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 04ff8b23b3b0260f68e75abd32efa80fb05f15a2..063f5d651076fdc13657402d9a5c207df6e073cc 100644 (file)
--- 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 156a1a07c3db958308949ad51dbac404f3edef4c..51decc07eeda1529de61a3e445df1ddc3ccd93c0 100644 (file)
--- 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 313abea4bf9d41282a2547b7822bf704f7b3af2b..29af0152a33790e6163580039d5b800ebf6edaf1 100644 (file)
--- 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");
+}