linux-kernel - [PATCH] media: venus: Replace zero-length array with flexible-array

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Date:   Thu, 7 May 2020 14:07:50 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Stanimir Varbanov <stanimir.varbanov@...aro.org>
Cc:     linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] media: venus: Replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 drivers/media/platform/qcom/venus/hfi_cmds.h |    2 +-
 drivers/media/platform/qcom/venus/hfi_msgs.h |   10 +++++-----
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/media/platform/qcom/venus/hfi_cmds.h b/drivers/media/platform/qcom/venus/hfi_cmds.h
index cae9d5d61c0c..83705e237f1c 100644
--- a/drivers/media/platform/qcom/venus/hfi_cmds.h
+++ b/drivers/media/platform/qcom/venus/hfi_cmds.h
@@ -107,7 +107,7 @@ struct hfi_session_abort_pkt {
 struct hfi_session_set_property_pkt {
 	struct hfi_session_hdr_pkt shdr;
 	u32 num_properties;
-	u32 data[0];
+	u32 data[];
 };
 
 struct hfi_session_set_buffers_pkt {
diff --git a/drivers/media/platform/qcom/venus/hfi_msgs.h b/drivers/media/platform/qcom/venus/hfi_msgs.h
index 7694b1d25d9d..526d9f5b487b 100644
--- a/drivers/media/platform/qcom/venus/hfi_msgs.h
+++ b/drivers/media/platform/qcom/venus/hfi_msgs.h
@@ -155,7 +155,7 @@ struct hfi_msg_session_empty_buffer_done_pkt {
 	u32 input_tag;
 	u32 packet_buffer;
 	u32 extradata_buffer;
-	u32 data[0];
+	u32 data[];
 };
 
 struct hfi_msg_session_fbd_compressed_pkt {
@@ -175,7 +175,7 @@ struct hfi_msg_session_fbd_compressed_pkt {
 	u32 picture_type;
 	u32 packet_buffer;
 	u32 extradata_buffer;
-	u32 data[0];
+	u32 data[];
 };
 
 struct hfi_msg_session_fbd_uncompressed_plane0_pkt {
@@ -202,7 +202,7 @@ struct hfi_msg_session_fbd_uncompressed_plane0_pkt {
 	u32 picture_type;
 	u32 packet_buffer;
 	u32 extradata_buffer;
-	u32 data[0];
+	u32 data[];
 };
 
 struct hfi_msg_session_fbd_uncompressed_plane1_pkt {
@@ -211,7 +211,7 @@ struct hfi_msg_session_fbd_uncompressed_plane1_pkt {
 	u32 filled_len;
 	u32 offset;
 	u32 packet_buffer2;
-	u32 data[0];
+	u32 data[];
 };
 
 struct hfi_msg_session_fbd_uncompressed_plane2_pkt {
@@ -220,7 +220,7 @@ struct hfi_msg_session_fbd_uncompressed_plane2_pkt {
 	u32 filled_len;
 	u32 offset;
 	u32 packet_buffer3;
-	u32 data[0];
+	u32 data[];
 };
 
 struct hfi_msg_session_parse_sequence_header_done_pkt {