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Subject[PATCH] kcsan: Add current->state to implicitly atomic accesses
From
Add volatile current->state to list of implicitly atomic accesses. This
is in preparation to eventually enable KCSAN on kernel/sched (which
currently still has KCSAN_SANITIZE := n).

Since accesses that match the special check in atomic.h are rare, it
makes more sense to move this check to the slow-path, avoiding the
additional compare in the fast-path. With the microbenchmark, a speedup
of ~6% is measured.

Signed-off-by: Marco Elver <elver@google.com>
---

Example data race that was reported with KCSAN enabled on kernel/sched:

write to 0xffff9e42c4400050 of 8 bytes by task 311 on cpu 7:
ttwu_do_wakeup.isra.0+0x48/0x1f0 kernel/sched/core.c:2222
ttwu_remote kernel/sched/core.c:2286 [inline]
try_to_wake_up+0x9f8/0xbe0 kernel/sched/core.c:2585
wake_up_process+0x1e/0x30 kernel/sched/core.c:2669
__up.isra.0+0xb5/0xe0 kernel/locking/semaphore.c:261
...

read to 0xffff9e42c4400050 of 8 bytes by task 310 on cpu 0:
sched_submit_work kernel/sched/core.c:4109 [inline] <--- current->state read
schedule+0x3a/0x1a0 kernel/sched/core.c:4153
schedule_timeout+0x202/0x250 kernel/time/timer.c:1872
...
---
kernel/kcsan/atomic.h | 21 +++++++--------------
kernel/kcsan/core.c | 22 +++++++++++++++-------
kernel/kcsan/debugfs.c | 27 ++++++++++++++++++---------
3 files changed, 40 insertions(+), 30 deletions(-)

diff --git a/kernel/kcsan/atomic.h b/kernel/kcsan/atomic.h
index a9c1930534914..be9e625227f3b 100644
--- a/kernel/kcsan/atomic.h
+++ b/kernel/kcsan/atomic.h
@@ -4,24 +4,17 @@
#define _KERNEL_KCSAN_ATOMIC_H

#include <linux/jiffies.h>
+#include <linux/sched.h>

/*
- * Helper that returns true if access to @ptr should be considered an atomic
- * access, even though it is not explicitly atomic.
- *
- * List all volatile globals that have been observed in races, to suppress
- * data race reports between accesses to these variables.
- *
- * For now, we assume that volatile accesses of globals are as strong as atomic
- * accesses (READ_ONCE, WRITE_ONCE cast to volatile). The situation is still not
- * entirely clear, as on some architectures (Alpha) READ_ONCE/WRITE_ONCE do more
- * than cast to volatile. Eventually, we hope to be able to remove this
- * function.
+ * Special rules for certain memory where concurrent conflicting accesses are
+ * common, however, the current convention is to not mark them; returns true if
+ * access to @ptr should be considered atomic. Called from slow-path.
*/
-static __always_inline bool kcsan_is_atomic(const volatile void *ptr)
+static bool kcsan_is_atomic_special(const volatile void *ptr)
{
- /* only jiffies for now */
- return ptr == &jiffies;
+ /* volatile globals that have been observed in data races. */
+ return ptr == &jiffies || ptr == &current->state;
}

#endif /* _KERNEL_KCSAN_ATOMIC_H */
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 065615df88eaa..eb30ecdc8c009 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -188,12 +188,13 @@ static __always_inline struct kcsan_ctx *get_ctx(void)
return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
}

+/* Rules for generic atomic accesses. Called from fast-path. */
static __always_inline bool
is_atomic(const volatile void *ptr, size_t size, int type)
{
struct kcsan_ctx *ctx;

- if ((type & KCSAN_ACCESS_ATOMIC) != 0)
+ if (type & KCSAN_ACCESS_ATOMIC)
return true;

/*
@@ -201,16 +202,16 @@ is_atomic(const volatile void *ptr, size_t size, int type)
* as atomic. This allows using them also in atomic regions, such as
* seqlocks, without implicitly changing their semantics.
*/
- if ((type & KCSAN_ACCESS_ASSERT) != 0)
+ if (type & KCSAN_ACCESS_ASSERT)
return false;

if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
- (type & KCSAN_ACCESS_WRITE) != 0 && size <= sizeof(long) &&
+ (type & KCSAN_ACCESS_WRITE) && size <= sizeof(long) &&
IS_ALIGNED((unsigned long)ptr, size))
return true; /* Assume aligned writes up to word size are atomic. */

ctx = get_ctx();
- if (unlikely(ctx->atomic_next > 0)) {
+ if (ctx->atomic_next > 0) {
/*
* Because we do not have separate contexts for nested
* interrupts, in case atomic_next is set, we simply assume that
@@ -224,10 +225,8 @@ is_atomic(const volatile void *ptr, size_t size, int type)
--ctx->atomic_next; /* in task, or outer interrupt */
return true;
}
- if (unlikely(ctx->atomic_nest_count > 0 || ctx->in_flat_atomic))
- return true;

- return kcsan_is_atomic(ptr);
+ return ctx->atomic_nest_count > 0 || ctx->in_flat_atomic;
}

static __always_inline bool
@@ -367,6 +366,15 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
if (!kcsan_is_enabled())
goto out;

+ /*
+ * Special atomic rules: unlikely to be true, so we check them here in
+ * the slow-path, and not in the fast-path in is_atomic(). Call after
+ * kcsan_is_enabled(), as we may access memory that is not yet
+ * initialized during early boot.
+ */
+ if (!is_assert && kcsan_is_atomic_special(ptr))
+ goto out;
+
if (!check_encodable((unsigned long)ptr, size)) {
kcsan_counter_inc(KCSAN_COUNTER_UNENCODABLE_ACCESSES);
goto out;
diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
index 2ff1961239778..72ee188ebc54a 100644
--- a/kernel/kcsan/debugfs.c
+++ b/kernel/kcsan/debugfs.c
@@ -74,25 +74,34 @@ void kcsan_counter_dec(enum kcsan_counter_id id)
*/
static noinline void microbenchmark(unsigned long iters)
{
+ const struct kcsan_ctx ctx_save = current->kcsan_ctx;
+ const bool was_enabled = READ_ONCE(kcsan_enabled);
cycles_t cycles;

+ /* We may have been called from an atomic region; reset context. */
+ memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
+ /*
+ * Disable to benchmark fast-path for all accesses, and (expected
+ * negligible) call into slow-path, but never set up watchpoints.
+ */
+ WRITE_ONCE(kcsan_enabled, false);
+
pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);

cycles = get_cycles();
while (iters--) {
- /*
- * We can run this benchmark from multiple tasks; this address
- * calculation increases likelyhood of some accesses
- * overlapping. Make the access type an atomic read, to never
- * set up watchpoints and test the fast-path only.
- */
- unsigned long addr =
- iters % (CONFIG_KCSAN_NUM_WATCHPOINTS * PAGE_SIZE);
- __kcsan_check_access((void *)addr, sizeof(long), KCSAN_ACCESS_ATOMIC);
+ unsigned long addr = iters & ((PAGE_SIZE << 8) - 1);
+ int type = !(iters & 0x7f) ? KCSAN_ACCESS_ATOMIC :
+ (!(iters & 0xf) ? KCSAN_ACCESS_WRITE : 0);
+ __kcsan_check_access((void *)addr, sizeof(long), type);
}
cycles = get_cycles() - cycles;

pr_info("KCSAN: %s end | cycles: %llu\n", __func__, cycles);
+
+ WRITE_ONCE(kcsan_enabled, was_enabled);
+ /* restore context */
+ current->kcsan_ctx = ctx_save;
}

/*
--
2.25.0.265.gbab2e86ba0-goog
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