kcsan: test: Adjust "expect" allocation type for kmalloc_obj

The call to kmalloc_obj(observed.lines) returns "char (*)[3][512]", a pointer to the whole 2D array. But "expect" wants to be "char (*)[512]", the decayed pointer type, as if it were observed.lines itself (though without the "3" bounds). This produces 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);
| ^

Instead of changing the "expect" type to "char (*)[3][512]" and requiring a dereference at each use (e.g. "(expect*)[0]"), just explicitly cast the return to the desired type.

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 <[email protected]>

Fixes 69050f8d6d07 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")

Signed-off-by Kees Cook <[email protected]>

---
Cc: Marco Elver <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: <[email protected]>

---
 kernel/kcsan/kcsan_test.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

kernel/kcsan/kcsan_test.c +1 -2

index 79e655ea4ca1..ae758150ccb9 100644
@@ -168,7 +168,7 @@ static bool __report_matches(const struct expect_report *r)
        if (!report_available())
                return false;
 
-       expect = kmalloc_obj(observed.lines);
+       expect = (typeof(expect))kmalloc_obj(observed.lines);
        if (WARN_ON(!expect))
                return false;
 
--

2.34.1