lib: stackinit: hide never-taken branch from compiler

[ Upstream commit 5c3793604f91123bf49bc792ce697a0bef4c173c ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/lib/test_stackinit.c b/lib/test_stackinit.c
index a3c74e6a21ffafc648ead9db6aa85f0053dd376e..56653c6a2a6152df506918d9634f408923225d90 100644 (file)
--- a/lib/test_stackinit.c
+++ b/lib/test_stackinit.c
@@ -259,6 +259,7 @@ static noinline __init int test_ ## name (void)                     \
 static noinline __init 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);        \