kcsan: permissive: Ignore data-racy 1-bit value changes

[linux-2.6-microblaze.git] / kernel / kcsan / permissive.h

diff --git a/kernel/kcsan/permissive.h b/kernel/kcsan/permissive.h
index f90e308..2c01fe4 100644 (file)
--- a/kernel/kcsan/permissive.h
+++ b/kernel/kcsan/permissive.h
@@ -12,6 +12,8 @@
 #ifndef _KERNEL_KCSAN_PERMISSIVE_H
 #define _KERNEL_KCSAN_PERMISSIVE_H
 
+#include <linux/bitops.h>
+#include <linux/sched.h>
 #include <linux/types.h>
 
 /*
@@ -22,7 +24,11 @@ static __always_inline bool kcsan_ignore_address(const volatile void *ptr)
        if (!IS_ENABLED(CONFIG_KCSAN_PERMISSIVE))
                return false;
 
-       return false;
+       /*
+        * Data-racy bitops on current->flags are too common, ignore completely
+        * for now.
+        */
+       return ptr == &current->flags;
 }
 
 /*
@@ -41,6 +47,47 @@ kcsan_ignore_data_race(size_t size, int type, u64 old, u64 new, u64 diff)
        if (type || size > sizeof(long))
                return false;
 
+       /*
+        * A common pattern is checking/setting just 1 bit in a variable; for
+        * example:
+        *
+        *      if (flags & SOME_FLAG) { ... }
+        *
+        * and elsewhere flags is updated concurrently:
+        *
+        *      flags |= SOME_OTHER_FLAG; // just 1 bit
+        *
+        * While it is still recommended that such accesses be marked
+        * appropriately, in many cases these types of data races are so common
+        * that marking them all is often unrealistic and left to maintainer
+        * preference.
+        *
+        * The assumption in all cases is that with all known compiler
+        * optimizations (including those that tear accesses), because no more
+        * than 1 bit changed, the plain accesses are safe despite the presence
+        * of data races.
+        *
+        * The rules here will ignore the data races if we observe no more than
+        * 1 bit changed.
+        *
+        * Of course many operations can effecively change just 1 bit, but the
+        * general assuption that data races involving 1-bit changes can be
+        * tolerated still applies.
+        *
+        * And in case a true bug is missed, the bug likely manifests as a
+        * reportable data race elsewhere.
+        */
+       if (hweight64(diff) == 1) {
+               /*
+                * Exception: Report data races where the values look like
+                * ordinary booleans (one of them was 0 and the 0th bit was
+                * changed) More often than not, they come with interesting
+                * memory ordering requirements, so let's report them.
+                */
+               if (!((!old || !new) && diff == 1))
+                       return true;
+       }
+
        return false;
 }