[PATCH v4 30/45] kcov: kmsan: unpoison area->list in kcov_remote_area_put()

[Alexander Potapenko <glider@google.com>]

KMSAN does not instrument kernel/kcov.c for performance reasons (with
CONFIG_KCOV=y virtually every place in the kernel invokes kcov
instrumentation). Therefore the tool may miss writes from kcov.c that
initialize memory.

When CONFIG_DEBUG_LIST is enabled, list pointers from kernel/kcov.c are
passed to instrumented helpers in lib/list_debug.c, resulting in false
positives.

To work around these reports, we unpoison the contents of area->list after
initializing it.

Signed-off-by: Alexander Potapenko <glider@google.com>
---

v4:
 -- change sizeof(type) to sizeof(*ptr)
 -- swap kcov: and kmsan: in the subject

Link: https://linux-review.googlesource.com/id/Ie17f2ee47a7af58f5cdf716d585ebf0769348a5a
---
 kernel/kcov.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index e19c84b02452e..e5cd09fd8a050 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -11,6 +11,7 @@
 #include <linux/fs.h>
 #include <linux/hashtable.h>
 #include <linux/init.h>
+#include <linux/kmsan-checks.h>
 #include <linux/mm.h>
 #include <linux/preempt.h>
 #include <linux/printk.h>
@@ -152,6 +153,12 @@ static void kcov_remote_area_put(struct kcov_remote_area *area,
 	INIT_LIST_HEAD(&area->list);
 	area->size = size;
 	list_add(&area->list, &kcov_remote_areas);
+	/*
+	 * KMSAN doesn't instrument this file, so it may not know area->list
+	 * is initialized. Unpoison it explicitly to avoid reports in
+	 * kcov_remote_area_get().
+	 */
+	kmsan_unpoison_memory(&area->list, sizeof(area->list));
 }
 
 static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t)
-- 
2.37.0.rc0.161.g10f37bed90-goog