[PATCH] videodev2.h: apply packed attribute to union

[PATCH] videodev2.h: apply packed attribute to union

[Devendra Tewari]

Fixes clang warning: field within 'v4l2_ext_control' is less than
'v4l2_ext_control::(anonymous union

Signed-off-by: Devendra Tewari <devendra.tewari@gmail.com>
---
 include/uapi/linux/videodev2.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
index 3768a0a80830..767c52c722cd 100644
--- a/include/uapi/linux/videodev2.h
+++ b/include/uapi/linux/videodev2.h
@@ -1765,7 +1765,7 @@ struct v4l2_ext_control {
 		struct v4l2_ctrl_vp9_compressed_hdr __user *p_vp9_compressed_hdr_probs;
 		struct v4l2_ctrl_vp9_frame __user *p_vp9_frame;
 		void __user *ptr;
-	};
+	} __attribute__ ((packed));
 } __attribute__ ((packed));
 
 struct v4l2_ext_controls {
-- 
2.25.1

Re: [PATCH] videodev2.h: apply packed attribute to union

[Kieran Bingham]

Hi Devendra,

Quoting Devendra Tewari (2022-04-22 20:20:31)
> Fixes clang warning: field within 'v4l2_ext_control' is less than

Can you detail which version of clang this occurs with? Have you tried
more than one version?


> 'v4l2_ext_control::(anonymous union
> 
> Signed-off-by: Devendra Tewari <devendra.tewari@gmail.com>
> ---
>  include/uapi/linux/videodev2.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 3768a0a80830..767c52c722cd 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -1765,7 +1765,7 @@ struct v4l2_ext_control {
>                 struct v4l2_ctrl_vp9_compressed_hdr __user *p_vp9_compressed_hdr_probs;
>                 struct v4l2_ctrl_vp9_frame __user *p_vp9_frame;
>                 void __user *ptr;
> -       };
> +       } __attribute__ ((packed));

This is a curious fix. It's applying a packed attribute to the union,
which I presume means that it's then applying the packed attribute to
any item in the union.

The items are all either: __s32, __s64, values - or pointers.

While applying this attribute here may fix the compiler warning, I'm not
sure it's clear why this is required. This file also has other
locations where a union inside a packed struct is not marked as packed.
Should all unions be marked with the attribute?

Is there any more context from the compiler warning beyond what is
reported above?

--
Kieran


>  } __attribute__ ((packed));
>  
>  struct v4l2_ext_controls {
> -- 
> 2.25.1
>

Re: [PATCH] videodev2.h: apply packed attribute to union

[Devendra Tewari]

> No dia 7 de out. de 2022, às 18:58, Kieran Bingham <kieran.bingham@ideasonboard.com> escreveu:
> 
> Hi Devendra,
> 
> Quoting Devendra Tewari (2022-04-22 20:20:31)
>> Fixes clang warning: field within 'v4l2_ext_control' is less than
> 
> Can you detail which version of clang this occurs with? Have you tried
> more than one version?
> 

This started happening with version 14.0.1 and I continue to see it with version 15.

> 
>> 'v4l2_ext_control::(anonymous union
>> 
>> Signed-off-by: Devendra Tewari <devendra.tewari@gmail.com>
>> ---
>> include/uapi/linux/videodev2.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
>> index 3768a0a80830..767c52c722cd 100644
>> --- a/include/uapi/linux/videodev2.h
>> +++ b/include/uapi/linux/videodev2.h
>> @@ -1765,7 +1765,7 @@ struct v4l2_ext_control {
>>                struct v4l2_ctrl_vp9_compressed_hdr __user *p_vp9_compressed_hdr_probs;
>>                struct v4l2_ctrl_vp9_frame __user *p_vp9_frame;
>>                void __user *ptr;
>> -       };
>> +       } __attribute__ ((packed));
> 
> This is a curious fix. It's applying a packed attribute to the union,
> which I presume means that it's then applying the packed attribute to
> any item in the union.
> 
> The items are all either: __s32, __s64, values - or pointers.
> 
> While applying this attribute here may fix the compiler warning, I'm not
> sure it's clear why this is required. This file also has other
> locations where a union inside a packed struct is not marked as packed.
> Should all unions be marked with the attribute?

Interesting - I need to look deeper into packed.

> Is there any more context from the compiler warning beyond what is
> reported above?

I'll post a more detailed log asap.

> 
> --
> Kieran
> 
> 
>> } __attribute__ ((packed));
>> 
>> struct v4l2_ext_controls {
>> -- 
>> 2.25.1
>> 
Thanks,
Devendra

Re: [PATCH] videodev2.h: apply packed attribute to union

[Devendra Tewari]

> On 7 Oct 2022, at 20:04, Devendra Tewari <devendra.tewari@gmail.com> wrote:
> 
> 
> 
>> No dia 7 de out. de 2022, às 18:58, Kieran Bingham <kieran.bingham@ideasonboard.com> escreveu:
>> 
>> Hi Devendra,
>> 
>> Quoting Devendra Tewari (2022-04-22 20:20:31)
>>> Fixes clang warning: field within 'v4l2_ext_control' is less than
>> 
>> Can you detail which version of clang this occurs with? Have you tried
>> more than one version?
>> 
> 
> This started happening with version 14.0.1 and I continue to see it with version 15.
> 
>> 
>>> 'v4l2_ext_control::(anonymous union
>>> 
>>> Signed-off-by: Devendra Tewari <devendra.tewari@gmail.com>
>>> ---
>>> include/uapi/linux/videodev2.h | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>> 
>>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
>>> index 3768a0a80830..767c52c722cd 100644
>>> --- a/include/uapi/linux/videodev2.h
>>> +++ b/include/uapi/linux/videodev2.h
>>> @@ -1765,7 +1765,7 @@ struct v4l2_ext_control {
>>>               struct v4l2_ctrl_vp9_compressed_hdr __user *p_vp9_compressed_hdr_probs;
>>>               struct v4l2_ctrl_vp9_frame __user *p_vp9_frame;
>>>               void __user *ptr;
>>> -       };
>>> +       } __attribute__ ((packed));
>> 
>> This is a curious fix. It's applying a packed attribute to the union,
>> which I presume means that it's then applying the packed attribute to
>> any item in the union.
>> 
>> The items are all either: __s32, __s64, values - or pointers.
>> 
>> While applying this attribute here may fix the compiler warning, I'm not
>> sure it's clear why this is required. This file also has other
>> locations where a union inside a packed struct is not marked as packed.
>> Should all unions be marked with the attribute?
> 
> Interesting - I need to look deeper into packed.
> 
>> Is there any more context from the compiler warning beyond what is
>> reported above?
> 
> I'll post a more detailed log asap.

This is the log with clang 15 compiler…

 ../git/src/libcamera/v4l2_device.cpp
| In file included from ../git/src/libcamera/v4l2_device.cpp:8:
| In file included from ../git/include/libcamera/internal/v4l2_device.h:15:
| ../git/include/linux/videodev2.h:1724:2: error: field  within 'v4l2_ext_control' is less aligned than 'v4l2_ext_control::(anonymous union at ../git/include/linux/videodev2.h:1724:2)' and is usually due to 'v4l2_ext_control' being packed, which can lead to unaligned accesses [-Werror,-Wunaligned-access]
|         union {
|         ^
| 1 error generated.


Compiler version...

# clang --version
clang version 15.0.1 (https://github.com/llvm/llvm-project 29d395a1b7a8176abb1d6278f7df98301fbe7744)
Target: x86_64-unknown-linux-gnu
Thread model: posix

> 
>> 
>> --
>> Kieran
>> 
>> 
>>> } __attribute__ ((packed));
>>> 
>>> struct v4l2_ext_controls {
>>> -- 
>>> 2.25.1
>>> 
> Thanks,
> Devendra
Resending because mailing list daemon is unhappy with rich text.

Thanks,
Devendra

Re: [PATCH] videodev2.h: apply packed attribute to union

[Devendra Tewari]

> On 7 Oct 2022, at 20:04, Devendra Tewari <devendra.tewari@gmail.com> wrote:
> 
> 
> 
>> No dia 7 de out. de 2022, às 18:58, Kieran Bingham <kieran.bingham@ideasonboard.com> escreveu:
>> 
>> Hi Devendra,
>> 
>> Quoting Devendra Tewari (2022-04-22 20:20:31)
>>> Fixes clang warning: field within 'v4l2_ext_control' is less than
>> 
>> Can you detail which version of clang this occurs with? Have you tried
>> more than one version?
>> 
> 
> This started happening with version 14.0.1 and I continue to see it with version 15.
> 
>> 
>>> 'v4l2_ext_control::(anonymous union
>>> 
>>> Signed-off-by: Devendra Tewari <devendra.tewari@gmail.com>
>>> ---
>>> include/uapi/linux/videodev2.h | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>> 
>>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
>>> index 3768a0a80830..767c52c722cd 100644
>>> --- a/include/uapi/linux/videodev2.h
>>> +++ b/include/uapi/linux/videodev2.h
>>> @@ -1765,7 +1765,7 @@ struct v4l2_ext_control {
>>>               struct v4l2_ctrl_vp9_compressed_hdr __user *p_vp9_compressed_hdr_probs;
>>>               struct v4l2_ctrl_vp9_frame __user *p_vp9_frame;
>>>               void __user *ptr;
>>> -       };
>>> +       } __attribute__ ((packed));
>> 
>> This is a curious fix. It's applying a packed attribute to the union,
>> which I presume means that it's then applying the packed attribute to
>> any item in the union.
>> 
>> The items are all either: __s32, __s64, values - or pointers.
>> 
>> While applying this attribute here may fix the compiler warning, I'm not
>> sure it's clear why this is required. This file also has other
>> locations where a union inside a packed struct is not marked as packed.
>> Should all unions be marked with the attribute?
> 
> Interesting - I need to look deeper into packed.

The only explanation I can think of is that this may be the only instance where a union inside a packed struct has other structs that are not packed.

> 
>> Is there any more context from the compiler warning beyond what is
>> reported above?
> 
> I'll post a more detailed log asap.
> 
>> 
>> --
>> Kieran
>> 
>> 
>>> } __attribute__ ((packed));
>>> 
>>> struct v4l2_ext_controls {
>>> -- 
>>> 2.25.1
>>> 
> Thanks,
> Devendra

Resending because mail daemon rejected rich text message.

Thanks,
Devendra

Re: [PATCH] videodev2.h: apply packed attribute to union

[Hans Verkuil]

Hi Devendra,

Cleaning up some old patches in

On 08/10/2022 13:48, Devendra Tewari wrote:
> 
> 
>> On 7 Oct 2022, at 20:04, Devendra Tewari <devendra.tewari@gmail.com> wrote:
>>
>>
>>
>>> No dia 7 de out. de 2022, às 18:58, Kieran Bingham <kieran.bingham@ideasonboard.com> escreveu:
>>>
>>> Hi Devendra,
>>>
>>> Quoting Devendra Tewari (2022-04-22 20:20:31)
>>>> Fixes clang warning: field within 'v4l2_ext_control' is less than
>>>
>>> Can you detail which version of clang this occurs with? Have you tried
>>> more than one version?
>>>
>>
>> This started happening with version 14.0.1 and I continue to see it with version 15.
>>
>>>
>>>> 'v4l2_ext_control::(anonymous union
>>>>
>>>> Signed-off-by: Devendra Tewari <devendra.tewari@gmail.com>
>>>> ---
>>>> include/uapi/linux/videodev2.h | 2 +-
>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
>>>> index 3768a0a80830..767c52c722cd 100644
>>>> --- a/include/uapi/linux/videodev2.h
>>>> +++ b/include/uapi/linux/videodev2.h
>>>> @@ -1765,7 +1765,7 @@ struct v4l2_ext_control {
>>>>               struct v4l2_ctrl_vp9_compressed_hdr __user *p_vp9_compressed_hdr_probs;
>>>>               struct v4l2_ctrl_vp9_frame __user *p_vp9_frame;
>>>>               void __user *ptr;
>>>> -       };
>>>> +       } __attribute__ ((packed));
>>>
>>> This is a curious fix. It's applying a packed attribute to the union,
>>> which I presume means that it's then applying the packed attribute to
>>> any item in the union.
>>>
>>> The items are all either: __s32, __s64, values - or pointers.
>>>
>>> While applying this attribute here may fix the compiler warning, I'm not
>>> sure it's clear why this is required. This file also has other
>>> locations where a union inside a packed struct is not marked as packed.
>>> Should all unions be marked with the attribute?
>>
>> Interesting - I need to look deeper into packed.
> 
> The only explanation I can think of is that this may be the only instance where a union inside a packed struct has other structs that are not packed.
> 
>>
>>> Is there any more context from the compiler warning beyond what is
>>> reported above?
>>
>> I'll post a more detailed log asap.

I haven't heard anything back, so I am rejecting this.

I am also afraid that changing this could cause unexpected ABI changes.

Regards,

	Hans

>>
>>>
>>> --
>>> Kieran
>>>
>>>
>>>> } __attribute__ ((packed));
>>>>
>>>> struct v4l2_ext_controls {
>>>> -- 
>>>> 2.25.1
>>>>
>> Thanks,
>> Devendra
> 
> Resending because mail daemon rejected rich text message.
> 
> Thanks,
> Devendra
>