linux-kernel - [PATCH 1/1] mm/rodata_test: Use READ

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Message-ID: <20241119112009.1286675-1-ptesarik@suse.com>
Date: Tue, 19 Nov 2024 12:20:09 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <kees@...nel.org>,
	Jinbum Park <jinb.park7@...il.com>
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Petr Tesarik <ptesarik@...e.com>
Subject: [PATCH 1/1] mm/rodata_test: Use READ_ONCE() to read const variable

The C compiler may optimize away the memory read of a const variable if its
value is known at compile time.

In particular, GCC14 with -O2 generates no code at all for test 1, and it
generates the following x86_64 instructions for test 3:

	cmpl	$195, 4(%rsp)
	je	.L14

That is, it replaces the read of rodata_test_data with an immediate value
and compares it to the value of the local variable "zero".

Use READ_ONCE() to undo any such compiler optimizations and enforce a
memory read.

Fixes: 2959a5f726f6 ("mm: add arch-independent testcases for RODATA")
Signed-off-by: Petr Tesarik <ptesarik@...e.com>
---
 mm/rodata_test.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/rodata_test.c b/mm/rodata_test.c
index 6d783436951f..3b60425d80fe 100644
--- a/mm/rodata_test.c
+++ b/mm/rodata_test.c
@@ -20,7 +20,7 @@ void rodata_test(void)
 
 	/* test 1: read the value */
 	/* If this test fails, some previous testrun has clobbered the state */
-	if (!rodata_test_data) {
+	if (!READ_ONCE(rodata_test_data)) {
 		pr_err("test 1 fails (start data)\n");
 		return;
 	}
@@ -33,7 +33,7 @@ void rodata_test(void)
 	}
 
 	/* test 3: check the value hasn't changed */
-	if (rodata_test_data == zero) {
+	if (READ_ONCE(rodata_test_data) == zero) {
 		pr_err("test data was changed\n");
 		return;
 	}
-- 
2.46.1