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

[Olivier Matz <olivier.matz@6wind.com>]

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.

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?

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
>