[PATCH 0/2] lib: scatterlist: fix sg_split() partial-coverage geometry + add KUnit tests

[Charles Pellegrini <c4ffein.work@gmail.com>]

sg_calculate_split() can decrement its nb_splits counter once too often
and miss the loop-termination check, corrupting the last output split's
geometry. This series fixes that and adds a KUnit suite.

Patch 1 is a one-line fix: the termination check tests `!nb_splits`, but
on partial coverage that ends mid-entry of a non-last input entry the
counter overshoots to -1, so the loop runs one iteration too many and
folds a trailing input entry into the last split (it ends up covering
more bytes than the caller requested). Widening the check to
`nb_splits <= 0` catches the overshoot.

Patch 2 adds a KUnit suite (16 cases) over the (input shape, skip, split
sizes) matrix: full / edge-aligned / mid-entry partial coverage, skip
variants, and multi-split runs. The mid-entry cases fail without patch 1
and pass with it.

Prior art / attribution. I hit this independently via property-based
testing and only afterwards found that Alexander Egorenkov had reported
it in 2021 [1], with an identical root-cause analysis. The independent
rediscovery and the matching diagnosis are good evidence this is a real
defect rather than a misreading of the partial-coverage contract. That
2021 posting went only to linux-kernel@vger, received no review, and was
never applied -- this file has no MAINTAINERS entry, which is likely why,
so I'm addressing the lib/ maintainer directly this time.

Egorenkov's proposed fix moved the termination check rather than widening
it, which fixes the mid-entry case but regresses edge-aligned partial
coverage (a split ending exactly on an input-entry boundary with further
entries trailing) -- a case the current code handles correctly. Test
sg_split_t10_edge_first_two_trailing in patch 2 fails his fix and passes
the one here, which is the concrete reason for choosing `nb_splits <= 0`
over the reorder. He is kept as Reported-by; the diagnosis is his.

Impact. No in-tree caller triggers this today. Four of the five callers
(SEC, pxa_camera, spi-omap2-mcspi, and sa2ul via its DMA-mapped re-split)
request full coverage; the one caller that requests partial coverage
(DTHEv2 AEAD) bounds its consumer by cryptlen, masking the wrong
geometry. The fix matters for callers that legitimately request partial
coverage, which sg_split() has documented as supported since it was added
("the union of spans of all resulting scatter lists is a subrange of the
span of the original scatter list").

Testing. Verified against the 16-case KUnit suite: 9 pass / 7 fail on
mainline, 16/16 with patch 1 applied. The seven failures are exactly the
mid-entry partial-coverage cases. The same 16 cases were first developed
and run as a standalone userspace harness against an extracted copy of
lib/sg_split.c, with identical results.

[1] https://lore.kernel.org/all/20210418143425.22944-1-egorenar-dev@posteo.net/

Charles Pellegrini (2):
  lib: scatterlist: fix sg_calculate_split() nb_splits overshoot on
    partial coverage
  lib: scatterlist: add KUnit tests for sg_split()

 lib/Kconfig               |  13 ++
 lib/Makefile              |   1 +
 lib/sg_split.c            |   2 +-
 lib/test_sg_split_kunit.c | 371 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 386 insertions(+), 1 deletion(-)
 create mode 100644 lib/test_sg_split_kunit.c

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2.47.3