Re: [PATCH bpf-next v3] libbpf: Fix potential uninitialized tail padding with LIBBPF_OPTS_RESET

[Daniel Borkmann <daniel@iogearbox.net>]

On 11/7/23 9:15 PM, Yonghong Song wrote:
> Martin reported that there is a libbpf complaining of non-zero-value tail
> padding with LIBBPF_OPTS_RESET macro if struct bpf_netkit_opts is modified
> to have a 4-byte tail padding. This only happens to clang compiler.
> The commend line is: ./test_progs -t tc_netkit_multi_links
> Martin and I did some investigation and found this indeed the case and
> the following are the investigation details.

[...]

Too bad we need this detour, but fix lgtm, thanks!

> diff --git a/tools/lib/bpf/libbpf_common.h b/tools/lib/bpf/libbpf_common.h
> index b7060f254486..8fe248e14eb6 100644
> --- a/tools/lib/bpf/libbpf_common.h
> +++ b/tools/lib/bpf/libbpf_common.h
> @@ -79,11 +79,14 @@
>    */
>   #define LIBBPF_OPTS_RESET(NAME, ...)					    \
>   	do {								    \
> -		memset(&NAME, 0, sizeof(NAME));				    \
> -		NAME = (typeof(NAME)) {					    \
> -			.sz = sizeof(NAME),				    \
> -			__VA_ARGS__					    \
> -		};							    \
> +		typeof(NAME) ___##NAME = ({ 				    \
> +			memset(&___##NAME, 0, sizeof(NAME));		    \
> +			(typeof(NAME)) {				    \
> +				.sz = sizeof(NAME),			    \
> +				__VA_ARGS__				    \
> +			};						    \
> +		});							    \
> +		memcpy(&NAME, &___##NAME, sizeof(NAME));		    \
>   	} while (0)
>   
>   #endif /* __LIBBPF_LIBBPF_COMMON_H */
>