Re: [PATCH v7 8/8] vhost: Add a KConfig knob to enable IOCTL VHOST_FORK_FROM_OWNER

[Jason Wang]

On Tue, Mar 4, 2025 at 1:33 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Mar 03, 2025 at 01:52:06PM +0800, Jason Wang wrote:
> > On Sun, Mar 2, 2025 at 10:34 PM Cindy Lu <lulu@xxxxxxxxxx> wrote:
> > >
> > > Introduce a new config knob `CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL`,
> > > to control the availability of the `VHOST_FORK_FROM_OWNER` ioctl.
> > > When CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL is set to n, the ioctl
> > > is disabled, and any attempt to use it will result in failure.
> > >
> > > Signed-off-by: Cindy Lu <lulu@xxxxxxxxxx>
> > > ---
> > >  drivers/vhost/Kconfig | 15 +++++++++++++++
> > >  drivers/vhost/vhost.c | 11 +++++++++++
> > >  2 files changed, 26 insertions(+)
> > >
> > > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > > index b455d9ab6f3d..e5b9dcbf31b6 100644
> > > --- a/drivers/vhost/Kconfig
> > > +++ b/drivers/vhost/Kconfig
> > > @@ -95,3 +95,18 @@ config VHOST_CROSS_ENDIAN_LEGACY
> > >           If unsure, say "N".
> > >
> > >  endif
> > > +
> > > +config VHOST_ENABLE_FORK_OWNER_IOCTL
> > > +       bool "Enable IOCTL VHOST_FORK_FROM_OWNER"
> > > +       default n
> > > +       help
> > > +         This option enables the IOCTL VHOST_FORK_FROM_OWNER, which allows
> > > +         userspace applications to modify the thread mode for vhost devices.
> > > +
> > > +          By default, `CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL` is set to `n`,
> > > +          meaning the ioctl is disabled and any operation using this ioctl
> > > +          will fail.
> > > +          When the configuration is enabled (y), the ioctl becomes
> > > +          available, allowing users to set the mode if needed.
> > > +
> > > +         If unsure, say "N".
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index fb0c7fb43f78..09e5e44dc516 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -2294,6 +2294,8 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> > >                 r = vhost_dev_set_owner(d);
> > >                 goto done;
> > >         }
> > > +
> > > +#ifdef CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL
> > >         if (ioctl == VHOST_FORK_FROM_OWNER) {
> > >                 u8 inherit_owner;
> > >                 /*inherit_owner can only be modified before owner is set*/
> > > @@ -2313,6 +2315,15 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> > >                 r = 0;
> > >                 goto done;
> > >         }
> > > +
> > > +#else
> > > +       if (ioctl == VHOST_FORK_FROM_OWNER) {
> > > +               /* When CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL is 'n', return error */
> > > +               r = -ENOTTY;
> > > +               goto done;
> > > +       }
>
> why do we need this? won't it fail as any other unsupported ioctl?
>
> > > +#endif
> > > +
> > >         /* You must be the owner to do anything else */
> > >         r = vhost_dev_check_owner(d);
> > >         if (r)
> > > --
> > > 2.45.0
> >
> > Do we need to change the default value of the inhert_owner? For example:
> >
> > #ifdef CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL
> > inherit_owner = false;
> > #else
> > inherit_onwer = true;
> > #endif
> >
> > ?
>
> I feel it is best to keep the default consistent.

Just want to make sure we are on the same page.

For "default", did you mean inherit_owner = false which is consistent
with behaviour without the vhost task?

Or inherit_onwer = true, then the new ioctl to make it false is
useless. And if legacy applications want kthread behaviour it needs to
be patched which seems self-contradictory.

> All the kconfig should do, is block the ioctl.
>

Thanks

>
> > Other patches look good to me.
> >
> > Thanks
> >
> > >
>