linux-hardening - [PATCH] kunit/fortify: Add back "volatile" for sizeof() constants

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Message-Id: <20250628234034.work.800-kees@kernel.org>
Date: Sat, 28 Jun 2025 16:40:38 -0700
From: Kees Cook <kees@...nel.org>
To: Jannik Glückert <jannik.glueckert@...il.com>,
	Nathan Chancellor <nathan@...nel.org>
Cc: Kees Cook <kees@...nel.org>,
	linux-hardening@...r.kernel.org,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: [PATCH] kunit/fortify: Add back "volatile" for sizeof() constants

It seems the Clang can see through OPTIMIZER_HIDE_VAR when the constant
is coming from sizeof. Adding "volatile" back to these variables solves
this false positive without reintroducing the issues that originally led
to switching to OPTIMIZER_HIDE_VAR in the first place[1].

Reported-by: Nathan Chancellor <nathan@...nel.org>
Closes: https://github.com/ClangBuiltLinux/linux/issues/2075 [1]
Cc: "Jannik Glückert" <jannik.glueckert@...il.com>
Suggested-by: Nathan Chancellor <nathan@...nel.org>
Fixes: 6ee149f61bcc ("kunit/fortify: Replace "volatile" with OPTIMIZER_HIDE_VAR()")
Signed-off-by: Kees Cook <kees@...nel.org>
---
Cc: <linux-hardening@...r.kernel.org>
---
 lib/tests/fortify_kunit.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/tests/fortify_kunit.c b/lib/tests/fortify_kunit.c
index 29ffc62a71e3..fc9c76f026d6 100644
--- a/lib/tests/fortify_kunit.c
+++ b/lib/tests/fortify_kunit.c
@@ -1003,8 +1003,8 @@ static void fortify_test_memcmp(struct kunit *test)
 {
 	char one[] = "My mind is going ...";
 	char two[] = "My mind is going ... I can feel it.";
-	size_t one_len = sizeof(one) - 1;
-	size_t two_len = sizeof(two) - 1;
+	volatile size_t one_len = sizeof(one) - 1;
+	volatile size_t two_len = sizeof(two) - 1;
 
 	OPTIMIZER_HIDE_VAR(one_len);
 	OPTIMIZER_HIDE_VAR(two_len);
-- 
2.34.1