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Commit c7fea488 authored by Kees Cook's avatar Kees Cook
Browse files

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: default avatarKees Cook <keescook@chromium.org>
parent 95925c99

drivers/misc/lkdtm.h

+2 −0
Original line number Diff line number Diff line
@@ -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);

drivers/misc/lkdtm_core.c

+2 −0
Original line number Diff line number Diff line
@@ -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),

drivers/misc/lkdtm_refcount.c

+44 −0
Original line number Diff line number Diff line
@@ -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");

}