Re: [dpdk-dev] [PATCH 2/3] mbuf: avoid cast-align warning in pktmbuf mtod offset macro

[Eli Britstein <elibr@nvidia.com>]

On 7/28/2021 6:28 PM, Olivier Matz wrote:
> External email: Use caution opening links or attachments
>
>
> On Tue, Jul 13, 2021 at 09:49:09AM +0300, Eli Britstein wrote:
>> In rte_pktmbuf_mtod_offset macro, there is a casting from char * to type
>> 't', which may cause cast-align warning when using gcc flags
>> '-Werror -Wcast-align':
>>
>> .../include/rte_mbuf_core.h:723:3: error: cast increases required alignment
>>      of target type [-Werror=cast-align]
>>    723 |  ((t)((char *)(m)->buf_addr + (m)->data_off + (o)))
>>        |   ^
>>
>> As the code assumes correct alignment, add first a (void *) casting, to
>> avoid the warning.
>>
>> Fixes: af75078fece3 ("first public release")
>> Cc: stable@dpdk.org
>>
>> Signed-off-by: Eli Britstein <elibr@nvidia.com>
> My initial thinking was that it's the problem of the application: if
> -Werror=cast-align is used, it is up to the application to cast the
> return value of rte_pktmbuf_mtod_offset() to (void *) before casting it
> to the network type.
>
> But, if I understand correctly, the problem is not about the application
> code itself, but about inlined code in the header files of dpdk
> (i.e. compiling an empty C file that just includes the dpdk headers with
> -Werror=cast-align). Is it correct? If yes I think it should be
> highlighted in the commit log.

I think yes, though in this specific patch it is not even an inline 
function, but a macro.

However, I don't have a synthetic application example to show those 
warnings, thus didn't put such in the commit msg.

>
> Out of curiosity, how did you find the errors? I mean, is it possible
> that some casts are missing some other headers, or is this patchset
> exhaustive?
Currently OVS-DPDK is compiled only with -Wno-cast-align.

Following complaint that a recent commit introduced a degradation in OVS 
[1], I compiled OVS without this warning deprecation.
The fixes in OVS are [2] and [3] (already merged). The fixes in DPDK are 
in this patch-set.

[1] https://mail.openvswitch.org/pipermail/ovs-dev/2021-July/385084.html
[2] https://mail.openvswitch.org/pipermail/ovs-dev/2021-July/386278.html
     e8cccd3a3589 ("netdev-offload-dpdk: Fix IPv6 rewrite cast-align 
warning.")
[3] https://mail.openvswitch.org/pipermail/ovs-dev/2021-July/386279.html
     1f7f557603a5 ("netdev-offload-dpdk: Fix vxlan vni cast-align 
warnings.")
> Thanks,
> Olivier
>
>
>> ---
>>   lib/mbuf/rte_mbuf_core.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/mbuf/rte_mbuf_core.h b/lib/mbuf/rte_mbuf_core.h
>> index bb38d7f581..dabdeee604 100644
>> --- a/lib/mbuf/rte_mbuf_core.h
>> +++ b/lib/mbuf/rte_mbuf_core.h
>> @@ -720,7 +720,7 @@ struct rte_mbuf_ext_shared_info {
>>    *   The type to cast the result into.
>>    */
>>   #define rte_pktmbuf_mtod_offset(m, t, o)     \
>> -     ((t)((char *)(m)->buf_addr + (m)->data_off + (o)))
>> +     ((t)(void *)((char *)(m)->buf_addr + (m)->data_off + (o)))
>>
>>   /**
>>    * A macro that points to the start of the data in the mbuf.
>> --
>> 2.28.0.2311.g225365fb51
>>