mm/rodata_test: use READ_ONCE() to read const variable

Patch series "Fix mm/rodata_test", v2.

Make sure that the test actually reads the read-only memory location.
Verify that the variable contains the expected value rather than any
non-zero value.

This patch (of 2):

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.

Link: https://lkml.kernel.org/r/cover.1732016064.git.ptesarik@suse.com
Link: https://lkml.kernel.org/r/2a66dee010151b25cb143efb39091ef7530aa00a.1732016064.git.ptesarik@suse.com
Fixes: 2959a5f726f6 ("mm: add arch-independent testcases for RODATA")
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Cc: Jinbum Park <jinb.park7@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/rodata_test.c b/mm/rodata_test.c
index 6d783436951f324f769c28ad6091c9a62bf85690..3b60425d80fea061ef257a5cf9b0ea610ffa2a5b 100644 (file)
--- 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;
        }