netdev - Re: [PATCH bpf-next v5 1/2] bpf: setup socket family and addresses in bpf_prog_test_run

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Message-ID: <20200802224850.ezn6tmcz4657ia4z@ast-mbp.dhcp.thefacebook.com>
Date:   Sun, 2 Aug 2020 15:48:50 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Dmitry Yakunin <zeil@...dex-team.ru>
Cc:     daniel@...earbox.net, netdev@...r.kernel.org, bpf@...r.kernel.org,
        sdf@...gle.com
Subject: Re: [PATCH bpf-next v5 1/2] bpf: setup socket family and addresses
 in bpf_prog_test_run_skb

On Mon, Aug 03, 2020 at 12:36:30AM +0300, Dmitry Yakunin wrote:
> Now it's impossible to test all branches of cgroup_skb bpf program which
> accesses skb->family and skb->{local,remote}_ip{4,6} fields because they
> are zeroed during socket allocation. This commit fills socket family and
> addresses from related fields in constructed skb.
> 
> v2:
>   - fix build without CONFIG_IPV6 (kernel test robot <lkp@...el.com>)
> 
> v3:
>   - check skb length before access to inet headers (Eric Dumazet)
> 
> Signed-off-by: Dmitry Yakunin <zeil@...dex-team.ru>
> ---
>  net/bpf/test_run.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index b03c469..8d69295 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -449,6 +449,27 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr,
>  	skb->protocol = eth_type_trans(skb, current->nsproxy->net_ns->loopback_dev);
>  	skb_reset_network_header(skb);
>  
> +	switch (skb->protocol) {
> +	case htons(ETH_P_IP):
> +		sk->sk_family = AF_INET;
> +		if (pskb_may_pull(skb, sizeof(struct iphdr))) {

skb was just inited with __skb_put(skb, size);
Looking at pskb_may_pull() messes with my brain too much,
since it should never go into __pskb_pull_tail path.
Can you open code the skb->len check instead?
if (sizeof(struct iphdr) <= skb_headlen(skb)) {

> +			sk->sk_rcv_saddr = ip_hdr(skb)->saddr;
> +			sk->sk_daddr = ip_hdr(skb)->daddr;
> +		}
> +		break;
> +#if IS_ENABLED(CONFIG_IPV6)
> +	case htons(ETH_P_IPV6):
> +		sk->sk_family = AF_INET6;
> +		if (pskb_may_pull(skb, sizeof(struct ipv6hdr))) {
> +			sk->sk_v6_rcv_saddr = ipv6_hdr(skb)->saddr;
> +			sk->sk_v6_daddr = ipv6_hdr(skb)->daddr;
> +		}
> +		break;
> +#endif
> +	default:
> +		break;
> +	}
> +
>  	if (is_l2)
>  		__skb_push(skb, hh_len);
>  	if (is_direct_pkt_access)
> -- 
> 2.7.4
>