Re: [PATCH bpf v3 2/2] bpf, sockmap: sk_prot needs inuse_idx set for proc stats

[Jakub Sitnicki <jakub@cloudflare.com>]

On Tue, Jul 06, 2021 at 06:31 PM CEST, John Fastabend wrote:
> Proc socket stats use sk_prot->inuse_idx value to record inuse sock stats.
> We currently do not set this correctly from sockmap side. The result is
> reading sock stats '/proc/net/sockstat' gives incorrect values. The
> socket counter is incremented correctly, but because we don't set the
> counter correctly when we replace sk_prot we may omit the decrement.
>
> Fixes: 604326b41a6fb ("bpf, sockmap: convert to generic sk_msg interface")
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
>  net/core/sock_map.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 60decd6420ca..27bdf768aa8c 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -185,10 +185,19 @@ static void sock_map_unref(struct sock *sk, void *link_raw)
>
>  static int sock_map_init_proto(struct sock *sk, struct sk_psock *psock)
>  {
> +	int err;
> +#ifdef CONFIG_PROC_FS
> +	int idx = sk->sk_prot->inuse_idx;
> +#endif
>  	if (!sk->sk_prot->psock_update_sk_prot)
>  		return -EINVAL;
>  	psock->psock_update_sk_prot = sk->sk_prot->psock_update_sk_prot;
> -	return sk->sk_prot->psock_update_sk_prot(sk, psock, false);
> +	err = sk->sk_prot->psock_update_sk_prot(sk, psock, false);
> +#ifdef CONFIG_PROC_FS
> +	if (!err)
> +		sk->sk_prot->inuse_idx = idx;
> +#endif
> +	return err;
>  }
>
>  static struct sk_psock *sock_map_psock_get_checked(struct sock *sk)

We could initialize inuse_idx just once in {tcp,udp}_bpf_rebuild_protos,
if we changed {tcp,udp}_bpf_v4_build_proto to be a late_initcall, so
that it runs after inet_init when {tcp,udp}_prot and udp_prot are
already registered and have inuse_idx assigned.