[PATCH] media: v4l2-event: Annotate struct v4l2_subscribed_event with __counted_by

[Kees Cook <keescook@chromium.org>]

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct v4l2_subscribed_event.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: lijian <lijian@yulong.com>
Cc: linux-media@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/media/v4l2-core/v4l2-event.c | 2 +-
 include/media/v4l2-event.h           | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/media/v4l2-core/v4l2-event.c b/drivers/media/v4l2-core/v4l2-event.c
index c5ce9f11ad7b..3898ff7edddb 100644
--- a/drivers/media/v4l2-core/v4l2-event.c
+++ b/drivers/media/v4l2-core/v4l2-event.c
@@ -238,6 +238,7 @@ int v4l2_event_subscribe(struct v4l2_fh *fh,
 	sev = kvzalloc(struct_size(sev, events, elems), GFP_KERNEL);
 	if (!sev)
 		return -ENOMEM;
+	sev->elems = elems;
 	for (i = 0; i < elems; i++)
 		sev->events[i].sev = sev;
 	sev->type = sub->type;
@@ -245,7 +246,6 @@ int v4l2_event_subscribe(struct v4l2_fh *fh,
 	sev->flags = sub->flags;
 	sev->fh = fh;
 	sev->ops = ops;
-	sev->elems = elems;
 
 	mutex_lock(&fh->subscribe_lock);
 
diff --git a/include/media/v4l2-event.h b/include/media/v4l2-event.h
index 4ffa914ade3a..3a0e2588361c 100644
--- a/include/media/v4l2-event.h
+++ b/include/media/v4l2-event.h
@@ -78,7 +78,7 @@ struct v4l2_subscribed_event {
 	unsigned int		elems;
 	unsigned int		first;
 	unsigned int		in_use;
-	struct v4l2_kevent	events[];
+	struct v4l2_kevent	events[] __counted_by(elems);
 };
 
 /**
-- 
2.34.1