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

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Message-ID: <CACGkMEt-ewTqeHDMq847WDEGiW+x-TEPG6GTDDUbayVmuiVvzg@mail.gmail.com>
Date: Mon, 21 Apr 2025 11:45:32 +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 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
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