From: Kees Cook <keescook@chromium.org>
Date: Fri, 21 Jul 2017 13:19:14 +0000 (-0700)
Subject: lkdtm: Provide timing tests for atomic_t vs refcount_t
X-Git-Tag: v4.14-rc1~146^2~144^2~2
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=c7fea48876773603721f545f8c1a2f894291ef85;p=platform%2Fkernel%2Flinux-exynos.git

lkdtm: Provide timing tests for atomic_t vs refcount_t

While not a crash test, this does provide two tight atomic_t and
refcount_t loops for performance comparisons:

	cd /sys/kernel/debug/provoke-crash
	perf stat -B -- cat <(echo ATOMIC_TIMING) > DIRECT
	perf stat -B -- cat <(echo REFCOUNT_TIMING) > DIRECT

Looking a CPU cycles is the best way to example the fast-path (rather
than instruction counts, since conditional jumps will be executed but
will be negligible due to branch-prediction).

Signed-off-by: Kees Cook <keescook@chromium.org>
---

diff --git a/drivers/misc/lkdtm.h b/drivers/misc/lkdtm.h
index 04ff8b2..063f5d6 100644
--- a/drivers/misc/lkdtm.h
+++ b/drivers/misc/lkdtm.h
@@ -61,6 +61,8 @@ void lkdtm_REFCOUNT_INC_NOT_ZERO_SATURATED(void);
 void lkdtm_REFCOUNT_ADD_NOT_ZERO_SATURATED(void);
 void lkdtm_REFCOUNT_DEC_AND_TEST_SATURATED(void);
 void lkdtm_REFCOUNT_SUB_AND_TEST_SATURATED(void);
+void lkdtm_REFCOUNT_TIMING(void);
+void lkdtm_ATOMIC_TIMING(void);
 
 /* lkdtm_rodata.c */
 void lkdtm_rodata_do_nothing(void);
diff --git a/drivers/misc/lkdtm_core.c b/drivers/misc/lkdtm_core.c
index 156a1a0..51decc0 100644
--- a/drivers/misc/lkdtm_core.c
+++ b/drivers/misc/lkdtm_core.c
@@ -238,6 +238,8 @@ struct crashtype crashtypes[] = {
 	CRASHTYPE(REFCOUNT_ADD_NOT_ZERO_SATURATED),
 	CRASHTYPE(REFCOUNT_DEC_AND_TEST_SATURATED),
 	CRASHTYPE(REFCOUNT_SUB_AND_TEST_SATURATED),
+	CRASHTYPE(REFCOUNT_TIMING),
+	CRASHTYPE(ATOMIC_TIMING),
 	CRASHTYPE(USERCOPY_HEAP_SIZE_TO),
 	CRASHTYPE(USERCOPY_HEAP_SIZE_FROM),
 	CRASHTYPE(USERCOPY_HEAP_FLAG_TO),
diff --git a/drivers/misc/lkdtm_refcount.c b/drivers/misc/lkdtm_refcount.c
index 313abea..29af015 100644
--- a/drivers/misc/lkdtm_refcount.c
+++ b/drivers/misc/lkdtm_refcount.c
@@ -354,3 +354,47 @@ void lkdtm_REFCOUNT_SUB_AND_TEST_SATURATED(void)
 
 	check_saturated(&sat);
 }
+
+/* Used to time the existing atomic_t when used for reference counting */
+void lkdtm_ATOMIC_TIMING(void)
+{
+	unsigned int i;
+	atomic_t count = ATOMIC_INIT(1);
+
+	for (i = 0; i < INT_MAX - 1; i++)
+		atomic_inc(&count);
+
+	for (i = INT_MAX; i > 0; i--)
+		if (atomic_dec_and_test(&count))
+			break;
+
+	if (i != 1)
+		pr_err("atomic timing: out of sync up/down cycle: %u\n", i - 1);
+	else
+		pr_info("atomic timing: done\n");
+}
+
+/*
+ * This can be compared to ATOMIC_TIMING when implementing fast refcount
+ * protections. Looking at the number of CPU cycles tells the real story
+ * about performance. For example:
+ *    cd /sys/kernel/debug/provoke-crash
+ *    perf stat -B -- cat <(echo REFCOUNT_TIMING) > DIRECT
+ */
+void lkdtm_REFCOUNT_TIMING(void)
+{
+	unsigned int i;
+	refcount_t count = REFCOUNT_INIT(1);
+
+	for (i = 0; i < INT_MAX - 1; i++)
+		refcount_inc(&count);
+
+	for (i = INT_MAX; i > 0; i--)
+		if (refcount_dec_and_test(&count))
+			break;
+
+	if (i != 1)
+		pr_err("refcount: out of sync up/down cycle: %u\n", i - 1);
+	else
+		pr_info("refcount timing: done\n");
+}