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

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Message-ID: <20200507211834.GA20715@embeddedor>
Date:   Thu, 7 May 2020 16:18:34 -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: Re: [PATCH] media: venus: Replace zero-length array with
 flexible-array

On Thu, May 07, 2020 at 11:39:51PM +0300, Stanimir Varbanov wrote:
> Hi Gustavo,
> 
> Two patches from you are already queued in media_tree for v5.8.
> 

Awesome. :)

Thanks, Stanimir.
--
Gustavo

> 0f61e171e4bbac4595175070c75707f1b12f4e37 media: venus: hfi_msgs.h:
> Replace zero-length array with flexible-array member
> 
> 380f3bbd9562dc93be2e3cadc329b15284fbedae media: venus: hfi_cmds.h:
> Replace zero-length array with flexible-array member
> 
> 
> On 5/7/20 10:07 PM, 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]
> > 
> > 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 {
> > 
> 
> -- 
> regards,
> Stan