+ mm-rodata_test-use-read_once-to-read-const-variable.patch added to mm-unstable branch

+ mm-rodata_test-use-read_once-to-read-const-variable.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/rodata_test: use READ_ONCE() to read const variable
has been added to the -mm mm-unstable branch.  Its filename is
     mm-rodata_test-use-read_once-to-read-const-variable.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-rodata_test-use-read_once-to-read-const-variable.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Petr Tesarik <ptesarik@suse.com>
Subject: mm/rodata_test: use READ_ONCE() to read const variable
Date: Tue, 19 Nov 2024 12:37:38 +0100

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>
---

 mm/rodata_test.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/rodata_test.c~mm-rodata_test-use-read_once-to-read-const-variable
+++ a/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;
 	}
_

Patches currently in -mm which might be from ptesarik@suse.com are

mm-rodata_test-use-read_once-to-read-const-variable.patch
mm-rodata_test-verify-test-data-is-unchanged-rather-than-non-zero.patch