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@xxxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> cc: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
> cc: Mike Marshall <hubcap@xxxxxxxxxxxx>
> cc: Gao Xiang <xiang@xxxxxxxxxx>
> cc: linux-afs@xxxxxxxxxxxxxxxxxxx
> cc: v9fs-developer@xxxxxxxxxxxxxxxxxxxxx
> cc: devel@xxxxxxxxxxxxxxxxxx
> cc: linux-erofs@xxxxxxxxxxxxxxxx
> cc: linux-cachefs@xxxxxxxxxx
> cc: linux-fsdevel@xxxxxxxxxxxxxxx
> ---
>
> 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@xxxxxxxxxx>
Tested-by: Jeff Layton <jlayton@xxxxxxxxxx>
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