Re: [PATCH v3 2/9] iov_iter: Add a general purpose iteration function

[Al Viro]

On Fri, May 27, 2022 at 11:44:09AM +0100, David Howells wrote:
> Add a function, iov_iter_scan(), to iterate over the buffers described by
> an I/O iterator, kmapping and passing each contiguous chunk the supplied
> scanner function in turn, up to the requested amount of data or until the
> scanner function returns an error.
> 
> This can be used, for example, to hash all the data in an iterator by
> having the scanner function call the appropriate crypto update function.

> +ssize_t iov_iter_scan(struct iov_iter *i, size_t bytes,
> +		      ssize_t (*scanner)(struct iov_iter *i, const void *p,
> +					 size_t len, size_t off, void *priv),
> +		      void *priv)
> +{
> +	ssize_t ret = 0, scanned = 0;
> +
> +	if (!bytes)
> +		return 0;
> +	if (iter_is_iovec(i))
> +		might_fault();
> +
> +	iterate_and_advance(
> +		i, bytes, base, len, off, ({
> +				ret = scanner(i, base, len, off, priv);
> +				if (ret < 0)
> +					break;
> +				scanned += ret;
> +			}), ({
> +				ret = scanner(i, base, len, off, priv);
> +				if (ret < 0)
> +					break;
> +				scanned += ret;
> +			})
> +	);
> +	return ret < 0 ? ret : scanned;
> +}

Have you even tried to run sparse on that?  How could that possibly work?
You are feeding the same callback both userland and kernel pointers;
that makes no sense.

NAK.