Re: [PATCH] block: Fix start and length check added to iov_iter_extract_bvecs()

[Keith Busch <kbusch@kernel.org>]

On Wed, Aug 26, 2026 at 08:24:05PM +0100, David Howells wrote:
> Keith Busch <kbusch@kernel.org> wrote:
> 
> > iov_iter_alignment loops over all the vectors when we only need to
> > examine the current one here.
> 
> Can the check be done earlier, then?

It used to be earlier, but the point was to reduce repeated iter
looping. It adds up, so I trying to co-locate validity checks with
places that have to iterate.

Would it be okay to special case the ubuf, iovec, and kvec types for the
simple check?

---
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 81e5c5e5121f7..b6b1e75352b0b 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1920,15 +1920,22 @@ ssize_t iov_iter_extract_bvecs(struct iov_iter *iter, struct bio_vec *bv,
 		unsigned short max_vecs, unsigned mem_align_mask,
 		iov_iter_extraction_t extraction_flags)
 {
-	unsigned long start = (unsigned long)iter_iov_addr(iter);
 	unsigned short entries_left = max_vecs - *nr_vecs;
 	unsigned short nr_pages, i = 0;
 	size_t left, offset, len;
 	struct page **pages;
 	ssize_t size;
 
-	if ((start | iter_iov_len(iter)) & mem_align_mask)
+	if (likely(iter_is_ubuf(iter) ||
+		   iter_is_iovec(iter) ||
+		   iov_iter_is_kvec(iter))) {
+		unsigned long start = (unsigned long)iter_iov_addr(iter);
+
+		if ((start | iter_iov_len(iter)) & mem_align_mask)
+			return -EINVAL;
+	} else if (iov_iter_alignment(iter) & mem_align_mask) {
 		return -EINVAL;
+	}
 
 	/*
 	 * Move page array up in the allocated memory for the bio vecs as far as
--