Re: [PATCH v3 1/9] kcsan: Add Kernel Concurrency Sanitizer infrastructure

Dmitry Vyukov
2019-11-06T09:38:21Z

On Mon, Nov 4, 2019 at 3:28 PM Marco Elver <elver@google.com> wrote:
>
> Kernel Concurrency Sanitizer (KCSAN) is a dynamic data-race detector for
> kernel space. KCSAN is a sampling watchpoint-based data-race detector.
> See the included Documentation/dev-tools/kcsan.rst for more details.
...
> +static inline atomic_long_t *find_watchpoint(unsigned long addr, size_t size,
> + bool expect_write,
> + long *encoded_watchpoint)
> +{
> + const int slot = watchpoint_slot(addr);
> + const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK;
> + atomic_long_t *watchpoint;
> + unsigned long wp_addr_masked;
> + size_t wp_size;
> + bool is_write;
> + int i;
> +
> + BUILD_BUG_ON(CONFIG_KCSAN_NUM_WATCHPOINTS < CHECK_NUM_SLOTS);
> +
> + for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
> + watchpoint = &watchpoints[SLOT_IDX(slot, i)];

The fast path code become much nicer!
I did another pass looking at how we can optimize the fast path.
Currently we still have 2 push/pop pairs on the fast path because of
register pressure. The logic in SLOT_IDX seems to be the main culprit.
We discussed several options offline:
1. Just check 1 slot and ignore all corner cases (we will miss racing
unaligned access to different addresses but overlapping and crossing
pages, which sounds pretty esoteric)
2. Check 3 slots in order and without wraparound (watchpoints[slot +
i], where i=-1,0,1), this will require adding dummy slots around the
array
3. An interesting option is to check just 2 slots (that's enough!), to
make this work we will need to slightly offset bucket number when
setting a watch point (namely, if an access goes to the very end of a
page, we set the watchpoint into the bucket corresponding to the
_next_ page)
All of these options remove push/pop in my experiments. Obviously
checking fewer slots will reduce dynamic overhead even more.

> + *encoded_watchpoint = atomic_long_read(watchpoint);
> + if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked,
> + &wp_size, &is_write))
> + continue;
> +
> + if (expect_write && !is_write)
> + continue;
> +
> + /* Check if the watchpoint matches the access. */
> + if (matching_access(wp_addr_masked, wp_size, addr_masked, size))
> + return watchpoint;
> + }
> +
> + return NULL;
> +}
> +
> +static inline atomic_long_t *insert_watchpoint(unsigned long addr, size_t size,
> + bool is_write)
> +{
> + const int slot = watchpoint_slot(addr);
> + const long encoded_watchpoint = encode_watchpoint(addr, size, is_write);
> + atomic_long_t *watchpoint;
> + int i;
> +
> + for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
> + long expect_val = INVALID_WATCHPOINT;
> +
> + /* Try to acquire this slot. */
> + watchpoint = &watchpoints[SLOT_IDX(slot, i)];

If we do this SLOT_IDX trickery to catch unaligned accesses crossing
pages, then I think we should not use it insert_watchpoint at all and
only set the watchpoint to the exact index. Otherwise, we will
actually miss the corner cases which defeats the whole purpose of
SLOT_IDX and 3 iterations.

> + if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val,
> + encoded_watchpoint))
> + return watchpoint;
> + }
> +
> + return NULL;
> +}
> +
> +/*
> + * Return true if watchpoint was successfully consumed, false otherwise.
> + *
> + * This may return false if:
> + *
> + * 1. another thread already consumed the watchpoint;
> + * 2. the thread that set up the watchpoint already removed it;
> + * 3. the watchpoint was removed and then re-used.
> + */
> +static inline bool try_consume_watchpoint(atomic_long_t *watchpoint,
> + long encoded_watchpoint)
> +{
> + return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint,
> + CONSUMED_WATCHPOINT);
> +}
> +
> +/*
> + * Return true if watchpoint was not touched, false if consumed.
> + */
> +static inline bool remove_watchpoint(atomic_long_t *watchpoint)
> +{
> + return atomic_long_xchg_relaxed(watchpoint, INVALID_WATCHPOINT) !=
> + CONSUMED_WATCHPOINT;
> +}
> +
> +static inline struct kcsan_ctx *get_ctx(void)
> +{
> + /*
> + * In interrupt, use raw_cpu_ptr to avoid unnecessary checks, that would
> + * also result in calls that generate warnings in uaccess regions.
> + */
> + return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
> +}
> +
> +static inline bool is_atomic(const volatile void *ptr)
> +{
> + struct kcsan_ctx *ctx = get_ctx();
> +
> + if (unlikely(ctx->atomic_next > 0)) {
> + --ctx->atomic_next;
> + return true;
> + }
> + if (unlikely(ctx->atomic_nest_count > 0 || ctx->in_flat_atomic))
> + return true;
> +
> + return kcsan_is_atomic(ptr);
> +}
> +
> +static inline bool should_watch(const volatile void *ptr, int type)
> +{
> + /*
> + * Never set up watchpoints when memory operations are atomic.
> + *
> + * Need to check this first, before kcsan_skip check below: (1) atomics
> + * should not count towards skipped instructions, and (2) to actually
> + * decrement kcsan_atomic_next for consecutive instruction stream.
> + */
> + if ((type & KCSAN_ACCESS_ATOMIC) != 0 || is_atomic(ptr))
> + return false;

should_watch and is_atomic are invoked on the fast path and do more
things than strictly necessary.
The minimal amount of actions would be:
- check and decrement ctx->atomic_next for atomic accesses
- decrement kcsan_skip

atomic_nest_count/in_flat_atomic/kcsan_is_atomic can be checked on
uninlined slow path.

It should not be necessary to set kcsan_skip to -1 if we _always_
resetup kcsan_skip on slow path.

> + if (this_cpu_dec_return(kcsan_skip) >= 0)
> + return false;
> +
> + /* avoid underflow if !kcsan_is_enabled() */
> + this_cpu_write(kcsan_skip, -1);
> +
> + /* this operation should be watched */
> + return true;
> +}
> +
> +static inline void reset_kcsan_skip(void)
> +{
> + long skip_count = CONFIG_KCSAN_SKIP_WATCH -
> + (IS_ENABLED(CONFIG_KCSAN_SKIP_WATCH_RANDOMIZE) ?
> + prandom_u32_max(CONFIG_KCSAN_SKIP_WATCH) :
> + 0);
> + this_cpu_write(kcsan_skip, skip_count);
> +}
> +
> +static inline bool kcsan_is_enabled(void)
> +{
> + return READ_ONCE(kcsan_enabled) && get_ctx()->disable_count == 0;
> +}
> +
> +static inline unsigned int get_delay(void)
> +{
> + unsigned int delay = in_task() ? CONFIG_KCSAN_UDELAY_TASK :
> + CONFIG_KCSAN_UDELAY_INTERRUPT;
> + return delay - (IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
> + prandom_u32_max(delay) :
> + 0);
> +}
> +
> +/*
> + * Pull everything together: check_access() below contains the performance
> + * critical operations; the fast-path (including check_access) functions should
> + * all be inlinable by the instrumentation functions.
> + *
> + * The slow-path (kcsan_found_watchpoint, kcsan_setup_watchpoint) are
> + * non-inlinable -- note that, we prefix these with "kcsan_" to ensure they can
> + * be filtered from the stacktrace, as well as give them unique names for the
> + * UACCESS whitelist of objtool. Each function uses user_access_save/restore(),
> + * since they do not access any user memory, but instrumentation is still
> + * emitted in UACCESS regions.
> + */
> +
> +static noinline void kcsan_found_watchpoint(const volatile void *ptr,
> + size_t size, bool is_write,
> + bool consumed)
> +{
> + unsigned long flags = user_access_save();
> + enum kcsan_report_type report_type;
> +
> + if (!consumed) {
> + /*
> + * The other thread may not print any diagnostics, as it has
> + * already removed the watchpoint, or another thread consumed
> + * the watchpoint before this thread.
> + */
> + kcsan_counter_inc(KCSAN_COUNTER_REPORT_RACES);
> + report_type = KCSAN_REPORT_RACE_CHECK_RACE;
> + } else {
> + report_type = KCSAN_REPORT_RACE_CHECK;
> + }
> +
> + kcsan_counter_inc(KCSAN_COUNTER_DATA_RACES);
> + kcsan_report(ptr, size, is_write, raw_smp_processor_id(), report_type);
> +
> + user_access_restore(flags);
> +}
> +
> +static noinline void kcsan_setup_watchpoint(const volatile void *ptr,
> + size_t size, bool is_write)
> +{
> + atomic_long_t *watchpoint;
> + union {
> + u8 _1;
> + u16 _2;
> + u32 _4;
> + u64 _8;
> + } expect_value;
> + bool is_expected = true;
> + unsigned long ua_flags = user_access_save();
> + unsigned long irq_flags;
> +
> + if (!check_encodable((unsigned long)ptr, size)) {
> + kcsan_counter_inc(KCSAN_COUNTER_UNENCODABLE_ACCESSES);
> + goto out;
> + }
> +
> + /*
> + * Disable interrupts & preemptions to avoid another thread on the same
> + * CPU accessing memory locations for the set up watchpoint; this is to
> + * avoid reporting races to e.g. CPU-local data.
> + *
> + * An alternative would be adding the source CPU to the watchpoint
> + * encoding, and checking that watchpoint-CPU != this-CPU. There are
> + * several problems with this:
> + * 1. we should avoid stealing more bits from the watchpoint encoding
> + * as it would affect accuracy, as well as increase performance
> + * overhead in the fast-path;
> + * 2. if we are preempted, but there *is* a genuine data race, we
> + * would *not* report it -- since this is the common case (vs.
> + * CPU-local data accesses), it makes more sense (from a data race
> + * detection point of view) to simply disable preemptions to ensure
> + * as many tasks as possible run on other CPUs.
> + */
> + local_irq_save(irq_flags);
> +
> + watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
> + if (watchpoint == NULL) {
> + /*
> + * Out of capacity: the size of `watchpoints`, and the frequency
> + * with which `should_watch()` returns true should be tweaked so
> + * that this case happens very rarely.
> + */
> + kcsan_counter_inc(KCSAN_COUNTER_NO_CAPACITY);
> + goto out_unlock;
> + }
> +
> + /*
> + * Reset kcsan_skip counter: only do this if we succeeded in setting up
> + * a watchpoint.
> + */
> + reset_kcsan_skip();
> +
> + kcsan_counter_inc(KCSAN_COUNTER_SETUP_WATCHPOINTS);
> + kcsan_counter_inc(KCSAN_COUNTER_USED_WATCHPOINTS);
> +
> + /*
> + * 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.
> + */
> + switch (size) {
> + case 1:
> + expect_value._1 = READ_ONCE(*(const u8 *)ptr);
> + break;
> + case 2:
> + expect_value._2 = READ_ONCE(*(const u16 *)ptr);
> + break;
> + case 4:
> + expect_value._4 = READ_ONCE(*(const u32 *)ptr);
> + break;
> + case 8:
> + expect_value._8 = READ_ONCE(*(const u64 *)ptr);
> + break;
> + default:
> + break; /* ignore; we do not diff the values */
> + }
> +
> + if (IS_ENABLED(CONFIG_KCSAN_DEBUG)) {
> + kcsan_disable_current();
> + pr_err("KCSAN: watching %s, size: %zu, addr: %px [slot: %d, encoded: %lx]\n",
> + is_write ? "write" : "read", size, ptr,
> + watchpoint_slot((unsigned long)ptr),
> + encode_watchpoint((unsigned long)ptr, size, is_write));
> + kcsan_enable_current();
> + }
> +
> + /*
> + * Delay this thread, to increase probability of observing a racy
> + * conflicting access.
> + */
> + udelay(get_delay());
> +
> + /*
> + * Re-read value, and check if it is as expected; if not, we infer a
> + * racy access.
> + */
> + switch (size) {
> + case 1:
> + is_expected = expect_value._1 == READ_ONCE(*(const u8 *)ptr);
> + break;
> + case 2:
> + is_expected = expect_value._2 == READ_ONCE(*(const u16 *)ptr);
> + break;
> + case 4:
> + is_expected = expect_value._4 == READ_ONCE(*(const u32 *)ptr);
> + break;
> + case 8:
> + is_expected = expect_value._8 == READ_ONCE(*(const u64 *)ptr);
> + break;
> + default:
> + break; /* ignore; we do not diff the values */
> + }
> +
> + /* Check if this access raced with another. */
> + if (!remove_watchpoint(watchpoint)) {
> + /*
> + * No need to increment 'data_races' counter, as the racing
> + * thread already did.
> + */
> + kcsan_report(ptr, size, is_write, smp_processor_id(),
> + KCSAN_REPORT_RACE_SETUP);
> + } else if (!is_expected) {
> + /* Inferring a race, since the value should not have changed. */
> + kcsan_counter_inc(KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN);
> + if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN))
> + kcsan_report(ptr, size, is_write, smp_processor_id(),
> + KCSAN_REPORT_RACE_UNKNOWN_ORIGIN);
> + }
> +
> + kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS);
> +out_unlock:
> + local_irq_restore(irq_flags);
> +out:
> + user_access_restore(ua_flags);
> +}
> +
> +static inline void check_access(const volatile void *ptr, size_t size, int type)
> +{
> + const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
> + atomic_long_t *watchpoint;
> + long encoded_watchpoint;
> +
> + if (IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE) && is_write)
> + type |= KCSAN_ACCESS_ATOMIC;
> +
> + /*
> + * Avoid user_access_save in fast-path: find_watchpoint is safe without
> + * user_access_save, as the address that ptr points to is only used to
> + * check if a watchpoint exists; ptr is never dereferenced.
> + */
> + watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
> + &encoded_watchpoint);
> +
> + /*
> + * It is safe to check kcsan_is_enabled() after find_watchpoint, but
> + * right before we would enter the slow-path: no state changes that
> + * cause a data race to be detected and reported have occurred yet.
> + */
> +
> + if (unlikely(watchpoint != NULL) && kcsan_is_enabled()) {

I would move kcsan_is_enabled and the rest of the code in the branch
into non-inlined slow path.
It makes the hot function much shorter.
There is a trick related to number of arguments, though. We would need
to pass ptr, size, is_write, watchpoint and encoded_watchpoint. That's
5 arguments. Only 4 are passed in registers. So it may make sense to
combine size and type into a single word. On the inlined fast path
compiler packs/unpacks that statically, so it does not matter. But for
the function call it will just forward a single const.

> + /*
> + * Try consume the watchpoint as soon after finding the
> + * watchpoint as possible; this must always be guarded by
> + * kcsan_is_enabled() check, as otherwise we might erroneously
> + * triggering reports when disabled.
> + */
> + const bool consumed =
> + try_consume_watchpoint(watchpoint, encoded_watchpoint);
> +
> + kcsan_found_watchpoint(ptr, size, is_write, consumed);
> + } else if (unlikely(should_watch(ptr, type)) && kcsan_is_enabled()) {

I would move kcsan_is_enabled check into kcsan_setup_watchpoint. It's
not executed on fast path, but bloats the host function code.

> + kcsan_setup_watchpoint(ptr, size, is_write);
> + }
> +}