lib: stackinit: hide never-taken branch from compiler

The never-taken branch leads to an invalid bounds condition, which is by
design. To avoid the unwanted warning from the compiler, hide the
variable from the optimizer.

../lib/stackinit_kunit.c: In function 'do_nothing_u16_zero':
../lib/stackinit_kunit.c:51:49: error: array subscript 1 is outside array bounds of 'u16[0]' {aka 'short unsigned int[]'} [-Werror=array-bounds=]
   51 | #define DO_NOTHING_RETURN_SCALAR(ptr)           *(ptr)
      |                                                 ^~~~~~
../lib/stackinit_kunit.c:219:24: note: in expansion of macro 'DO_NOTHING_RETURN_SCALAR'
  219 |                 return DO_NOTHING_RETURN_ ## which(ptr + 1);    \
      |                        ^~~~~~~~~~~~~~~~~~

Link: https://lkml.kernel.org/r/20241117113813.work.735-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/lib/stackinit_kunit.c b/lib/stackinit_kunit.c
index c14c6f8e6308df7f7669b2f6ba1f275f85a315c3..c40818ec9c1801186f007595107f9faf41ca8c5f 100644 (file)
--- a/lib/stackinit_kunit.c
+++ b/lib/stackinit_kunit.c
@@ -212,6 +212,7 @@ static noinline void test_ ## name (struct kunit *test)             \
 static noinline DO_NOTHING_TYPE_ ## which(var_type)            \
 do_nothing_ ## name(var_type *ptr)                             \
 {                                                              \
+       OPTIMIZER_HIDE_VAR(ptr);                                \
        /* Will always be true, but compiler doesn't know. */   \
        if ((unsigned long)ptr > 0x2)                           \
                return DO_NOTHING_RETURN_ ## which(ptr);        \