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

[Al Viro <viro@zeniv.linux.org.uk>]

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)
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,