linux-kernel - Re: [PATCH RESEND V12 2/8] fuse: 32-bit user space ioctl compat for fuse device

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Date:   Mon, 1 Mar 2021 12:26:26 +0000
From:   Alessio Balsini <balsini@...roid.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Alessio Balsini <balsini@...roid.com>,
        Akilesh Kailash <akailash@...gle.com>,
        Amir Goldstein <amir73il@...il.com>,
        Antonio SJ Musumeci <trapexit@...wn.link>,
        David Anderson <dvander@...gle.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Martijn Coenen <maco@...roid.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Lawrence <paullawrence@...gle.com>,
        Peng Tao <bergwolf@...il.com>,
        Stefano Duo <duostefano93@...il.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>, wuyan <wu-yan@....com>,
        fuse-devel <fuse-devel@...ts.sourceforge.net>,
        kernel-team <kernel-team@...roid.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND V12 2/8] fuse: 32-bit user space ioctl compat for
 fuse device

On Wed, Feb 17, 2021 at 11:21:04AM +0100, Miklos Szeredi wrote:
> On Mon, Jan 25, 2021 at 4:31 PM Alessio Balsini <balsini@...roid.com> wrote:
> >
> > With a 64-bit kernel build the FUSE device cannot handle ioctl requests
> > coming from 32-bit user space.
> > This is due to the ioctl command translation that generates different
> > command identifiers that thus cannot be used for direct comparisons
> > without proper manipulation.
> >
> > Explicitly extract type and number from the ioctl command to enable
> > 32-bit user space compatibility on 64-bit kernel builds.
> >
> > Signed-off-by: Alessio Balsini <balsini@...roid.com>
> > ---
> >  fs/fuse/dev.c             | 29 ++++++++++++++++++-----------
> >  include/uapi/linux/fuse.h |  3 ++-
> >  2 files changed, 20 insertions(+), 12 deletions(-)
> >
> > diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> > index 588f8d1240aa..ff9f3b83f879 100644
> > --- a/fs/fuse/dev.c
> > +++ b/fs/fuse/dev.c
> > @@ -2233,37 +2233,44 @@ static int fuse_device_clone(struct fuse_conn *fc, struct file *new)
> >  static long fuse_dev_ioctl(struct file *file, unsigned int cmd,
> >                            unsigned long arg)
> >  {
> > -       int err = -ENOTTY;
> > +       int res;
> > +       int oldfd;
> > +       struct fuse_dev *fud = NULL;
> >
> > -       if (cmd == FUSE_DEV_IOC_CLONE) {
> > -               int oldfd;
> > +       if (_IOC_TYPE(cmd) != FUSE_DEV_IOC_MAGIC)
> > +               return -EINVAL;
> 
> Why change ENOTTY to EINVAL?
> 
> Thanks,
> MIklos

For the magic number mismatch I was thinking that EINVAL would have been
more appropriate, meaning: "this ioctl is definitely something this file
doesn't support".  ENOTTY, could be more specific to the subset of
ioctls supported by the file, so I kept this in the default case of the
switch.
After counting the use of ENOTTY vs EINVAL for these _IOC_TYPE checks,
EINVALs were less than half ENOTTYs, so I'm totally fine with switching
back to ENOTTY in V13.

Thanks!
Alessio