kcsan: Fix misreporting if concurrent races on same address
If there are at least 4 threads racing on the same address, it can
happen that one of the readers may observe another matching reader in
other_info. To avoid locking up, we have to consume 'other_info'
regardless, but skip the report. See the added comment for more details.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index 3bc590e..abf6852 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -422,6 +422,44 @@ static bool prepare_report(unsigned long *flags, const volatile void *ptr,
return false;
}
+ access_type |= other_info.access_type;
+ if ((access_type & KCSAN_ACCESS_WRITE) == 0) {
+ /*
+ * While the address matches, this is not the other_info
+ * from the thread that consumed our watchpoint, since
+ * neither this nor the access in other_info is a write.
+ * It is invalid to continue with the report, since we
+ * only have information about reads.
+ *
+ * This can happen due to concurrent races on the same
+ * address, with at least 4 threads. To avoid locking up
+ * other_info and all other threads, we have to consume
+ * it regardless.
+ *
+ * A concrete case to illustrate why we might lock up if
+ * we do not consume other_info:
+ *
+ * We have 4 threads, all accessing the same address
+ * (or matching address ranges). Assume the following
+ * watcher and watchpoint consumer pairs:
+ * write1-read1, read2-write2. The first to populate
+ * other_info is write2, however, write1 consumes it,
+ * resulting in a report of write1-write2. This report
+ * is valid, however, now read1 populates other_info;
+ * read2-read1 is an invalid conflict, yet, no other
+ * conflicting access is left. Therefore, we must
+ * consume read1's other_info.
+ *
+ * Since this case is assumed to be rare, it is
+ * reasonable to omit this report: one of the other
+ * reports includes information about the same shared
+ * data, and at this point the likelihood that we
+ * re-report the same race again is high.
+ */
+ release_report(flags, KCSAN_REPORT_RACE_SIGNAL);
+ return false;
+ }
+
/*
* Matching & usable access in other_info: keep other_info_lock
* locked, as this thread consumes it to print the full report;
