linux-kernel - [PATCH][next] mei: hdcp: Replace one-element array with flexible-array member

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Message-ID: <20200722181534.GA31357@embeddedor>
Date:   Wed, 22 Jul 2020 13:15:34 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>
Subject: [PATCH][next] mei: hdcp: Replace one-element array with
 flexible-array member

There is a regular need in the kernel to provide a way to declare
having a dynamically sized set of trailing elements in a structure.
Kernel code should always use “flexible array members”[1] for these
cases. The older style of one-element or zero-length arrays should
no longer be used[2].

Also, make use of the array_size() helper instead of the open-coded
version in memcpy(). These sorts of multiplication factors need to
be wrapped in array_size().

And while there, use the preferred form for passing a size of a struct.
The alternative form where struct name is spelled out hurts readability
and introduces an opportunity for a bug when the pointer variable type is
changed but the corresponding sizeof that is passed as argument is not.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://github.com/KSPP/linux/issues/79

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 drivers/misc/mei/hdcp/mei_hdcp.c | 2 +-
 drivers/misc/mei/hdcp/mei_hdcp.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/misc/mei/hdcp/mei_hdcp.c b/drivers/misc/mei/hdcp/mei_hdcp.c
index e6c3dc595617..d1d3e025ca0e 100644
--- a/drivers/misc/mei/hdcp/mei_hdcp.c
+++ b/drivers/misc/mei/hdcp/mei_hdcp.c
@@ -572,7 +572,7 @@ static int mei_hdcp_verify_mprime(struct device *dev,
 	       HDCP_2_2_MPRIME_LEN);
 	drm_hdcp_cpu_to_be24(verify_mprime_in.seq_num_m, data->seq_num_m);
 	memcpy(verify_mprime_in.streams, data->streams,
-	       (data->k * sizeof(struct hdcp2_streamid_type)));
+	       array_size(data->k, sizeof(*data->streams)));
 
 	verify_mprime_in.k = cpu_to_be16(data->k);
 
diff --git a/drivers/misc/mei/hdcp/mei_hdcp.h b/drivers/misc/mei/hdcp/mei_hdcp.h
index 18ffc773fa18..834757f5e072 100644
--- a/drivers/misc/mei/hdcp/mei_hdcp.h
+++ b/drivers/misc/mei/hdcp/mei_hdcp.h
@@ -358,7 +358,7 @@ struct wired_cmd_repeater_auth_stream_req_in {
 	u8				seq_num_m[HDCP_2_2_SEQ_NUM_LEN];
 	u8				m_prime[HDCP_2_2_MPRIME_LEN];
 	__be16				k;
-	struct hdcp2_streamid_type	streams[1];
+	struct hdcp2_streamid_type	streams[];
 } __packed;
 
 struct wired_cmd_repeater_auth_stream_req_out {
-- 
2.27.0