On Sat, Jun 11, 2022 at 12:56:27PM +0000, Al Viro wrote:
> On Sat, Jun 11, 2022 at 12:37:44PM +0000, Al Viro wrote:
> > On Sat, Jun 11, 2022 at 12:12:47PM +0000, Al Viro wrote:
> >
> >
> > > At a guess, should be
> > > return min((size_t)nr * PAGE_SIZE - offset, maxsize);
> > >
> > > in both places. I'm more than half-asleep right now; could you verify that it
> > > (as the last lines of both iter_xarray_get_pages() and iter_xarray_get_pages_alloc())
> > > builds correctly?
> >
> > No, I'm misreading it - it's unsigned * unsigned long - unsigned vs. size_t.
> > On arm it ends up with unsigned long vs. unsigned int; functionally it *is*
> > OK (both have the same range there), but it triggers the tests. Try
> >
> > return min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
> >
> > there (both places).
>
> The reason we can't overflow on multiplication there, BTW, is that we have
> nr <= count, and count has come from weirdly open-coded
> DIV_ROUND_UP(size + offset, PAGE_SIZE)
That is often done to avoid possible overflows. Is size + offset
guaranteed to be smaller than ULONG_MAX ?
Guenter
> IMO we'd better make it explicit, so how about the following:
>
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index dda6d5f481c1..150dbd314d25 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1445,15 +1445,7 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
> offset = pos & ~PAGE_MASK;
> *_start_offset = offset;
>
> - count = 1;
> - if (size > PAGE_SIZE - offset) {
> - size -= PAGE_SIZE - offset;
> - count += size >> PAGE_SHIFT;
> - size &= ~PAGE_MASK;
> - if (size)
> - count++;
> - }
> -
> + count = DIV_ROUND_UP(size + offset, PAGE_SIZE);
> if (count > maxpages)
> count = maxpages;
>
> @@ -1461,7 +1453,7 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
> if (nr == 0)
> return 0;
>
> - return min(nr * PAGE_SIZE - offset, maxsize);
> + return min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
> }
>
> /* must be done on non-empty ITER_IOVEC one */
> @@ -1607,15 +1599,7 @@ static ssize_t iter_xarray_get_pages_alloc(struct iov_iter *i,
> offset = pos & ~PAGE_MASK;
> *_start_offset = offset;
>
> - count = 1;
> - if (size > PAGE_SIZE - offset) {
> - size -= PAGE_SIZE - offset;
> - count += size >> PAGE_SHIFT;
> - size &= ~PAGE_MASK;
> - if (size)
> - count++;
> - }
> -
> + count = DIV_ROUND_UP(size + offset, PAGE_SIZE);
> p = get_pages_array(count);
> if (!p)
> return -ENOMEM;
> @@ -1625,7 +1609,7 @@ static ssize_t iter_xarray_get_pages_alloc(struct iov_iter *i,
> if (nr == 0)
> return 0;
>
> - return min(nr * PAGE_SIZE - offset, maxsize);
> + return min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
> }
>
> ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
--
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