[RFC PATCH] fs: Add vfs_masks_device_ioctl*() helpers

[Arnd Bergmann]

On Mon, Feb 19, 2024, at 19:35, Mickaël Salaün wrote:
> vfs_masks_device_ioctl() and vfs_masks_device_ioctl_compat() are useful
> to differenciate between device driver IOCTL implementations and
> filesystem ones.  The goal is to be able to filter well-defined IOCTLs
> from per-device (i.e. namespaced) IOCTLs and control such access.
>
> Add a new ioctl_compat() helper, similar to vfs_ioctl(), to wrap
> compat_ioctl() calls and handle error conversions.

I'm still a bit confused by what your goal is here. I see
the code getting more complex but don't see the payoff in this
patch.

> Cc: Arnd Bergmann <arnd at arndb.de>
> Cc: Christian Brauner <brauner at kernel.org>
> Cc: Günther Noack <gnoack at google.com>

I assume the missing Signed-off-by is intentional while you are
still gathering feedback?

> +/*
> + * Safeguard to maintain a list of valid IOCTLs handled by 
> do_vfs_ioctl()
> + * instead of def_blk_fops or def_chr_fops (see init_special_inode).
> + */
> +__attribute_const__ bool vfs_masked_device_ioctl(const unsigned int 
> cmd)
> +{
> +	switch (cmd) {
> +	case FIOCLEX:
> +	case FIONCLEX:
> +	case FIONBIO:
> +	case FIOASYNC:
> +	case FIOQSIZE:
> +	case FIFREEZE:
> +	case FITHAW:
> +	case FS_IOC_FIEMAP:
> +	case FIGETBSZ:
> +	case FICLONE:
> +	case FICLONERANGE:
> +	case FIDEDUPERANGE:
> +	/* FIONREAD is forwarded to device implementations. */
> +	case FS_IOC_GETFLAGS:
> +	case FS_IOC_SETFLAGS:
> +	case FS_IOC_FSGETXATTR:
> +	case FS_IOC_FSSETXATTR:
> +	/* file_ioctl()'s IOCTLs are forwarded to device implementations. */
> +		return true;
> +	default:
> +		return false;
> +	}
> +}
> +EXPORT_SYMBOL(vfs_masked_device_ioctl);

It looks like this gets added into the hot path of every
ioctl command, which is not ideal, especially when this
no longer gets inlined into the caller.

> +	inode = file_inode(f.file);
> +	is_device = S_ISBLK(inode->i_mode) || S_ISCHR(inode->i_mode);
> +	if (is_device && !vfs_masked_device_ioctl(cmd)) {
> +		error = vfs_ioctl(f.file, cmd, arg);
> +		goto out;
> +	}

The S_ISBLK() || S_ISCHR() check here looks like it changes
behavior, at least for sockets. If that is intentional,
it should probably be a separate patch with a detailed
explanation.

>  	error = do_vfs_ioctl(f.file, fd, cmd, arg);
> -	if (error == -ENOIOCTLCMD)
> +	if (error == -ENOIOCTLCMD) {
> +		WARN_ON_ONCE(is_device);
>  		error = vfs_ioctl(f.file, cmd, arg);
> +	}

The WARN_ON_ONCE() looks like it can be triggered from
userspace, which is generally a bad idea.
 
> +extern __attribute_const__ bool vfs_masked_device_ioctl(const unsigned 
> int cmd);
> +#ifdef CONFIG_COMPAT
> +extern __attribute_const__ bool
> +vfs_masked_device_ioctl_compat(const unsigned int cmd);
> +#else /* CONFIG_COMPAT */
> +static inline __attribute_const__ bool
> +vfs_masked_device_ioctl_compat(const unsigned int cmd)
> +{
> +	return vfs_masked_device_ioctl(cmd);
> +}
> +#endif /* CONFIG_COMPAT */

I don't understand the purpose of the #else path here,
this should not be needed.

      Arnd