[PATCH][next] media: venus: hfi_msgs.h: Replace zero-length array with flexible-array member

[PATCH][next] media: venus: hfi_msgs.h: Replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

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]

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 <gustavo@embeddedor.com>
---
 drivers/media/platform/qcom/venus/hfi_msgs.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

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 {
-- 
2.23.0

Re: [PATCH][next] media: venus: hfi_msgs.h: Replace zero-length array with flexible-array member

[Stanimir Varbanov]

Hi Gustavo,

Thanks for the patch!

On 3/20/20 12:22 AM, Gustavo A. R. Silva wrote:
> 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]
> 
> 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 <gustavo@embeddedor.com>
> ---
>  drivers/media/platform/qcom/venus/hfi_msgs.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)

Acked-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>

-- 
regards,
Stan