[PATCH] lkdtm/bugs: Remove volatile from element_count

[Nathan Chancellor <nathan@kernel.org>]

After a recent improvement in clang to warn on unsequenced accesses to
volatile variables [1], an instance appears in lkdtm_FAM_BOUNDS(), which
can break the build with CONFIG_WERROR=y / W=e:

  drivers/misc/lkdtm/bugs.c:493:31: error: unsequenced accesses to volatile qualified 'element_count' [-Werror,-Wunsequenced]
    493 |         inst->array[element_count] = element_count;
        |                     ~~~~~~~~~~~~~    ^

Per C11 5.1.2.3.2, accessing a volatile object is a side effect. Per C11
6.5.2, unsequenced side effects on the same scalar object are undefined
behavior. Per C11 6.5.16.3, the value computations of the left and
right operands of the assignment operator are unsequenced.

This variable is marked volatile to hide its value from the compiler at
compile time. The same thing can be accomplished with
OPTIMIZER_HIDE_VAR. Drop volatile from element_count to clear up the
warning. The tests still pass and there are no warnings at compile time.

Cc: stable@vger.kernel.org
Fixes: 5d207e83ca41 ("lkdtm: Add FAM_BOUNDS test for __counted_by")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202611140405.wMowxoPE-lkp@intel.com/
Closes: https://github.com/ClangBuiltLinux/linux/issues/2194
Link: https://github.com/llvm/llvm-project/commit/955715460eaf9188c241c74750637fc82dfcc423 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 drivers/misc/lkdtm/bugs.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index 2761685cf4f8..7c799fef89a8 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -472,12 +472,14 @@ struct lkdtm_cb_fam {
 	int array[] __counted_by(count);
 };
 
-static volatile int element_count = 4;
+static int element_count = 4;
 
 static void lkdtm_FAM_BOUNDS(void)
 {
 	struct lkdtm_cb_fam *inst;
 
+	OPTIMIZER_HIDE_VAR(element_count);
+
 	inst = kzalloc_flex(*inst, array, element_count + 1);
 	if (!inst) {
 		pr_err("FAIL: could not allocate test struct!\n");
@@ -534,6 +536,8 @@ static void lkdtm_PTR_BOUNDS(void)
 {
 	struct lkdtm_cb_ptr *inst;
 
+	OPTIMIZER_HIDE_VAR(element_count);
+
 	inst = kzalloc_obj(*inst);
 	if (!inst) {
 		pr_err("FAIL: could not allocate struct lkdtm_cb_ptr!\n");

---
base-commit: 7475ea8b58616b33c04afb976cf0fbaad9dd63fb
base-tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
change-id: 20261114-lkdtm-bugs-wunsequenced-7c18499c2771

Best regards,
--  
Cheers,
Nathan