[PATCH v3 0/5] Fix bugs in extract_iter_to_sg()

[Christian A. Ehrhardt]

Fix bugs in the kvec and user variants of extract_iter_to_sg.
This series is growing due to useful remarks made by sashiko.dev.

The main bugs are:
- The length for an sglist entry when extracting from
  a kvec can exceed the number of bytes in the page. This
  is obviously not intended.
- When extracting a user buffer the sglist is temporarily
  used as a scratch buffer for extracted page pointers.
  If the sglist already contains some elements this scratch
  buffer could overlap with existing entries in the sglist.

The series adds test cases to the kunit_iov_iter test that
demonstrate all of these bugs. Additionally, there is a
memory leak fix for the test itself.

The bugs were orignally introduced into kernel v6.3 where the
function lived in fs/netfs/iterator.c. It was later moved
to lib/scatterlist.c in v6.5. Thus the actual fix is only
marked for backports to v6.5+.

---
Changes in v2:
Addresss valid issues raised by AI review
https://sashiko.dev/#/patchset/20260323212350.807118-1-lk@xxxxxxx:
- Add kunit assertions for OOM conditions in the test
- Reorder commits.
- Fix sg_max == 0 case.
- Fix return value if we run out of sg entries.
- Adjust tests to catch these cases, too.

Changes in v3:
- Add fix and test for extract_user_to_sg()
- Fix memory leak in the unit test.
- Re-order commits: Code fixes first, then tests.
---

Christian A. Ehrhardt (5):
  lib/scatterlist: Fix length calculations in extract_kvec_to_sg
  lib/scatterlist: Fix temp buffer in extract_user_to_sg()
  lib: kunit_iov_iter: Fix memory leaks
  lib: kunit_iov_iter: Improve error detection
  lib: kunit_iov_iter: Add tests for extract_iter_to_sg

 lib/scatterlist.c          |   8 +-
 lib/tests/kunit_iov_iter.c | 224 +++++++++++++++++++++++++++++++++++--
 2 files changed, 221 insertions(+), 11 deletions(-)

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2.43.0