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kcsan: Introduce kcsan_value_change type

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Introduces kcsan_value_change type, which explicitly points out if we
either observed a value-change (TRUE), or we could not observe one but
cannot rule out a value-change happened (MAYBE). The MAYBE state can
either be reported or not, depending on configuration preferences.

A follow-up patch introduces the FALSE state, which should never be
reported.

No functional change intended.

Acked-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Marco Elver 2020-02-11 17:04:21 +01:00 committed by Ingo Molnar
parent b968a08f24
commit b738f6169f
3 changed files with 54 additions and 29 deletions
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38
kernel/kcsan/core.c
View File

@ -341,7 +341,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
u32 _4;
u64 _8;
} expect_value;
bool value_change = false;
enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
unsigned long ua_flags = user_access_save();
unsigned long irq_flags;
@ -398,6 +398,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
* Read the current value, to later check and infer a race if the data
* was modified via a non-instrumented access, e.g. from a device.
*/
expect_value._8 = 0;
switch (size) {
case 1:
expect_value._1 = READ_ONCE(*(const u8 *)ptr);
@ -436,23 +437,36 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
*/
switch (size) {
case 1:
value_change = expect_value._1 != READ_ONCE(*(const u8 *)ptr);
expect_value._1 ^= READ_ONCE(*(const u8 *)ptr);
break;
case 2:
value_change = expect_value._2 != READ_ONCE(*(const u16 *)ptr);
expect_value._2 ^= READ_ONCE(*(const u16 *)ptr);
break;
case 4:
value_change = expect_value._4 != READ_ONCE(*(const u32 *)ptr);
expect_value._4 ^= READ_ONCE(*(const u32 *)ptr);
break;
case 8:
value_change = expect_value._8 != READ_ONCE(*(const u64 *)ptr);
expect_value._8 ^= READ_ONCE(*(const u64 *)ptr);
break;
default:
break; /* ignore; we do not diff the values */
}
/* Were we able to observe a value-change? */
if (expect_value._8 != 0)
value_change = KCSAN_VALUE_CHANGE_TRUE;
/* Check if this access raced with another. */
if (!remove_watchpoint(watchpoint)) {
/*
* Depending on the access type, map a value_change of MAYBE to
* TRUE (require reporting).
*/
if (value_change == KCSAN_VALUE_CHANGE_MAYBE && (size > 8 || is_assert)) {
/* Always assume a value-change. */
value_change = KCSAN_VALUE_CHANGE_TRUE;
}
/*
* No need to increment 'data_races' counter, as the racing
* thread already did.
@ -461,20 +475,12 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
* therefore both this thread and the racing thread may
* increment this counter.
*/
if (is_assert)
if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE)
kcsan_counter_inc(KCSAN_COUNTER_ASSERT_FAILURES);
/*
* - If we were not able to observe a value change due to size
* constraints, always assume a value change.
* - If the access type is an assertion, we also always assume a
* value change to always report the race.
*/
value_change = value_change || size > 8 || is_assert;
kcsan_report(ptr, size, type, value_change, smp_processor_id(),
KCSAN_REPORT_RACE_SIGNAL);
} else if (value_change) {
} else if (value_change == KCSAN_VALUE_CHANGE_TRUE) {
/* Inferring a race, since the value should not have changed. */
kcsan_counter_inc(KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN);
@ -482,7 +488,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
kcsan_counter_inc(KCSAN_COUNTER_ASSERT_FAILURES);
if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert)
kcsan_report(ptr, size, type, true,
kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_TRUE,
smp_processor_id(),
KCSAN_REPORT_RACE_UNKNOWN_ORIGIN);
}

19
kernel/kcsan/kcsan.h
View File

@ -88,6 +88,22 @@ extern void kcsan_counter_dec(enum kcsan_counter_id id);
*/
extern bool kcsan_skip_report_debugfs(unsigned long func_addr);
/*
* Value-change states.
*/
enum kcsan_value_change {
/*
* Did not observe a value-change, however, it is valid to report the
* race, depending on preferences.
*/
KCSAN_VALUE_CHANGE_MAYBE,
/*
* The value was observed to change, and the race should be reported.
*/
KCSAN_VALUE_CHANGE_TRUE,
};
enum kcsan_report_type {
/*
* The thread that set up the watchpoint and briefly stalled was
@ -111,6 +127,7 @@ enum kcsan_report_type {
* Print a race report from thread that encountered the race.
*/
extern void kcsan_report(const volatile void *ptr, size_t size, int access_type,
bool value_change, int cpu_id, enum kcsan_report_type type);
enum kcsan_value_change value_change, int cpu_id,
enum kcsan_report_type type);
#endif /* _KERNEL_KCSAN_KCSAN_H */

26
kernel/kcsan/report.c
View File

@ -130,26 +130,27 @@ static bool rate_limit_report(unsigned long frame1, unsigned long frame2)
* Special rules to skip reporting.
*/
static bool
skip_report(bool value_change, unsigned long top_frame)
skip_report(enum kcsan_value_change value_change, unsigned long top_frame)
{
/*
* The first call to skip_report always has value_change==true, since we
* The first call to skip_report always has value_change==TRUE, since we
* cannot know the value written of an instrumented access. For the 2nd
* call there are 6 cases with CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY:
*
* 1. read watchpoint, conflicting write (value_change==true): report;
* 2. read watchpoint, conflicting write (value_change==false): skip;
* 3. write watchpoint, conflicting write (value_change==true): report;
* 4. write watchpoint, conflicting write (value_change==false): skip;
* 5. write watchpoint, conflicting read (value_change==false): skip;
* 6. write watchpoint, conflicting read (value_change==true): report;
* 1. read watchpoint, conflicting write (value_change==TRUE): report;
* 2. read watchpoint, conflicting write (value_change==MAYBE): skip;
* 3. write watchpoint, conflicting write (value_change==TRUE): report;
* 4. write watchpoint, conflicting write (value_change==MAYBE): skip;
* 5. write watchpoint, conflicting read (value_change==MAYBE): skip;
* 6. write watchpoint, conflicting read (value_change==TRUE): report;
*
* Cases 1-4 are intuitive and expected; case 5 ensures we do not report
* data races where the write may have rewritten the same value; case 6
* is possible either if the size is larger than what we check value
* changes for or the access type is KCSAN_ACCESS_ASSERT.
*/
if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) && !value_change) {
if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) &&
value_change == KCSAN_VALUE_CHANGE_MAYBE) {
/*
* The access is a write, but the data value did not change.
*
@ -245,7 +246,7 @@ static int sym_strcmp(void *addr1, void *addr2)
* Returns true if a report was generated, false otherwise.
*/
static bool print_report(const volatile void *ptr, size_t size, int access_type,
bool value_change, int cpu_id,
enum kcsan_value_change value_change, int cpu_id,
enum kcsan_report_type type)
{
unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
@ -258,7 +259,7 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
/*
* Must check report filter rules before starting to print.
*/
if (skip_report(true, stack_entries[skipnr]))
if (skip_report(KCSAN_VALUE_CHANGE_TRUE, stack_entries[skipnr]))
return false;
if (type == KCSAN_REPORT_RACE_SIGNAL) {
@ -477,7 +478,8 @@ static bool prepare_report(unsigned long *flags, const volatile void *ptr,
}
void kcsan_report(const volatile void *ptr, size_t size, int access_type,
bool value_change, int cpu_id, enum kcsan_report_type type)
enum kcsan_value_change value_change, int cpu_id,
enum kcsan_report_type type)
{
unsigned long flags = 0;