Re: mainline build failure due to 6c77676645ad ("iov_iter: Fix iter_xarray_get_pages{,_alloc}()")

[Guenter Roeck <linux@roeck-us.net>]

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@zeniv.linux.org.uk>
> ---
> 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,