[PATCH v2 1/5] kcsan: Move interfaces that affects checks to kcsan-checks.h

[PATCH v2 1/5] kcsan: Move interfaces that affects checks to kcsan-checks.h

[Marco Elver]

This moves functions that affect state changing the behaviour of
kcsan_check_access() to kcsan-checks.h. Since these are likely used with
kcsan_check_access() it makes more sense to have them in kcsan-checks.h,
to avoid including all of 'include/linux/kcsan.h'.

No functional change intended.

Signed-off-by: Marco Elver <elver@google.com>
---
 include/linux/kcsan-checks.h | 48 ++++++++++++++++++++++++++++++++++--
 include/linux/kcsan.h        | 41 ------------------------------
 2 files changed, 46 insertions(+), 43 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index cf6961794e9a1..8675411c8dbcd 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -32,10 +32,54 @@
  */
 void __kcsan_check_access(const volatile void *ptr, size_t size, int type);
 
-#else
+/**
+ * kcsan_nestable_atomic_begin - begin nestable atomic region
+ *
+ * Accesses within the atomic region may appear to race with other accesses but
+ * should be considered atomic.
+ */
+void kcsan_nestable_atomic_begin(void);
+
+/**
+ * kcsan_nestable_atomic_end - end nestable atomic region
+ */
+void kcsan_nestable_atomic_end(void);
+
+/**
+ * kcsan_flat_atomic_begin - begin flat atomic region
+ *
+ * Accesses within the atomic region may appear to race with other accesses but
+ * should be considered atomic.
+ */
+void kcsan_flat_atomic_begin(void);
+
+/**
+ * kcsan_flat_atomic_end - end flat atomic region
+ */
+void kcsan_flat_atomic_end(void);
+
+/**
+ * kcsan_atomic_next - consider following accesses as atomic
+ *
+ * Force treating the next n memory accesses for the current context as atomic
+ * operations.
+ *
+ * @n number of following memory accesses to treat as atomic.
+ */
+void kcsan_atomic_next(int n);
+
+#else /* CONFIG_KCSAN */
+
 static inline void __kcsan_check_access(const volatile void *ptr, size_t size,
 					int type) { }
-#endif
+
+static inline void kcsan_nestable_atomic_begin(void)	{ }
+static inline void kcsan_nestable_atomic_end(void)	{ }
+static inline void kcsan_flat_atomic_begin(void)	{ }
+static inline void kcsan_flat_atomic_end(void)		{ }
+static inline void kcsan_atomic_next(int n)		{ }
+
+#endif /* CONFIG_KCSAN */
 
 /*
  * kcsan_*: Only calls into the runtime when the particular compilation unit has
diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
index 1019e3a2c6897..7a614ca558f65 100644
--- a/include/linux/kcsan.h
+++ b/include/linux/kcsan.h
@@ -56,52 +56,11 @@ void kcsan_disable_current(void);
  */
 void kcsan_enable_current(void);
 
-/**
- * kcsan_nestable_atomic_begin - begin nestable atomic region
- *
- * Accesses within the atomic region may appear to race with other accesses but
- * should be considered atomic.
- */
-void kcsan_nestable_atomic_begin(void);
-
-/**
- * kcsan_nestable_atomic_end - end nestable atomic region
- */
-void kcsan_nestable_atomic_end(void);
-
-/**
- * kcsan_flat_atomic_begin - begin flat atomic region
- *
- * Accesses within the atomic region may appear to race with other accesses but
- * should be considered atomic.
- */
-void kcsan_flat_atomic_begin(void);
-
-/**
- * kcsan_flat_atomic_end - end flat atomic region
- */
-void kcsan_flat_atomic_end(void);
-
-/**
- * kcsan_atomic_next - consider following accesses as atomic
- *
- * Force treating the next n memory accesses for the current context as atomic
- * operations.
- *
- * @n number of following memory accesses to treat as atomic.
- */
-void kcsan_atomic_next(int n);
-
 #else /* CONFIG_KCSAN */
 
 static inline void kcsan_init(void)			{ }
 static inline void kcsan_disable_current(void)		{ }
 static inline void kcsan_enable_current(void)		{ }
-static inline void kcsan_nestable_atomic_begin(void)	{ }
-static inline void kcsan_nestable_atomic_end(void)	{ }
-static inline void kcsan_flat_atomic_begin(void)	{ }
-static inline void kcsan_flat_atomic_end(void)		{ }
-static inline void kcsan_atomic_next(int n)		{ }
 
 #endif /* CONFIG_KCSAN */
 
-- 
2.25.0.225.g125e21ebc7-goog

[PATCH v2 2/5] compiler.h, seqlock.h: Remove unnecessary kcsan.h includes

[Marco Elver]

No we longer have to include kcsan.h, since the required KCSAN interface
for both compiler.h and seqlock.h are now provided by kcsan-checks.h.

Signed-off-by: Marco Elver <elver@google.com>
---
 include/linux/compiler.h | 2 --
 include/linux/seqlock.h  | 2 +-
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index c1bdf37571cb8..f504edebd5d71 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -313,8 +313,6 @@ unsigned long read_word_at_a_time(const void *addr)
 	__u.__val;					\
 })
 
-#include <linux/kcsan.h>
-
 /**
  * data_race - mark an expression as containing intentional data races
  *
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 239701cae3764..8b97204f35a77 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -37,7 +37,7 @@
 #include <linux/preempt.h>
 #include <linux/lockdep.h>
 #include <linux/compiler.h>
-#include <linux/kcsan.h>
+#include <linux/kcsan-checks.h>
 #include <asm/processor.h>
 
 /*
-- 
2.25.0.225.g125e21ebc7-goog

[PATCH v2 3/5] kcsan: Introduce kcsan_value_change type

[Marco Elver]

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.

Signed-off-by: Marco Elver <elver@google.com>
---
 kernel/kcsan/core.c   | 38 ++++++++++++++++++++++----------------
 kernel/kcsan/kcsan.h  | 19 ++++++++++++++++++-
 kernel/kcsan/report.c | 26 ++++++++++++++------------
 3 files changed, 54 insertions(+), 29 deletions(-)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 498b1eb3c1cda..3f89801161d33 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -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);
 	}
diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
index 50078e7d43c32..83a79b08b550e 100644
--- a/kernel/kcsan/kcsan.h
+++ b/kernel/kcsan/kcsan.h
@@ -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 */
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index abf6852dff72f..d871476dc1348 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -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;
 
-- 
2.25.0.225.g125e21ebc7-goog

[PATCH v2 4/5] kcsan: Add kcsan_set_access_mask() support

[Marco Elver]

When setting up an access mask with kcsan_set_access_mask(), KCSAN will
only report races if concurrent changes to bits set in access_mask are
observed. Conveying access_mask via a separate call avoids introducing
overhead in the common-case fast-path.

Signed-off-by: Marco Elver <elver@google.com>
---
 include/linux/kcsan-checks.h | 11 +++++++++
 include/linux/kcsan.h        |  5 +++++
 init/init_task.c             |  1 +
 kernel/kcsan/core.c          | 43 ++++++++++++++++++++++++++++++++----
 kernel/kcsan/kcsan.h         |  5 +++++
 kernel/kcsan/report.c        | 13 ++++++++++-
 6 files changed, 73 insertions(+), 5 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 8675411c8dbcd..4ef5233ff3f04 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -68,6 +68,16 @@ void kcsan_flat_atomic_end(void);
  */
 void kcsan_atomic_next(int n);
 
+/**
+ * kcsan_set_access_mask - set access mask
+ *
+ * Set the access mask for all accesses for the current context if non-zero.
+ * Only value changes to bits set in the mask will be reported.
+ *
+ * @mask bitmask
+ */
+void kcsan_set_access_mask(unsigned long mask);
+
 #else /* CONFIG_KCSAN */
 
 static inline void __kcsan_check_access(const volatile void *ptr, size_t size,
@@ -78,6 +88,7 @@ static inline void kcsan_nestable_atomic_end(void)	{ }
 static inline void kcsan_flat_atomic_begin(void)	{ }
 static inline void kcsan_flat_atomic_end(void)		{ }
 static inline void kcsan_atomic_next(int n)		{ }
+static inline void kcsan_set_access_mask(unsigned long mask) { }
 
 #endif /* CONFIG_KCSAN */
 
diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
index 7a614ca558f65..3b84606e1e675 100644
--- a/include/linux/kcsan.h
+++ b/include/linux/kcsan.h
@@ -35,6 +35,11 @@ struct kcsan_ctx {
 	 */
 	int atomic_nest_count;
 	bool in_flat_atomic;
+
+	/*
+	 * Access mask for all accesses if non-zero.
+	 */
+	unsigned long access_mask;
 };
 
 /**
diff --git a/init/init_task.c b/init/init_task.c
index 2b4fe98b0f095..096191d177d5c 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -167,6 +167,7 @@ struct task_struct init_task
 		.atomic_next		= 0,
 		.atomic_nest_count	= 0,
 		.in_flat_atomic		= false,
+		.access_mask		= 0,
 	},
 #endif
 #ifdef CONFIG_TRACE_IRQFLAGS
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 3f89801161d33..589b1e7f0f253 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -39,6 +39,7 @@ static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = {
 	.atomic_next		= 0,
 	.atomic_nest_count	= 0,
 	.in_flat_atomic		= false,
+	.access_mask		= 0,
 };
 
 /*
@@ -298,6 +299,15 @@ static noinline void kcsan_found_watchpoint(const volatile void *ptr,
 
 	if (!kcsan_is_enabled())
 		return;
+
+	/*
+	 * The access_mask check relies on value-change comparison. To avoid
+	 * reporting a race where e.g. the writer set up the watchpoint, but the
+	 * reader has access_mask!=0, we have to ignore the found watchpoint.
+	 */
+	if (get_ctx()->access_mask != 0)
+		return;
+
 	/*
 	 * Consume the watchpoint as soon as possible, to minimize the chances
 	 * of !consumed. Consuming the watchpoint must always be guarded by
@@ -341,6 +351,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 		u32 _4;
 		u64 _8;
 	} expect_value;
+	unsigned long access_mask;
 	enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
 	unsigned long ua_flags = user_access_save();
 	unsigned long irq_flags;
@@ -435,18 +446,27 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	 * Re-read value, and check if it is as expected; if not, we infer a
 	 * racy access.
 	 */
+	access_mask = get_ctx()->access_mask;
 	switch (size) {
 	case 1:
 		expect_value._1 ^= READ_ONCE(*(const u8 *)ptr);
+		if (access_mask)
+			expect_value._1 &= (u8)access_mask;
 		break;
 	case 2:
 		expect_value._2 ^= READ_ONCE(*(const u16 *)ptr);
+		if (access_mask)
+			expect_value._2 &= (u16)access_mask;
 		break;
 	case 4:
 		expect_value._4 ^= READ_ONCE(*(const u32 *)ptr);
+		if (access_mask)
+			expect_value._4 &= (u32)access_mask;
 		break;
 	case 8:
 		expect_value._8 ^= READ_ONCE(*(const u64 *)ptr);
+		if (access_mask)
+			expect_value._8 &= (u64)access_mask;
 		break;
 	default:
 		break; /* ignore; we do not diff the values */
@@ -460,11 +480,20 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	if (!remove_watchpoint(watchpoint)) {
 		/*
 		 * Depending on the access type, map a value_change of MAYBE to
-		 * TRUE (require reporting).
+		 * TRUE (always report) or FALSE (never report).
 		 */
-		if (value_change == KCSAN_VALUE_CHANGE_MAYBE && (size > 8 || is_assert)) {
-			/* Always assume a value-change. */
-			value_change = KCSAN_VALUE_CHANGE_TRUE;
+		if (value_change == KCSAN_VALUE_CHANGE_MAYBE) {
+			if (access_mask != 0) {
+				/*
+				 * For access with access_mask, we require a
+				 * value-change, as it is likely that races on
+				 * ~access_mask bits are expected.
+				 */
+				value_change = KCSAN_VALUE_CHANGE_FALSE;
+			} else if (size > 8 || is_assert) {
+				/* Always assume a value-change. */
+				value_change = KCSAN_VALUE_CHANGE_TRUE;
+			}
 		}
 
 		/*
@@ -622,6 +651,12 @@ void kcsan_atomic_next(int n)
 }
 EXPORT_SYMBOL(kcsan_atomic_next);
 
+void kcsan_set_access_mask(unsigned long mask)
+{
+	get_ctx()->access_mask = mask;
+}
+EXPORT_SYMBOL(kcsan_set_access_mask);
+
 void __kcsan_check_access(const volatile void *ptr, size_t size, int type)
 {
 	check_access(ptr, size, type);
diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
index 83a79b08b550e..892de5120c1b6 100644
--- a/kernel/kcsan/kcsan.h
+++ b/kernel/kcsan/kcsan.h
@@ -98,6 +98,11 @@ enum kcsan_value_change {
 	 */
 	KCSAN_VALUE_CHANGE_MAYBE,
 
+	/*
+	 * Did not observe a value-change, and it is invalid to report the race.
+	 */
+	KCSAN_VALUE_CHANGE_FALSE,
+
 	/*
 	 * The value was observed to change, and the race should be reported.
 	 */
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index d871476dc1348..11c791b886f3c 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -132,6 +132,9 @@ static bool rate_limit_report(unsigned long frame1, unsigned long frame2)
 static bool
 skip_report(enum kcsan_value_change value_change, unsigned long top_frame)
 {
+	/* Should never get here if value_change==FALSE. */
+	WARN_ON_ONCE(value_change == KCSAN_VALUE_CHANGE_FALSE);
+
 	/*
 	 * 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
@@ -493,7 +496,15 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,
 
 	kcsan_disable_current();
 	if (prepare_report(&flags, ptr, size, access_type, cpu_id, type)) {
-		if (print_report(ptr, size, access_type, value_change, cpu_id, type) && panic_on_warn)
+		/*
+		 * Never report if value_change is FALSE, only if we it is
+		 * either TRUE or MAYBE. In case of MAYBE, further filtering may
+		 * be done once we know the full stack trace in print_report().
+		 */
+		bool reported = value_change != KCSAN_VALUE_CHANGE_FALSE &&
+				print_report(ptr, size, access_type, value_change, cpu_id, type);
+
+		if (reported && panic_on_warn)
 			panic("panic_on_warn set ...\n");
 
 		release_report(&flags, type);
-- 
2.25.0.225.g125e21ebc7-goog

[PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

[Marco Elver]

This introduces ASSERT_EXCLUSIVE_BITS(var, mask).
ASSERT_EXCLUSIVE_BITS(var, mask) will cause KCSAN to assume that the
following access is safe w.r.t. data races (however, please see the
docbook comment for disclaimer here).

For more context on why this was considered necessary, please see:
  http://lkml.kernel.org/r/1580995070-25139-1-git-send-email-cai@lca.pw

In particular, before this patch, data races between reads (that use
@mask bits of an access that should not be modified concurrently) and
writes (that change ~@mask bits not used by the readers) would have been
annotated with "data_race()" (or "READ_ONCE()"). However, doing so would
then hide real problems: we would no longer be able to detect harmful
races between reads to @mask bits and writes to @mask bits.

Therefore, by using ASSERT_EXCLUSIVE_BITS(var, mask), we accomplish:

  1. Avoid proliferation of specific macros at the call sites: by
     including a single mask in the argument list, we can use the same
     macro in a wide variety of call sites, regardless of how and which
     bits in a field each call site actually accesses.

  2. The existing code does not need to be modified (although READ_ONCE()
     may still be advisable if we cannot prove that the data race is
     always safe).

  3. We catch bugs where the exclusive bits are modified concurrently.

  4. We document properties of the current code.

Signed-off-by: Marco Elver <elver@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Qian Cai <cai@lca.pw>
---
v2:
* Update API documentation to be clearer about how this compares to the
  existing assertions, and update use-cases. [Based on suggestions from
  John Hubbard]
* Update commit message. [Suggestions from John Hubbard]
---
 include/linux/kcsan-checks.h | 69 ++++++++++++++++++++++++++++++++----
 kernel/kcsan/debugfs.c       | 15 +++++++-
 2 files changed, 77 insertions(+), 7 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 4ef5233ff3f04..1b8aac5d6a0b5 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -152,9 +152,9 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
 #endif
 
 /**
- * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
+ * ASSERT_EXCLUSIVE_WRITER - assert no concurrent writes to @var
  *
- * Assert that there are no other threads writing @var; other readers are
+ * Assert that there are no concurrent writes to @var; other readers are
  * allowed. This assertion can be used to specify properties of concurrent code,
  * where violation cannot be detected as a normal data race.
  *
@@ -171,11 +171,11 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
 	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
 
 /**
- * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
+ * ASSERT_EXCLUSIVE_ACCESS - assert no concurrent accesses to @var
  *
- * Assert that no other thread is accessing @var (no readers nor writers). This
- * assertion can be used to specify properties of concurrent code, where
- * violation cannot be detected as a normal data race.
+ * Assert that there are no concurrent accesses to @var (no readers nor
+ * writers). This assertion can be used to specify properties of concurrent
+ * code, where violation cannot be detected as a normal data race.
  *
  * For example, in a reference-counting algorithm where exclusive access is
  * expected after the refcount reaches 0. We can check that this property
@@ -191,4 +191,61 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
 #define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
 	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
 
+/**
+ * ASSERT_EXCLUSIVE_BITS - assert no concurrent writes to subset of bits in @var
+ *
+ * Bit-granular variant of ASSERT_EXCLUSIVE_WRITER(var).
+ *
+ * Assert that there are no concurrent writes to a subset of bits in @var;
+ * concurrent readers are permitted. This assertion captures more detailed
+ * bit-level properties, compared to the other (word granularity) assertions.
+ * Only the bits set in @mask are checked for concurrent modifications, while
+ * ignoring the remaining bits, i.e. concurrent writes (or reads) to ~@mask bits
+ * are ignored.
+ *
+ * Use this for variables, where some bits must not be modified concurrently,
+ * yet other bits are expected to be modified concurrently.
+ *
+ * For example, variables where, after initialization, some bits are read-only,
+ * but other bits may still be modified concurrently. A reader may wish to
+ * assert that this is true as follows:
+ *
+ *	ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
+ *	foo = (READ_ONCE(flags) & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
+ *
+ *   Note: The access that immediately follows ASSERT_EXCLUSIVE_BITS() is
+ *   assumed to access the masked bits only, and KCSAN optimistically assumes it
+ *   is therefore safe, even in the presence of data races, and marking it with
+ *   READ_ONCE() is optional from KCSAN's point-of-view. We caution, however,
+ *   that it may still be advisable to do so, since we cannot reason about all
+ *   compiler optimizations when it comes to bit manipulations (on the reader
+ *   and writer side). If you are sure nothing can go wrong, we can write the
+ *   above simply as:
+ *
+ * 	ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
+ *	foo = (flags & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
+ *
+ * Another example, where this may be used, is when certain bits of @var may
+ * only be modified when holding the appropriate lock, but other bits may still
+ * be modified concurrently. Writers, where other bits may change concurrently,
+ * could use the assertion as follows:
+ *
+ *	spin_lock(&foo_lock);
+ *	ASSERT_EXCLUSIVE_BITS(flags, FOO_MASK);
+ *	old_flags = READ_ONCE(flags);
+ *	new_flags = (old_flags & ~FOO_MASK) | (new_foo << FOO_SHIFT);
+ *	if (cmpxchg(&flags, old_flags, new_flags) != old_flags) { ... }
+ *	spin_unlock(&foo_lock);
+ *
+ * @var variable to assert on
+ * @mask only check for modifications to bits set in @mask
+ */
+#define ASSERT_EXCLUSIVE_BITS(var, mask)                                       \
+	do {                                                                   \
+		kcsan_set_access_mask(mask);                                   \
+		__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT);\
+		kcsan_set_access_mask(0);                                      \
+		kcsan_atomic_next(1);                                          \
+	} while (0)
+
 #endif /* _LINUX_KCSAN_CHECKS_H */
diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
index 9bbba0e57c9b3..2ff1961239778 100644
--- a/kernel/kcsan/debugfs.c
+++ b/kernel/kcsan/debugfs.c
@@ -100,8 +100,10 @@ static noinline void microbenchmark(unsigned long iters)
  * debugfs file from multiple tasks to generate real conflicts and show reports.
  */
 static long test_dummy;
+static long test_flags;
 static noinline void test_thread(unsigned long iters)
 {
+	const long CHANGE_BITS = 0xff00ff00ff00ff00L;
 	const struct kcsan_ctx ctx_save = current->kcsan_ctx;
 	cycles_t cycles;
 
@@ -109,16 +111,27 @@ static noinline void test_thread(unsigned long iters)
 	memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
 
 	pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);
+	pr_info("test_dummy@%px, test_flags@%px\n", &test_dummy, &test_flags);
 
 	cycles = get_cycles();
 	while (iters--) {
+		/* These all should generate reports. */
 		__kcsan_check_read(&test_dummy, sizeof(test_dummy));
-		__kcsan_check_write(&test_dummy, sizeof(test_dummy));
 		ASSERT_EXCLUSIVE_WRITER(test_dummy);
 		ASSERT_EXCLUSIVE_ACCESS(test_dummy);
 
+		ASSERT_EXCLUSIVE_BITS(test_flags, ~CHANGE_BITS); /* no report */
+		__kcsan_check_read(&test_flags, sizeof(test_flags)); /* no report */
+
+		ASSERT_EXCLUSIVE_BITS(test_flags, CHANGE_BITS); /* report */
+		__kcsan_check_read(&test_flags, sizeof(test_flags)); /* no report */
+
 		/* not actually instrumented */
 		WRITE_ONCE(test_dummy, iters);  /* to observe value-change */
+		__kcsan_check_write(&test_dummy, sizeof(test_dummy));
+
+		test_flags ^= CHANGE_BITS; /* generate value-change */
+		__kcsan_check_write(&test_flags, sizeof(test_flags));
 	}
 	cycles = get_cycles() - cycles;
 
-- 
2.25.0.225.g125e21ebc7-goog

Re: [PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

[John Hubbard]

On 2/11/20 8:04 AM, Marco Elver wrote:
> This introduces ASSERT_EXCLUSIVE_BITS(var, mask).
> ASSERT_EXCLUSIVE_BITS(var, mask) will cause KCSAN to assume that the
> following access is safe w.r.t. data races (however, please see the
> docbook comment for disclaimer here).
> 
> For more context on why this was considered necessary, please see:
>   http://lkml.kernel.org/r/1580995070-25139-1-git-send-email-cai@lca.pw
> 
> In particular, before this patch, data races between reads (that use
> @mask bits of an access that should not be modified concurrently) and
> writes (that change ~@mask bits not used by the readers) would have been
> annotated with "data_race()" (or "READ_ONCE()"). However, doing so would
> then hide real problems: we would no longer be able to detect harmful
> races between reads to @mask bits and writes to @mask bits.
> 
> Therefore, by using ASSERT_EXCLUSIVE_BITS(var, mask), we accomplish:
> 
>   1. Avoid proliferation of specific macros at the call sites: by
>      including a single mask in the argument list, we can use the same
>      macro in a wide variety of call sites, regardless of how and which
>      bits in a field each call site actually accesses.
> 
>   2. The existing code does not need to be modified (although READ_ONCE()
>      may still be advisable if we cannot prove that the data race is
>      always safe).
> 
>   3. We catch bugs where the exclusive bits are modified concurrently.
> 
>   4. We document properties of the current code.


API looks good to me. (I'm not yet familiar enough with KCSAN to provide
any useful review of about the various kcsan*() calls that implement the 
new macro.)

btw, it might be helpful for newcomers if you mentioned which tree this
is based on. I poked around briefly and failed several times to find one. :)

You can add:

Acked-by: John Hubbard <jhubbard@nvidia.com>


thanks,
-- 
John Hubbard
NVIDIA
> 
> Signed-off-by: Marco Elver <elver@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Qian Cai <cai@lca.pw>
> ---
> v2:
> * Update API documentation to be clearer about how this compares to the
>   existing assertions, and update use-cases. [Based on suggestions from
>   John Hubbard]
> * Update commit message. [Suggestions from John Hubbard]
> ---
>  include/linux/kcsan-checks.h | 69 ++++++++++++++++++++++++++++++++----
>  kernel/kcsan/debugfs.c       | 15 +++++++-
>  2 files changed, 77 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> index 4ef5233ff3f04..1b8aac5d6a0b5 100644
> --- a/include/linux/kcsan-checks.h
> +++ b/include/linux/kcsan-checks.h
> @@ -152,9 +152,9 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
>  #endif
>  
>  /**
> - * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
> + * ASSERT_EXCLUSIVE_WRITER - assert no concurrent writes to @var
>   *
> - * Assert that there are no other threads writing @var; other readers are
> + * Assert that there are no concurrent writes to @var; other readers are
>   * allowed. This assertion can be used to specify properties of concurrent code,
>   * where violation cannot be detected as a normal data race.
>   *
> @@ -171,11 +171,11 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
>  	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
>  
>  /**
> - * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
> + * ASSERT_EXCLUSIVE_ACCESS - assert no concurrent accesses to @var
>   *
> - * Assert that no other thread is accessing @var (no readers nor writers). This
> - * assertion can be used to specify properties of concurrent code, where
> - * violation cannot be detected as a normal data race.
> + * Assert that there are no concurrent accesses to @var (no readers nor
> + * writers). This assertion can be used to specify properties of concurrent
> + * code, where violation cannot be detected as a normal data race.
>   *
>   * For example, in a reference-counting algorithm where exclusive access is
>   * expected after the refcount reaches 0. We can check that this property
> @@ -191,4 +191,61 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
>  #define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
>  	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
>  
> +/**
> + * ASSERT_EXCLUSIVE_BITS - assert no concurrent writes to subset of bits in @var
> + *
> + * Bit-granular variant of ASSERT_EXCLUSIVE_WRITER(var).
> + *
> + * Assert that there are no concurrent writes to a subset of bits in @var;
> + * concurrent readers are permitted. This assertion captures more detailed
> + * bit-level properties, compared to the other (word granularity) assertions.
> + * Only the bits set in @mask are checked for concurrent modifications, while
> + * ignoring the remaining bits, i.e. concurrent writes (or reads) to ~@mask bits
> + * are ignored.
> + *
> + * Use this for variables, where some bits must not be modified concurrently,
> + * yet other bits are expected to be modified concurrently.
> + *
> + * For example, variables where, after initialization, some bits are read-only,
> + * but other bits may still be modified concurrently. A reader may wish to
> + * assert that this is true as follows:
> + *
> + *	ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
> + *	foo = (READ_ONCE(flags) & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
> + *
> + *   Note: The access that immediately follows ASSERT_EXCLUSIVE_BITS() is
> + *   assumed to access the masked bits only, and KCSAN optimistically assumes it
> + *   is therefore safe, even in the presence of data races, and marking it with
> + *   READ_ONCE() is optional from KCSAN's point-of-view. We caution, however,
> + *   that it may still be advisable to do so, since we cannot reason about all
> + *   compiler optimizations when it comes to bit manipulations (on the reader
> + *   and writer side). If you are sure nothing can go wrong, we can write the
> + *   above simply as:
> + *
> + * 	ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
> + *	foo = (flags & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
> + *
> + * Another example, where this may be used, is when certain bits of @var may
> + * only be modified when holding the appropriate lock, but other bits may still
> + * be modified concurrently. Writers, where other bits may change concurrently,
> + * could use the assertion as follows:
> + *
> + *	spin_lock(&foo_lock);
> + *	ASSERT_EXCLUSIVE_BITS(flags, FOO_MASK);
> + *	old_flags = READ_ONCE(flags);
> + *	new_flags = (old_flags & ~FOO_MASK) | (new_foo << FOO_SHIFT);
> + *	if (cmpxchg(&flags, old_flags, new_flags) != old_flags) { ... }
> + *	spin_unlock(&foo_lock);
> + *
> + * @var variable to assert on
> + * @mask only check for modifications to bits set in @mask
> + */
> +#define ASSERT_EXCLUSIVE_BITS(var, mask)                                       \
> +	do {                                                                   \
> +		kcsan_set_access_mask(mask);                                   \
> +		__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT);\
> +		kcsan_set_access_mask(0);                                      \
> +		kcsan_atomic_next(1);                                          \
> +	} while (0)
> +
>  #endif /* _LINUX_KCSAN_CHECKS_H */
> diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
> index 9bbba0e57c9b3..2ff1961239778 100644
> --- a/kernel/kcsan/debugfs.c
> +++ b/kernel/kcsan/debugfs.c
> @@ -100,8 +100,10 @@ static noinline void microbenchmark(unsigned long iters)
>   * debugfs file from multiple tasks to generate real conflicts and show reports.
>   */
>  static long test_dummy;
> +static long test_flags;
>  static noinline void test_thread(unsigned long iters)
>  {
> +	const long CHANGE_BITS = 0xff00ff00ff00ff00L;
>  	const struct kcsan_ctx ctx_save = current->kcsan_ctx;
>  	cycles_t cycles;
>  
> @@ -109,16 +111,27 @@ static noinline void test_thread(unsigned long iters)
>  	memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
>  
>  	pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);
> +	pr_info("test_dummy@%px, test_flags@%px\n", &test_dummy, &test_flags);
>  
>  	cycles = get_cycles();
>  	while (iters--) {
> +		/* These all should generate reports. */
>  		__kcsan_check_read(&test_dummy, sizeof(test_dummy));
> -		__kcsan_check_write(&test_dummy, sizeof(test_dummy));
>  		ASSERT_EXCLUSIVE_WRITER(test_dummy);
>  		ASSERT_EXCLUSIVE_ACCESS(test_dummy);
>  
> +		ASSERT_EXCLUSIVE_BITS(test_flags, ~CHANGE_BITS); /* no report */
> +		__kcsan_check_read(&test_flags, sizeof(test_flags)); /* no report */
> +
> +		ASSERT_EXCLUSIVE_BITS(test_flags, CHANGE_BITS); /* report */
> +		__kcsan_check_read(&test_flags, sizeof(test_flags)); /* no report */
> +
>  		/* not actually instrumented */
>  		WRITE_ONCE(test_dummy, iters);  /* to observe value-change */
> +		__kcsan_check_write(&test_dummy, sizeof(test_dummy));
> +
> +		test_flags ^= CHANGE_BITS; /* generate value-change */
> +		__kcsan_check_write(&test_flags, sizeof(test_flags));
>  	}
>  	cycles = get_cycles() - cycles;
>  
> 

Re: [PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

[Marco Elver]

On Tue, 11 Feb 2020 at 22:41, John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 2/11/20 8:04 AM, Marco Elver wrote:
> > This introduces ASSERT_EXCLUSIVE_BITS(var, mask).
> > ASSERT_EXCLUSIVE_BITS(var, mask) will cause KCSAN to assume that the
> > following access is safe w.r.t. data races (however, please see the
> > docbook comment for disclaimer here).
> >
> > For more context on why this was considered necessary, please see:
> >   http://lkml.kernel.org/r/1580995070-25139-1-git-send-email-cai@lca.pw
> >
> > In particular, before this patch, data races between reads (that use
> > @mask bits of an access that should not be modified concurrently) and
> > writes (that change ~@mask bits not used by the readers) would have been
> > annotated with "data_race()" (or "READ_ONCE()"). However, doing so would
> > then hide real problems: we would no longer be able to detect harmful
> > races between reads to @mask bits and writes to @mask bits.
> >
> > Therefore, by using ASSERT_EXCLUSIVE_BITS(var, mask), we accomplish:
> >
> >   1. Avoid proliferation of specific macros at the call sites: by
> >      including a single mask in the argument list, we can use the same
> >      macro in a wide variety of call sites, regardless of how and which
> >      bits in a field each call site actually accesses.
> >
> >   2. The existing code does not need to be modified (although READ_ONCE()
> >      may still be advisable if we cannot prove that the data race is
> >      always safe).
> >
> >   3. We catch bugs where the exclusive bits are modified concurrently.
> >
> >   4. We document properties of the current code.
>
>
> API looks good to me. (I'm not yet familiar enough with KCSAN to provide
> any useful review of about the various kcsan*() calls that implement the
> new macro.)
>
> btw, it might be helpful for newcomers if you mentioned which tree this
> is based on. I poked around briefly and failed several times to find one. :)

KCSAN is currently in -rcu (kcsan branch has the latest version),
-tip, and -next.

> You can add:
>
> Acked-by: John Hubbard <jhubbard@nvidia.com>

Thank you!
-- Marco

>
> thanks,
> --
> John Hubbard
> NVIDIA
> >
> > Signed-off-by: Marco Elver <elver@google.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Jan Kara <jack@suse.cz>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Qian Cai <cai@lca.pw>
> > ---
> > v2:
> > * Update API documentation to be clearer about how this compares to the
> >   existing assertions, and update use-cases. [Based on suggestions from
> >   John Hubbard]
> > * Update commit message. [Suggestions from John Hubbard]
> > ---
> >  include/linux/kcsan-checks.h | 69 ++++++++++++++++++++++++++++++++----
> >  kernel/kcsan/debugfs.c       | 15 +++++++-
> >  2 files changed, 77 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> > index 4ef5233ff3f04..1b8aac5d6a0b5 100644
> > --- a/include/linux/kcsan-checks.h
> > +++ b/include/linux/kcsan-checks.h
> > @@ -152,9 +152,9 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> >  #endif
> >
> >  /**
> > - * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
> > + * ASSERT_EXCLUSIVE_WRITER - assert no concurrent writes to @var
> >   *
> > - * Assert that there are no other threads writing @var; other readers are
> > + * Assert that there are no concurrent writes to @var; other readers are
> >   * allowed. This assertion can be used to specify properties of concurrent code,
> >   * where violation cannot be detected as a normal data race.
> >   *
> > @@ -171,11 +171,11 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> >       __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
> >
> >  /**
> > - * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
> > + * ASSERT_EXCLUSIVE_ACCESS - assert no concurrent accesses to @var
> >   *
> > - * Assert that no other thread is accessing @var (no readers nor writers). This
> > - * assertion can be used to specify properties of concurrent code, where
> > - * violation cannot be detected as a normal data race.
> > + * Assert that there are no concurrent accesses to @var (no readers nor
> > + * writers). This assertion can be used to specify properties of concurrent
> > + * code, where violation cannot be detected as a normal data race.
> >   *
> >   * For example, in a reference-counting algorithm where exclusive access is
> >   * expected after the refcount reaches 0. We can check that this property
> > @@ -191,4 +191,61 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> >  #define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
> >       __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
> >
> > +/**
> > + * ASSERT_EXCLUSIVE_BITS - assert no concurrent writes to subset of bits in @var
> > + *
> > + * Bit-granular variant of ASSERT_EXCLUSIVE_WRITER(var).
> > + *
> > + * Assert that there are no concurrent writes to a subset of bits in @var;
> > + * concurrent readers are permitted. This assertion captures more detailed
> > + * bit-level properties, compared to the other (word granularity) assertions.
> > + * Only the bits set in @mask are checked for concurrent modifications, while
> > + * ignoring the remaining bits, i.e. concurrent writes (or reads) to ~@mask bits
> > + * are ignored.
> > + *
> > + * Use this for variables, where some bits must not be modified concurrently,
> > + * yet other bits are expected to be modified concurrently.
> > + *
> > + * For example, variables where, after initialization, some bits are read-only,
> > + * but other bits may still be modified concurrently. A reader may wish to
> > + * assert that this is true as follows:
> > + *
> > + *   ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
> > + *   foo = (READ_ONCE(flags) & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
> > + *
> > + *   Note: The access that immediately follows ASSERT_EXCLUSIVE_BITS() is
> > + *   assumed to access the masked bits only, and KCSAN optimistically assumes it
> > + *   is therefore safe, even in the presence of data races, and marking it with
> > + *   READ_ONCE() is optional from KCSAN's point-of-view. We caution, however,
> > + *   that it may still be advisable to do so, since we cannot reason about all
> > + *   compiler optimizations when it comes to bit manipulations (on the reader
> > + *   and writer side). If you are sure nothing can go wrong, we can write the
> > + *   above simply as:
> > + *
> > + *   ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
> > + *   foo = (flags & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
> > + *
> > + * Another example, where this may be used, is when certain bits of @var may
> > + * only be modified when holding the appropriate lock, but other bits may still
> > + * be modified concurrently. Writers, where other bits may change concurrently,
> > + * could use the assertion as follows:
> > + *
> > + *   spin_lock(&foo_lock);
> > + *   ASSERT_EXCLUSIVE_BITS(flags, FOO_MASK);
> > + *   old_flags = READ_ONCE(flags);
> > + *   new_flags = (old_flags & ~FOO_MASK) | (new_foo << FOO_SHIFT);
> > + *   if (cmpxchg(&flags, old_flags, new_flags) != old_flags) { ... }
> > + *   spin_unlock(&foo_lock);
> > + *
> > + * @var variable to assert on
> > + * @mask only check for modifications to bits set in @mask
> > + */
> > +#define ASSERT_EXCLUSIVE_BITS(var, mask)                                       \
> > +     do {                                                                   \
> > +             kcsan_set_access_mask(mask);                                   \
> > +             __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT);\
> > +             kcsan_set_access_mask(0);                                      \
> > +             kcsan_atomic_next(1);                                          \
> > +     } while (0)
> > +
> >  #endif /* _LINUX_KCSAN_CHECKS_H */
> > diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
> > index 9bbba0e57c9b3..2ff1961239778 100644
> > --- a/kernel/kcsan/debugfs.c
> > +++ b/kernel/kcsan/debugfs.c
> > @@ -100,8 +100,10 @@ static noinline void microbenchmark(unsigned long iters)
> >   * debugfs file from multiple tasks to generate real conflicts and show reports.
> >   */
> >  static long test_dummy;
> > +static long test_flags;
> >  static noinline void test_thread(unsigned long iters)
> >  {
> > +     const long CHANGE_BITS = 0xff00ff00ff00ff00L;
> >       const struct kcsan_ctx ctx_save = current->kcsan_ctx;
> >       cycles_t cycles;
> >
> > @@ -109,16 +111,27 @@ static noinline void test_thread(unsigned long iters)
> >       memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
> >
> >       pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);
> > +     pr_info("test_dummy@%px, test_flags@%px\n", &test_dummy, &test_flags);
> >
> >       cycles = get_cycles();
> >       while (iters--) {
> > +             /* These all should generate reports. */
> >               __kcsan_check_read(&test_dummy, sizeof(test_dummy));
> > -             __kcsan_check_write(&test_dummy, sizeof(test_dummy));
> >               ASSERT_EXCLUSIVE_WRITER(test_dummy);
> >               ASSERT_EXCLUSIVE_ACCESS(test_dummy);
> >
> > +             ASSERT_EXCLUSIVE_BITS(test_flags, ~CHANGE_BITS); /* no report */
> > +             __kcsan_check_read(&test_flags, sizeof(test_flags)); /* no report */
> > +
> > +             ASSERT_EXCLUSIVE_BITS(test_flags, CHANGE_BITS); /* report */
> > +             __kcsan_check_read(&test_flags, sizeof(test_flags)); /* no report */
> > +
> >               /* not actually instrumented */
> >               WRITE_ONCE(test_dummy, iters);  /* to observe value-change */
> > +             __kcsan_check_write(&test_dummy, sizeof(test_dummy));
> > +
> > +             test_flags ^= CHANGE_BITS; /* generate value-change */
> > +             __kcsan_check_write(&test_flags, sizeof(test_flags));
> >       }
> >       cycles = get_cycles() - cycles;
> >
> >
>
> --
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Re: [PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

[Qian Cai]

> On Feb 12, 2020, at 5:57 AM, Marco Elver <elver@google.com> wrote:
> 
> KCSAN is currently in -rcu (kcsan branch has the latest version),
> -tip, and -next.

It would like be nice to at least have this patchset can be applied against the linux-next, so I can try it a spin.

Maybe a better question to Paul if he could push all the latest kcsan code base to linux-next soon since we are now past the merging window. I also noticed some data races in rcu but only found out some of them had already been fixed in rcu tree but not in linux-next.

Re: [PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

[Paul E. McKenney]

On Wed, Feb 12, 2020 at 07:30:16AM -0500, Qian Cai wrote:
> 
> 
> > On Feb 12, 2020, at 5:57 AM, Marco Elver <elver@google.com> wrote:
> > 
> > KCSAN is currently in -rcu (kcsan branch has the latest version),
> > -tip, and -next.
> 
> It would like be nice to at least have this patchset can be applied against the linux-next, so I can try it a spin.
> 
> Maybe a better question to Paul if he could push all the latest kcsan code base to linux-next soon since we are now past the merging window. I also noticed some data races in rcu but only found out some of them had already been fixed in rcu tree but not in linux-next.

I have pushed all that I have queued other than the last set of five,
which I will do tomorrow (Prague time) if testing goes well.

Could you please check the -rcu "dev" branch to see if I am missing any
of the KCSAN patches?

							Thanx, Paul

Re: [PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

[Qian Cai]

> On Feb 12, 2020, at 4:40 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> On Wed, Feb 12, 2020 at 07:30:16AM -0500, Qian Cai wrote:
>> 
>> 
>>> On Feb 12, 2020, at 5:57 AM, Marco Elver <elver@google.com> wrote:
>>> 
>>> KCSAN is currently in -rcu (kcsan branch has the latest version),
>>> -tip, and -next.
>> 
>> It would like be nice to at least have this patchset can be applied against the linux-next, so I can try it a spin.
>> 
>> Maybe a better question to Paul if he could push all the latest kcsan code base to linux-next soon since we are now past the merging window. I also noticed some data races in rcu but only found out some of them had already been fixed in rcu tree but not in linux-next.
> 
> I have pushed all that I have queued other than the last set of five,
> which I will do tomorrow (Prague time) if testing goes well.
> 
> Could you please check the -rcu "dev" branch to see if I am missing any
> of the KCSAN patches?

Nope. It looks good to me.

Re: [PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

[Paul E. McKenney]

On Wed, Feb 12, 2020 at 07:48:15PM -0500, Qian Cai wrote:
> 
> 
> > On Feb 12, 2020, at 4:40 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
> > 
> > On Wed, Feb 12, 2020 at 07:30:16AM -0500, Qian Cai wrote:
> >> 
> >> 
> >>> On Feb 12, 2020, at 5:57 AM, Marco Elver <elver@google.com> wrote:
> >>> 
> >>> KCSAN is currently in -rcu (kcsan branch has the latest version),
> >>> -tip, and -next.
> >> 
> >> It would like be nice to at least have this patchset can be applied against the linux-next, so I can try it a spin.
> >> 
> >> Maybe a better question to Paul if he could push all the latest kcsan code base to linux-next soon since we are now past the merging window. I also noticed some data races in rcu but only found out some of them had already been fixed in rcu tree but not in linux-next.
> > 
> > I have pushed all that I have queued other than the last set of five,
> > which I will do tomorrow (Prague time) if testing goes well.
> > 
> > Could you please check the -rcu "dev" branch to see if I am missing any
> > of the KCSAN patches?
> 
> Nope. It looks good to me.

Thank you for checking!

							Thanx, Paul

Re: [PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

[Paul E. McKenney]

On Tue, Feb 11, 2020 at 01:41:14PM -0800, John Hubbard wrote:
> On 2/11/20 8:04 AM, Marco Elver wrote:
> > This introduces ASSERT_EXCLUSIVE_BITS(var, mask).
> > ASSERT_EXCLUSIVE_BITS(var, mask) will cause KCSAN to assume that the
> > following access is safe w.r.t. data races (however, please see the
> > docbook comment for disclaimer here).
> > 
> > For more context on why this was considered necessary, please see:
> >   http://lkml.kernel.org/r/1580995070-25139-1-git-send-email-cai@lca.pw
> > 
> > In particular, before this patch, data races between reads (that use
> > @mask bits of an access that should not be modified concurrently) and
> > writes (that change ~@mask bits not used by the readers) would have been
> > annotated with "data_race()" (or "READ_ONCE()"). However, doing so would
> > then hide real problems: we would no longer be able to detect harmful
> > races between reads to @mask bits and writes to @mask bits.
> > 
> > Therefore, by using ASSERT_EXCLUSIVE_BITS(var, mask), we accomplish:
> > 
> >   1. Avoid proliferation of specific macros at the call sites: by
> >      including a single mask in the argument list, we can use the same
> >      macro in a wide variety of call sites, regardless of how and which
> >      bits in a field each call site actually accesses.
> > 
> >   2. The existing code does not need to be modified (although READ_ONCE()
> >      may still be advisable if we cannot prove that the data race is
> >      always safe).
> > 
> >   3. We catch bugs where the exclusive bits are modified concurrently.
> > 
> >   4. We document properties of the current code.
> 
> 
> API looks good to me. (I'm not yet familiar enough with KCSAN to provide
> any useful review of about the various kcsan*() calls that implement the 
> new macro.)
> 
> btw, it might be helpful for newcomers if you mentioned which tree this
> is based on. I poked around briefly and failed several times to find one. :)
> 
> You can add:
> 
> Acked-by: John Hubbard <jhubbard@nvidia.com>

Queued for testing and further review, thank you both!

							Thanx, Paul

> thanks,
> -- 
> John Hubbard
> NVIDIA
> > 
> > Signed-off-by: Marco Elver <elver@google.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Jan Kara <jack@suse.cz>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Qian Cai <cai@lca.pw>
> > ---
> > v2:
> > * Update API documentation to be clearer about how this compares to the
> >   existing assertions, and update use-cases. [Based on suggestions from
> >   John Hubbard]
> > * Update commit message. [Suggestions from John Hubbard]
> > ---
> >  include/linux/kcsan-checks.h | 69 ++++++++++++++++++++++++++++++++----
> >  kernel/kcsan/debugfs.c       | 15 +++++++-
> >  2 files changed, 77 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> > index 4ef5233ff3f04..1b8aac5d6a0b5 100644
> > --- a/include/linux/kcsan-checks.h
> > +++ b/include/linux/kcsan-checks.h
> > @@ -152,9 +152,9 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> >  #endif
> >  
> >  /**
> > - * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
> > + * ASSERT_EXCLUSIVE_WRITER - assert no concurrent writes to @var
> >   *
> > - * Assert that there are no other threads writing @var; other readers are
> > + * Assert that there are no concurrent writes to @var; other readers are
> >   * allowed. This assertion can be used to specify properties of concurrent code,
> >   * where violation cannot be detected as a normal data race.
> >   *
> > @@ -171,11 +171,11 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> >  	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
> >  
> >  /**
> > - * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
> > + * ASSERT_EXCLUSIVE_ACCESS - assert no concurrent accesses to @var
> >   *
> > - * Assert that no other thread is accessing @var (no readers nor writers). This
> > - * assertion can be used to specify properties of concurrent code, where
> > - * violation cannot be detected as a normal data race.
> > + * Assert that there are no concurrent accesses to @var (no readers nor
> > + * writers). This assertion can be used to specify properties of concurrent
> > + * code, where violation cannot be detected as a normal data race.
> >   *
> >   * For example, in a reference-counting algorithm where exclusive access is
> >   * expected after the refcount reaches 0. We can check that this property
> > @@ -191,4 +191,61 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> >  #define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
> >  	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
> >  
> > +/**
> > + * ASSERT_EXCLUSIVE_BITS - assert no concurrent writes to subset of bits in @var
> > + *
> > + * Bit-granular variant of ASSERT_EXCLUSIVE_WRITER(var).
> > + *
> > + * Assert that there are no concurrent writes to a subset of bits in @var;
> > + * concurrent readers are permitted. This assertion captures more detailed
> > + * bit-level properties, compared to the other (word granularity) assertions.
> > + * Only the bits set in @mask are checked for concurrent modifications, while
> > + * ignoring the remaining bits, i.e. concurrent writes (or reads) to ~@mask bits
> > + * are ignored.
> > + *
> > + * Use this for variables, where some bits must not be modified concurrently,
> > + * yet other bits are expected to be modified concurrently.
> > + *
> > + * For example, variables where, after initialization, some bits are read-only,
> > + * but other bits may still be modified concurrently. A reader may wish to
> > + * assert that this is true as follows:
> > + *
> > + *	ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
> > + *	foo = (READ_ONCE(flags) & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
> > + *
> > + *   Note: The access that immediately follows ASSERT_EXCLUSIVE_BITS() is
> > + *   assumed to access the masked bits only, and KCSAN optimistically assumes it
> > + *   is therefore safe, even in the presence of data races, and marking it with
> > + *   READ_ONCE() is optional from KCSAN's point-of-view. We caution, however,
> > + *   that it may still be advisable to do so, since we cannot reason about all
> > + *   compiler optimizations when it comes to bit manipulations (on the reader
> > + *   and writer side). If you are sure nothing can go wrong, we can write the
> > + *   above simply as:
> > + *
> > + * 	ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
> > + *	foo = (flags & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
> > + *
> > + * Another example, where this may be used, is when certain bits of @var may
> > + * only be modified when holding the appropriate lock, but other bits may still
> > + * be modified concurrently. Writers, where other bits may change concurrently,
> > + * could use the assertion as follows:
> > + *
> > + *	spin_lock(&foo_lock);
> > + *	ASSERT_EXCLUSIVE_BITS(flags, FOO_MASK);
> > + *	old_flags = READ_ONCE(flags);
> > + *	new_flags = (old_flags & ~FOO_MASK) | (new_foo << FOO_SHIFT);
> > + *	if (cmpxchg(&flags, old_flags, new_flags) != old_flags) { ... }
> > + *	spin_unlock(&foo_lock);
> > + *
> > + * @var variable to assert on
> > + * @mask only check for modifications to bits set in @mask
> > + */
> > +#define ASSERT_EXCLUSIVE_BITS(var, mask)                                       \
> > +	do {                                                                   \
> > +		kcsan_set_access_mask(mask);                                   \
> > +		__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT);\
> > +		kcsan_set_access_mask(0);                                      \
> > +		kcsan_atomic_next(1);                                          \
> > +	} while (0)
> > +
> >  #endif /* _LINUX_KCSAN_CHECKS_H */
> > diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
> > index 9bbba0e57c9b3..2ff1961239778 100644
> > --- a/kernel/kcsan/debugfs.c
> > +++ b/kernel/kcsan/debugfs.c
> > @@ -100,8 +100,10 @@ static noinline void microbenchmark(unsigned long iters)
> >   * debugfs file from multiple tasks to generate real conflicts and show reports.
> >   */
> >  static long test_dummy;
> > +static long test_flags;
> >  static noinline void test_thread(unsigned long iters)
> >  {
> > +	const long CHANGE_BITS = 0xff00ff00ff00ff00L;
> >  	const struct kcsan_ctx ctx_save = current->kcsan_ctx;
> >  	cycles_t cycles;
> >  
> > @@ -109,16 +111,27 @@ static noinline void test_thread(unsigned long iters)
> >  	memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
> >  
> >  	pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);
> > +	pr_info("test_dummy@%px, test_flags@%px\n", &test_dummy, &test_flags);
> >  
> >  	cycles = get_cycles();
> >  	while (iters--) {
> > +		/* These all should generate reports. */
> >  		__kcsan_check_read(&test_dummy, sizeof(test_dummy));
> > -		__kcsan_check_write(&test_dummy, sizeof(test_dummy));
> >  		ASSERT_EXCLUSIVE_WRITER(test_dummy);
> >  		ASSERT_EXCLUSIVE_ACCESS(test_dummy);
> >  
> > +		ASSERT_EXCLUSIVE_BITS(test_flags, ~CHANGE_BITS); /* no report */
> > +		__kcsan_check_read(&test_flags, sizeof(test_flags)); /* no report */
> > +
> > +		ASSERT_EXCLUSIVE_BITS(test_flags, CHANGE_BITS); /* report */
> > +		__kcsan_check_read(&test_flags, sizeof(test_flags)); /* no report */
> > +
> >  		/* not actually instrumented */
> >  		WRITE_ONCE(test_dummy, iters);  /* to observe value-change */
> > +		__kcsan_check_write(&test_dummy, sizeof(test_dummy));
> > +
> > +		test_flags ^= CHANGE_BITS; /* generate value-change */
> > +		__kcsan_check_write(&test_flags, sizeof(test_flags));
> >  	}
> >  	cycles = get_cycles() - cycles;
> >  
> >