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

[Stefano Garzarella]

On Mon, Mar 03, 2025 at 01:52:06PM +0800, Jason Wang wrote:
On Sun, Mar 2, 2025 at 10:34 PM Cindy Lu <l...@redhat.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 <l...@redhat.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