linux-kernel - Re: [PATCH v7 8/8] vhost: Add a KConfig knob to enable IOCTL VHOST_FORK_FROM

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Message-ID: <svi5ui3ea55mor5cav7jirrttd6lkv4xkjnjj57tnjdyiwmr5c@p2hhfwuokyv5>
Date: Mon, 3 Mar 2025 10:12:06 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Cindy Lu <lulu@...hat.com>, mst@...hat.com, 
	michael.christie@...cle.com, linux-kernel@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH v7 8/8] vhost: Add a KConfig knob to enable IOCTL
 VHOST_FORK_FROM_OWNER

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@...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.
>>
>> Signed-off-by: Cindy Lu <lulu@...hat.com>
>> ---
>>  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;
>>         }
>> +

nit: this empyt line is not needed

>> +#else
>> +       if (ioctl == VHOST_FORK_FROM_OWNER) {
>> +               /* When CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL is 'n', return error */
>> +               r = -ENOTTY;
>> +               goto done;
>> +       }
>> +#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'm not sure about this honestly, the user space has no way to figure 
out the default value and still has to do the IOCTL.
So IMHO better to have a default value that is independent of the kernel 
configuration and consistent with the current behavior.

Thanks,
Stefano

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