Merge tag 'kcsan.2023.04.04a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull KCSAN updates from Paul McKenney:
 "Kernel concurrency sanitizer (KCSAN) updates for v6.4

  This fixes kernel-doc warnings and also updates instrumentation from
  READ_ONCE() to volatile in order to avoid unaligned load-acquire
  instructions on arm64 in kernels built with LTO"

* tag 'kcsan.2023.04.04a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  kcsan: Avoid READ_ONCE() in read_instrumented_memory()
  instrumented.h: Fix all kernel-doc format warnings

include/linux/instrumented.h

@@ -15,12 +15,11 @@
 
 /**
  * instrument_read - instrument regular read access
+ * @v: address of access
+ * @size: size of access
  *
  * Instrument a regular read access. The instrumentation should be inserted
  * before the actual read happens.
- *
- * @ptr address of access
- * @size size of access
  */
 static __always_inline void instrument_read(const volatile void *v, size_t size)
 {
@@ -30,12 +29,11 @@ static __always_inline void instrument_read(const volatile void *v, size_t size)
 
 /**
  * instrument_write - instrument regular write access
+ * @v: address of access
+ * @size: size of access
  *
  * Instrument a regular write access. The instrumentation should be inserted
  * before the actual write happens.
- *
- * @ptr address of access
- * @size size of access
  */
 static __always_inline void instrument_write(const volatile void *v, size_t size)
 {
@@ -45,12 +43,11 @@ static __always_inline void instrument_write(const volatile void *v, size_t size
 
 /**
  * instrument_read_write - instrument regular read-write access
+ * @v: address of access
+ * @size: size of access
  *
  * Instrument a regular write access. The instrumentation should be inserted
  * before the actual write happens.
- *
- * @ptr address of access
- * @size size of access
  */
 static __always_inline void instrument_read_write(const volatile void *v, size_t size)
 {
@@ -60,12 +57,11 @@ static __always_inline void instrument_read_write(const volatile void *v, size_t
 
 /**
  * instrument_atomic_read - instrument atomic read access
+ * @v: address of access
+ * @size: size of access
  *
  * Instrument an atomic read access. The instrumentation should be inserted
  * before the actual read happens.
- *
- * @ptr address of access
- * @size size of access
  */
 static __always_inline void instrument_atomic_read(const volatile void *v, size_t size)
 {
@@ -75,12 +71,11 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 
 /**
  * instrument_atomic_write - instrument atomic write access
+ * @v: address of access
+ * @size: size of access
  *
  * Instrument an atomic write access. The instrumentation should be inserted
  * before the actual write happens.
- *
- * @ptr address of access
- * @size size of access
  */
 static __always_inline void instrument_atomic_write(const volatile void *v, size_t size)
 {
@@ -90,12 +85,11 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 
 /**
  * instrument_atomic_read_write - instrument atomic read-write access
+ * @v: address of access
+ * @size: size of access
  *
  * Instrument an atomic read-write access. The instrumentation should be
  * inserted before the actual write happens.
- *
- * @ptr address of access
- * @size size of access
  */
 static __always_inline void instrument_atomic_read_write(const volatile void *v, size_t size)
 {
@@ -105,13 +99,12 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 
 /**
  * instrument_copy_to_user - instrument reads of copy_to_user
+ * @to: destination address
+ * @from: source address
+ * @n: number of bytes to copy
  *
  * Instrument reads from kernel memory, that are due to copy_to_user (and
  * variants). The instrumentation must be inserted before the accesses.
- *
- * @to destination address
- * @from source address
- * @n number of bytes to copy
  */
 static __always_inline void
 instrument_copy_to_user(void __user *to, const void *from, unsigned long n)
@@ -123,13 +116,12 @@ instrument_copy_to_user(void __user *to, const void *from, unsigned long n)
 
 /**
  * instrument_copy_from_user_before - add instrumentation before copy_from_user
+ * @to: destination address
+ * @from: source address
+ * @n: number of bytes to copy
  *
  * Instrument writes to kernel memory, that are due to copy_from_user (and
  * variants). The instrumentation should be inserted before the accesses.
- *
- * @to destination address
- * @from source address
- * @n number of bytes to copy
  */
 static __always_inline void
 instrument_copy_from_user_before(const void *to, const void __user *from, unsigned long n)
@@ -140,14 +132,13 @@ instrument_copy_from_user_before(const void *to, const void __user *from, unsign
 
 /**
  * instrument_copy_from_user_after - add instrumentation after copy_from_user
+ * @to: destination address
+ * @from: source address
+ * @n: number of bytes to copy
+ * @left: number of bytes not copied (as returned by copy_from_user)
  *
  * Instrument writes to kernel memory, that are due to copy_from_user (and
  * variants). The instrumentation should be inserted after the accesses.
- *
- * @to destination address
- * @from source address
- * @n number of bytes to copy
- * @left number of bytes not copied (as returned by copy_from_user)
  */
 static __always_inline void
 instrument_copy_from_user_after(const void *to, const void __user *from,
@@ -158,12 +149,11 @@ instrument_copy_from_user_after(const void *to, const void __user *from,
 
 /**
  * instrument_get_user() - add instrumentation to get_user()-like macros
+ * @to: destination variable, may not be address-taken
  *
  * get_user() and friends are fragile, so it may depend on the implementation
  * whether the instrumentation happens before or after the data is copied from
  * the userspace.
- *
- * @to destination variable, may not be address-taken
  */
 #define instrument_get_user(to)				\
 ({							\
@@ -175,14 +165,13 @@ instrument_copy_from_user_after(const void *to, const void __user *from,
 
 /**
  * instrument_put_user() - add instrumentation to put_user()-like macros
+ * @from: source address
+ * @ptr: userspace pointer to copy to
+ * @size: number of bytes to copy
  *
  * put_user() and friends are fragile, so it may depend on the implementation
  * whether the instrumentation happens before or after the data is copied from
  * the userspace.
- *
- * @from source address
- * @ptr userspace pointer to copy to
- * @size number of bytes to copy
  */
 #define instrument_put_user(from, ptr, size)			\
 ({								\

kernel/kcsan/core.c

@@ -337,11 +337,20 @@ static void delay_access(int type)
  */
 static __always_inline u64 read_instrumented_memory(const volatile void *ptr, size_t size)
 {
+	/*
+	 * In the below we don't necessarily need the read of the location to
+	 * be atomic, and we don't use READ_ONCE(), since all we need for race
+	 * detection is to observe 2 different values.
+	 *
+	 * Furthermore, on certain architectures (such as arm64), READ_ONCE()
+	 * may turn into more complex instructions than a plain load that cannot
+	 * do unaligned accesses.
+	 */
 	switch (size) {
-	case 1:  return READ_ONCE(*(const u8 *)ptr);
-	case 2:  return READ_ONCE(*(const u16 *)ptr);
-	case 4:  return READ_ONCE(*(const u32 *)ptr);
-	case 8:  return READ_ONCE(*(const u64 *)ptr);
+	case 1:  return *(const volatile u8 *)ptr;
+	case 2:  return *(const volatile u16 *)ptr;
+	case 4:  return *(const volatile u32 *)ptr;
+	case 8:  return *(const volatile u64 *)ptr;
 	default: return 0; /* Ignore; we do not diff the values. */
 	}
 }