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