Rodrigues <rgoldwyn@suse.de>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Amir Goldstein <amir73il@gmail.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
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Subject: Re: [PATCH] fs: Evaluate O_WRONLY | O_RDWR to O_RDWR
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On Thu, Dec 13, 2018 at 01:16:45PM +0100, Dmitry Vyukov wrote:
> On Thu, Dec 13, 2018 at 1:10 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Thu, Dec 13, 2018 at 05:57:17AM -0600, Goldwyn Rodrigues wrote:
> > > A user can open(O_WRONLY | O_RDWR) and the options are valid.
> > > However, OPEN_FMODE() evaluates both FMODE_READ and FMODE_WRITE,
> > > as negative. We also need to protect the lower layers from this
> > > anomaly.
> > >
> > > Solve it by dropping O_WRONLY, so O_RDWR takes precedence.
> >
> > Congratulations, you've broken fdutils...  Passing 3 in lower bits
> > of open() flags is *not* the same as O_RDWR; behaiviour is
> > different and deliberately chosen by existing userland code.
> >
> > IOW, NAK.
> 
> I am eager to hear what it means then.

Open for ioctls, basically.  No read/write allowed, no checks for
media writability, etc. done at open() time, both read and write
*permissions* required from device node.

Think of the things like formatting a floppy, for example...