Re: [PATCH v2 01/18] vfs: add miscattr ops

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Mar 22, 2021 at 03:48:59PM +0100, Miklos Szeredi wrote:

minor nit: copy_fsxattr_{to,from}_user() might be better.

> +int fsxattr_copy_to_user(const struct miscattr *ma, struct fsxattr __user *ufa)
> +{
> +	struct fsxattr fa = {
> +		.fsx_xflags	= ma->fsx_xflags,
> +		.fsx_extsize	= ma->fsx_extsize,
> +		.fsx_nextents	= ma->fsx_nextents,
> +		.fsx_projid	= ma->fsx_projid,
> +		.fsx_cowextsize	= ma->fsx_cowextsize,
> +	};

That wants a comment along the lines of "guaranteed to be gap-free",
since otherwise you'd need memset() to avoid an infoleak.

> +static int ioctl_getflags(struct file *file, void __user *argp)
> +{
> +	struct miscattr ma = { .flags_valid = true }; /* hint only */
> +	unsigned int flags;
> +	int err;
> +
> +	err = vfs_miscattr_get(file_dentry(file), &ma);

Umm...  Just to clarify - do we plan to have that ever called via
ovl_real_ioctl()?  IOW, is file_dentry() anything other than a way
to spell ->f_path.dentry here?

> +struct miscattr {
> +	u32	flags;		/* flags (FS_IOC_GETFLAGS/FS_IOC_SETFLAGS) */
> +	/* struct fsxattr: */
> +	u32	fsx_xflags;	/* xflags field value (get/set) */
> +	u32	fsx_extsize;	/* extsize field value (get/set)*/
> +	u32	fsx_nextents;	/* nextents field value (get)	*/
> +	u32	fsx_projid;	/* project identifier (get/set) */
> +	u32	fsx_cowextsize;	/* CoW extsize field value (get/set)*/
> +	/* selectors: */
> +	bool	flags_valid:1;
> +	bool	xattr_valid:1;
> +};

OK as long as it stays kernel-only, but if we ever expose that to userland, we'd
better remember to turn the last two into an u32 with explicit bitmasks.