Re: [PATCH 04/29] vfs: strengthen checking of file range inputs to generic_remap_checks

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

On Wed, Oct 17, 2018 at 03:44:43PM -0700, Darrick J. Wong wrote:
> +static int generic_access_check_limits(struct file *file, loff_t pos,
> +				       loff_t *count)
> +{
> +	struct inode *inode = file->f_mapping->host;
> +
> +	/* Don't exceed the LFS limits. */
> +	if (unlikely(pos + *count > MAX_NON_LFS &&
> +				!(file->f_flags & O_LARGEFILE))) {
> +		if (pos >= MAX_NON_LFS)
> +			return -EFBIG;
> +		*count = min(*count, (loff_t)MAX_NON_LFS - pos);

	Can that can be different from MAX_NON_LFS - pos?

> +	}
> +
> +	/*
> +	 * Don't operate on ranges the page cache doesn't support.
> +	 *
> +	 * If we have written data it becomes a short write.  If we have
> +	 * exceeded without writing data we send a signal and return EFBIG.
> +	 * Linus frestrict idea will clean these up nicely..
> +	 */
> +	if (unlikely(pos >= inode->i_sb->s_maxbytes))
> +		return -EFBIG;
> +
> +	*count = min(*count, inode->i_sb->s_maxbytes - pos);
> +	return 0;
> +}

Anyway, I would rather do this here:

	struct inode *inode = file->f_mapping->host;
	loff_t max_size = inode->i_sb->s_maxbytes;

	if (!(file->f_flags & O_LARGEFILE))
		max_size = MAX_NON_LFS;

	if (unlikely(pos >= max_size))
		return -EFBIG;
	*count = min(*count, max_size - pos);
	return 0;