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

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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: 2959a5f726 ("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>
This commit is contained in:
Petr Tesarik 2024-11-19 12:37:38 +01:00 committed by Andrew Morton
parent fa5cd95ce9
commit 3e29764ee2
1 changed files with 2 additions and 2 deletions
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4
mm/rodata_test.c
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@ -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;
}