[PATCH v1] fs: Fix inconsistent f_mode

[Al Viro]

On Sat, Mar 12, 2022 at 10:34:27AM +0900, Tetsuo Handa wrote:
> On 2022/03/12 7:15, Paul Moore wrote:
> > The silence on this has been deafening :/  No thoughts on fixing, or
> > not fixing OPEN_FMODE(), Al?
> 
> On 2022/03/01 19:15, Mickaël Salaün wrote:
> > 
> > On 01/03/2022 10:22, Christian Brauner wrote:
> >> That specific part seems a bit risky at first glance. Given that the
> >> patch referenced is from 2009 this means we've been allowing O_WRONLY |
> >> O_RDWR to succeed for almost 13 years now.
> >
> > Yeah, it's an old bug, but we should keep in mind that a file descriptor
> > created with such flags cannot be used to read nor write. However,
> > unfortunately, it can be used for things like ioctl, fstat, chdir… I
> > don't know if there is any user of this trick.
> 
> I got a reply from Al at https://lkml.kernel.org/r/20090212032821.GD28946@ZenIV.linux.org.uk
> that sys_open(path, 3) is for ioctls only. And I'm using this trick when opening something
> for ioctls only.

... so it's not just fdutils.  Cast in stone, IOW.