Re: [PATCH] selftests: net: ipmr: Avoid memcpy() from NULL in nl_add_rtattr()

[Hangbin Liu <hangbin.liu@linux.dev>]

On Tue, Sep 01, 2026 at 07:13:53AM +0000, Chaithanya Lagisetty wrote:
> nl_add_rtattr() unconditionally does memcpy(RTA_DATA(rta), data, len).
> For zero-length attributes the callers pass data == NULL and len == 0,
> for example the RTA_PREFSRC attribute added for proxy MFC entries:
> 
> 	if (mfc_attr->proxy)
> 		rta = nl_add_rtattr(nlmsg, rta, RTA_PREFSRC, NULL, 0);
> 
> Passing a NULL pointer to memcpy() is undefined behaviour even when the
> length is zero, because its source parameter is marked
> __attribute__((nonnull)); it is flagged by fortify/-Wnonnull.
> 
> Only call memcpy() when len is non-zero.
> 
> Fixes: 05068eaa67b2 ("selftest: net: Add basic functionality tests for ipmr.")
> Signed-off-by: Chaithanya Lagisetty <nagachaithanya9911@gmail.com>
> ---
>  tools/testing/selftests/net/forwarding/ipmr.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/net/forwarding/ipmr.c b/tools/testing/selftests/net/forwarding/ipmr.c
> index 9cd9f70de132..d3e26341821c 100644
> --- a/tools/testing/selftests/net/forwarding/ipmr.c
> +++ b/tools/testing/selftests/net/forwarding/ipmr.c
> @@ -120,7 +120,8 @@ static struct rtattr *nl_add_rtattr(struct nlmsghdr *nlmsg, struct rtattr *rta,
>  
>  	rta->rta_type = type;
>  	rta->rta_len = RTA_LENGTH(len);
> -	memcpy(RTA_DATA(rta), data, len);
> +	if (len)
> +		memcpy(RTA_DATA(rta), data, len);
>  
>  	nlmsg->nlmsg_len += NLMSG_ALIGN(rta->rta_len);
>  
> -- 
> 2.43.0
> 

Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>