linux-kernel - Re: [PATCH v9 4/4] vhost: Add a KConfig knob to enable IOCTL VHOST_FORK_FROM

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Message-ID: <CACGkMEte6Lobr+tFM9ZmrDWYOpMtN6Xy=rzvTy=YxSPkHaVdPA@mail.gmail.com>
Date: Mon, 21 Apr 2025 11:46:04 +0800
From: Jason Wang <jasowang@...hat.com>
To: Cindy Lu <lulu@...hat.com>
Cc: mst@...hat.com, michael.christie@...cle.com, sgarzare@...hat.com, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v9 4/4] vhost: Add a KConfig knob to enable IOCTL VHOST_FORK_FROM_OWNER

On Mon, Apr 21, 2025 at 11:45 AM Jason Wang <jasowang@...hat.com> wrote:
>
> On Mon, Apr 21, 2025 at 10:45 AM Cindy Lu <lulu@...hat.com> 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.
>
> I think we need to describe why the default value was chosen to be false.
>
> What's more, should we document the implications here?
>
> inherit_owner was set to false: this means "legacy" userspace may

I meant "true" actually.

> still be broken unless it can do VHOST_FORK_FROM_OWNER which is
> impractical. Does this defeat the purpose of this series actually?
>
> >
> > Signed-off-by: Cindy Lu <lulu@...hat.com>
> > ---
> >  drivers/vhost/Kconfig | 15 +++++++++++++++
> >  drivers/vhost/vhost.c |  3 +++
> >  2 files changed, 18 insertions(+)
> >
> > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > index 020d4fbb947c..bc8fadb06f98 100644
> > --- a/drivers/vhost/Kconfig
> > +++ b/drivers/vhost/Kconfig
> > @@ -96,3 +96,18 @@ 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..568e43cb54a9 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 +2315,7 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> >                 r = 0;
> >                 goto done;
> >         }
> > +#endif
> >         /* You must be the owner to do anything else */
> >         r = vhost_dev_check_owner(d);
> >         if (r)
> > --
> > 2.45.0
> >
>
> Thanks

Thanks