Re: [PATCH v2 2/3] lib: Fix length calculations in extract_kvec_to_sg

[Josh Law <objecting@objecting.org>]

On 24 March 2026 20:34:52 GMT, "Christian A. Ehrhardt" <lk@c--e.de> wrote:
>When extracting from a kvec to a scatterlist, do not
>cross page boundaries. The required length is already
>calculated but not used as intended.
>
>Adjust the copied length if the loop runs out auf
>sglist entries.

Nit: "auf" > of?? (Probably a typo)

>While there return immediately from extract_iter_to_sg
>if there are no sglist entries at all.

Suggestion: You could change it  to "While there**,** return immediately from extract_iter_to_sg if there are no sglist entries at all." It is something a bit "silly" so it doesn't matter much

>The changes to the kunit_iov_iter.c in the next commit
>demonstrate that the patch is necessary.
>
>Cc: David Howells <dhowells@redhat.com>
>Cc: Andrew Morton <akpm@linux-foundation.org>
>Cc: stable@vger.kernel.org # v6.5+
>Fixes: 018584697533 ("netfs: Add a function to extract an iterator into a scatterlist")
>Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
>---
> lib/scatterlist.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
>diff --git a/lib/scatterlist.c b/lib/scatterlist.c
>index d773720d11bf..befdc4b9c11d 100644
>--- a/lib/scatterlist.c
>+++ b/lib/scatterlist.c
>@@ -1247,7 +1247,7 @@ static ssize_t extract_kvec_to_sg(struct iov_iter *iter,
> 			else
> 				page = virt_to_page((void *)kaddr);
> 
>-			sg_set_page(sg, page, len, off);
>+			sg_set_page(sg, page, seg, off);
> 			sgtable->nents++;
> 			sg++;
> 			sg_max--;
>@@ -1256,6 +1256,7 @@ static ssize_t extract_kvec_to_sg(struct iov_iter *iter,
> 			kaddr += PAGE_SIZE;
> 			off = 0;
> 		} while (len > 0 && sg_max > 0);
>+		ret -= len;
> 
> 		if (maxsize <= 0 || sg_max == 0)
> 			break;
>@@ -1409,7 +1410,7 @@ ssize_t extract_iter_to_sg(struct iov_iter *iter, size_t maxsize,
> 			   struct sg_table *sgtable, unsigned int sg_max,
> 			   iov_iter_extraction_t extraction_flags)
> {
>-	if (maxsize == 0)
>+	if (maxsize == 0 || sg_max == 0)
> 		return 0;
> 
> 	switch (iov_iter_type(iter)) {


This C is absolutely perfect! No nits here


V/R 


Josh Law