[PATCH v2 2/2] lkdtm: Provide timing tests for atomic_t vs refcount_t

From: Kees Cook
Date: Tue Jul 25 2017 - 18:14:00 EST


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@xxxxxxxxxxxx>
---
drivers/misc/lkdtm.h | 2 ++
drivers/misc/lkdtm_core.c | 2 ++
drivers/misc/lkdtm_refcount.c | 44 +++++++++++++++++++++++++++++++++++++++++++
3 files changed, 48 insertions(+)

diff --git a/drivers/misc/lkdtm.h b/drivers/misc/lkdtm.h
index 04ff8b23b3b0..063f5d651076 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 156a1a07c3db..51decc07eeda 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 313abea4bf9d..494d35c433cc 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 - 1; i >= 0; i--)
+ if (atomic_dec_and_test(&count))
+ break;
+
+ if (i != 0)
+ pr_err("atomic timing: out of sync up/down cycle: %u\n", i);
+ 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 - 1; i >= 0; i--)
+ if (refcount_dec_and_test(&count))
+ break;
+
+ if (i != 0)
+ pr_err("refcount: out of sync up/down cycle: %u\n", i);
+ else
+ pr_info("refcount timing: done\n");
+}
--
2.7.4