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

[Kees Cook <kees@kernel.org>]

On Tue, Feb 24, 2026 at 01:48:51PM -0800, Kees Cook wrote:
> 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;

Or:

	expect = kmalloc_objs(*observed.lines, ARRAY_SIZE(observed.lines));

I think the quoted cast is probably better...

-- 
Kees Cook