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Subject[PATCH kcsan 18/32] kcsan: Add test to generate conflicts via debugfs
Date
From: Marco Elver <elver@google.com>

Add 'test=<iters>' option to KCSAN's debugfs interface to invoke KCSAN
checks on a dummy variable. By writing 'test=<iters>' to the debugfs
file from multiple tasks, we can generate real conflicts, and trigger
data race reports.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/kcsan/debugfs.c | 51 +++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 46 insertions(+), 5 deletions(-)

diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
index a9dad44..9bbba0e 100644
--- a/kernel/kcsan/debugfs.c
+++ b/kernel/kcsan/debugfs.c
@@ -6,6 +6,7 @@
#include <linux/debugfs.h>
#include <linux/init.h>
#include <linux/kallsyms.h>
+#include <linux/sched.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/sort.h>
@@ -69,9 +70,9 @@ void kcsan_counter_dec(enum kcsan_counter_id id)
/*
* The microbenchmark allows benchmarking KCSAN core runtime only. To run
* multiple threads, pipe 'microbench=<iters>' from multiple tasks into the
- * debugfs file.
+ * debugfs file. This will not generate any conflicts, and tests fast-path only.
*/
-static void microbenchmark(unsigned long iters)
+static noinline void microbenchmark(unsigned long iters)
{
cycles_t cycles;

@@ -81,18 +82,52 @@ static void microbenchmark(unsigned long iters)
while (iters--) {
/*
* We can run this benchmark from multiple tasks; this address
- * calculation increases likelyhood of some accesses overlapping
- * (they still won't conflict because all are reads).
+ * 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_read((void *)addr, sizeof(long));
+ __kcsan_check_access((void *)addr, sizeof(long), KCSAN_ACCESS_ATOMIC);
}
cycles = get_cycles() - cycles;

pr_info("KCSAN: %s end | cycles: %llu\n", __func__, cycles);
}

+/*
+ * Simple test to create conflicting accesses. Write 'test=<iters>' to KCSAN's
+ * debugfs file from multiple tasks to generate real conflicts and show reports.
+ */
+static long test_dummy;
+static noinline void test_thread(unsigned long iters)
+{
+ const struct kcsan_ctx ctx_save = current->kcsan_ctx;
+ cycles_t cycles;
+
+ /* We may have been called from an atomic region; reset context. */
+ memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
+
+ pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);
+
+ cycles = get_cycles();
+ while (iters--) {
+ __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);
+
+ /* not actually instrumented */
+ WRITE_ONCE(test_dummy, iters); /* to observe value-change */
+ }
+ cycles = get_cycles() - cycles;
+
+ pr_info("KCSAN: %s end | cycles: %llu\n", __func__, cycles);
+
+ /* restore context */
+ current->kcsan_ctx = ctx_save;
+}
+
static int cmp_filterlist_addrs(const void *rhs, const void *lhs)
{
const unsigned long a = *(const unsigned long *)rhs;
@@ -242,6 +277,12 @@ debugfs_write(struct file *file, const char __user *buf, size_t count, loff_t *o
if (kstrtoul(&arg[sizeof("microbench=") - 1], 0, &iters))
return -EINVAL;
microbenchmark(iters);
+ } else if (!strncmp(arg, "test=", sizeof("test=") - 1)) {
+ unsigned long iters;
+
+ if (kstrtoul(&arg[sizeof("test=") - 1], 0, &iters))
+ return -EINVAL;
+ test_thread(iters);
} else if (!strcmp(arg, "whitelist")) {
set_report_filterlist_whitelist(true);
} else if (!strcmp(arg, "blacklist")) {
--
2.9.5
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