nfc: virtual_ncidev: Add missing ioctl compat handler

The compat handler for ioctls to the virtual nci device is missing. So,
nci-specific ioctls of a compat task return with -1 and errno set to
ENOTTY. Add a handler.

The handling of an ioctl() call of a compat task to get the index of
virtual nci device (IOCTL_GET_NCIDEV_IDX) lands in the default case of
the ioctl compat handler (see fs/ioctl.c):

COMPAT_SYSCALL_DEFINE3(ioctl, ...)
{
	...
	default:
       		error = do_vfs_ioctl(fd_file(f), fd, cmd, ...);
		if (error != -ENOIOCTLCMD)
			break;

		if (fd_file(f)->f_op->compat_ioctl)
			error = fd_file(f)->f_op->compat_ioctl(fd_file(f), cmd, arg);
		if (error == -ENOIOCTLCMD)
			error = -ENOTTY;
	...
}

There, do_vfs_ioctl() returns -ENOIOCTLCMD and compat_ioctl is not
set for virtual_ncidev_fops, i.e. f_op->compat_ioctl == NULL. So, the
ioctl() syscall returns with -1 and errno set to ENOTTY to the compat
task.

To fix this, use the compat_ptr_ioctl helper for compat handling here.
It shall be used for ioctls that "either ignore the argument or pass a
pointer to a compatible data type". The driver's sole ioctl takes a user
void pointer and copies nfc_dev->idx to it, a 4-byte integer across all
ABIs.

This issue has been found by running the nci_dev kernel selftest as
rv64 binary on top of a CHERI kernel, where the ioctl() ends up in
the ioctl compat handler, similar to a 32-bit application on top of a
64-bit kernel.

Fixes: e624e6c3e7 ("nfc: Add a virtual nci device driver")
Signed-off-by: Chris Gellermann <christian.gellermann@codasip.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260904164252.18351-1-christian.gellermann@codasip.com
Signed-off-by: David Heidelberg <david@ixit.cz>

drivers/nfc/virtual_ncidev.c

@@ -195,7 +195,8 @@ static const struct file_operations virtual_ncidev_fops = {
 	.write = virtual_ncidev_write,
 	.open = virtual_ncidev_open,
 	.release = virtual_ncidev_close,
-	.unlocked_ioctl = virtual_ncidev_ioctl
+	.unlocked_ioctl = virtual_ncidev_ioctl,
+	.compat_ioctl = compat_ptr_ioctl,
 };
 
 static struct miscdevice miscdev = {