linux-hardening - [PATCH] lib: stackinit: Hide never-taken branch from compiler

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Message-Id: <20241117113813.work.735-kees@kernel.org>
Date: Sun, 17 Nov 2024 03:38:13 -0800
From: Kees Cook <kees@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH] 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);    \
      |                        ^~~~~~~~~~~~~~~~~~

Signed-off-by: Kees Cook <kees@...nel.org>
---
 lib/stackinit_kunit.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/stackinit_kunit.c b/lib/stackinit_kunit.c
index c14c6f8e6308..c40818ec9c18 100644
--- 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);	\
-- 
2.34.1