Re: [PATCH] kcsan: test: Adjust "expect" allocation type for kmalloc_obj

[Kees Cook <kees@kernel.org>]

On Tue, Feb 24, 2026 at 11:09:44AM +0100, Marco Elver wrote:
> On Mon, 23 Feb 2026 at 23:22, Kees Cook <kees@kernel.org> wrote:
> >
> > Instead of depending on the implicit case between a pointer to pointers
> > and pointer to arrays, use the assigned variable type for the allocation
> > type so they correctly match. Solves the following build error:
> >
> > ../kernel/kcsan/kcsan_test.c: In function '__report_matches':
> > ../kernel/kcsan/kcsan_test.c:171:16: error: assignment to 'char (*)[512]' from incompatible pointer type 'char (*)[3][512]'
> > [-Wincompatible-pointer-types]
> >   171 |         expect = kmalloc_obj(observed.lines);
> >       |                ^
> >
> > Tested with:
> >
> > $ ./tools/testing/kunit/kunit.py run \
> >         --kconfig_add CONFIG_DEBUG_KERNEL=y \
> >         --kconfig_add CONFIG_KCSAN=y \
> >         --kconfig_add CONFIG_KCSAN_KUNIT_TEST=y \
> >         --arch=x86_64 --qemu_args '-smp 2' kcsan
> >
> > Reported-by: Nathan Chancellor <nathan@kernel.org>
> > Fixes: 69050f8d6d07 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")
> > Signed-off-by: Kees Cook <kees@kernel.org>
> > ---
> > Cc: Marco Elver <elver@google.com>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> > Cc: <kasan-dev@googlegroups.com>
> > ---
> >  kernel/kcsan/kcsan_test.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
> > index 79e655ea4ca1..056fa859ad9a 100644
> > --- a/kernel/kcsan/kcsan_test.c
> > +++ b/kernel/kcsan/kcsan_test.c
> > @@ -168,7 +168,7 @@ static bool __report_matches(const struct expect_report *r)
> >         if (!report_available())
> >                 return false;
> >
> > -       expect = kmalloc_obj(observed.lines);
> > +       expect = kmalloc_obj(*expect);
> 
> This is wrong. Instead of allocating 3x512 bytes it's now only
> allocating 512 bytes, so we get OOB below with this change. 'expect'
> is a pointer to a 3-dimensional array of 512-char arrays (matching
> observed.lines).

Why did running the kunit test not trip over this? :(

Hmpf, getting arrays allocated without an explicit cast seems to be
impossible. How about this:


diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
index 056fa859ad9a..ae758150ccb9 100644
--- a/kernel/kcsan/kcsan_test.c
+++ b/kernel/kcsan/kcsan_test.c
@@ -168,7 +168,7 @@ static bool __report_matches(const struct expect_report *r)
 	if (!report_available())
 		return false;
 
-	expect = kmalloc_obj(*expect);
+	expect = (typeof(expect))kmalloc_obj(observed.lines);
 	if (WARN_ON(!expect))
 		return false;
 



-- 
Kees Cook