[PATCH] iov_iter: Fix iter_xarray_get_pages{,_alloc}()

[Jeff Layton]

On Thu, 2022-06-09 at 09:07 +0100, David Howells wrote:
> The maths at the end of iter_xarray_get_pages() to calculate the actual
> size doesn't work under some circumstances, such as when it's been asked to
> extract a partial single page.  Various terms of the equation cancel out
> and you end up with actual == offset.  The same issue exists in
> iter_xarray_get_pages_alloc().
> 
> Fix these to just use min() to select the lesser amount from between the
> amount of page content transcribed into the buffer, minus the offset, and
> the size limit specified.
> 
> This doesn't appear to have caused a problem yet upstream because network
> filesystems aren't getting the pages from an xarray iterator, but rather
> passing it directly to the socket, which just iterates over it.  Cachefiles
> *does* do DIO from one to/from ext4/xfs/btrfs/etc. but it always asks for
> whole pages to be written or read.
> 
> Fixes: 7ff5062079ef ("iov_iter: Add ITER_XARRAY")
> Reported-by: Jeff Layton <jlayton at kernel.org>
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: Alexander Viro <viro at zeniv.linux.org.uk>
> cc: Dominique Martinet <asmadeus at codewreck.org>
> cc: Mike Marshall <hubcap at omnibond.com>
> cc: Gao Xiang <xiang at kernel.org>
> cc: linux-afs at lists.infradead.org
> cc: v9fs-developer at lists.sourceforge.net
> cc: devel at lists.orangefs.org
> cc: linux-erofs at lists.ozlabs.org
> cc: linux-cachefs at redhat.com
> cc: linux-fsdevel at vger.kernel.org
> ---
> 
>  lib/iov_iter.c |   20 ++++----------------
>  1 file changed, 4 insertions(+), 16 deletions(-)
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 834e1e268eb6..814f65fd0c42 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1434,7 +1434,7 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
>  {
>  	unsigned nr, offset;
>  	pgoff_t index, count;
> -	size_t size = maxsize, actual;
> +	size_t size = maxsize;
>  	loff_t pos;
>  
>  	if (!size || !maxpages)
> @@ -1461,13 +1461,7 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
>  	if (nr == 0)
>  		return 0;
>  
> -	actual = PAGE_SIZE * nr;
> -	actual -= offset;
> -	if (nr == count && size > 0) {
> -		unsigned last_offset = (nr > 1) ? 0 : offset;
> -		actual -= PAGE_SIZE - (last_offset + size);
> -	}
> -	return actual;
> +	return min(nr * PAGE_SIZE - offset, maxsize);
>  }
>  
>  /* must be done on non-empty ITER_IOVEC one */
> @@ -1602,7 +1596,7 @@ static ssize_t iter_xarray_get_pages_alloc(struct iov_iter *i,
>  	struct page **p;
>  	unsigned nr, offset;
>  	pgoff_t index, count;
> -	size_t size = maxsize, actual;
> +	size_t size = maxsize;
>  	loff_t pos;
>  
>  	if (!size)
> @@ -1631,13 +1625,7 @@ static ssize_t iter_xarray_get_pages_alloc(struct iov_iter *i,
>  	if (nr == 0)
>  		return 0;
>  
> -	actual = PAGE_SIZE * nr;
> -	actual -= offset;
> -	if (nr == count && size > 0) {
> -		unsigned last_offset = (nr > 1) ? 0 : offset;
> -		actual -= PAGE_SIZE - (last_offset + size);
> -	}
> -	return actual;
> +	return min(nr * PAGE_SIZE - offset, maxsize);
>  }
>  
>  ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
> 
> 

This seems to fix the bug I was hitting. Thanks!

Reviewed-by: Jeff Layton <jlayton at kernel.org>
Tested-by: Jeff Layton <jlayton at kernel.org>