Subject: Re: [PATCH RESEND bpf 2/4] bpf, sockmap: Fix missing BPF_F_INGRESS flag when using apply_bytes

From: "Pengcheng Yang" <[email-masked]>
Date: Tue, 29 Nov 2022 16:02:48 +0800

Jakub Sitnicki <[email-masked]> wrote:
> On Wed, Nov 23, 2022 at 02:01 PM +08, Pengcheng Yang wrote:
> > John Fastabend <[email-masked]> wrote:
> >>
> >> 		if (!psock->apply_bytes) {
> >> 			/* Clean up before releasing the sock lock. */
> >> 			eval = psock->eval;
> >> 			psock->eval = __SK_NONE;
> >> 			psock->sk_redir = NULL;
> >> 		}
> >>
> >> Now that we have a psock->flags we should clera that as
> >> well right?
> >
> > According to my understanding, it is not necessary (but can) to clear
> > psock->flags here, because psock->flags will be overwritten by msg->flags
> > at the beginning of each redirection (in sk_psock_msg_verdict()).
> 
> 1. We should at least document that psock->flags value can be garbage
>    (undefined) if psock->sk_redir is null.
> 
> 2. 'flags' is amiguous (flags for what?). I'd suggest to rename to
>    something like redir_flags.
> 
>    Also, since we don't care about all flags, but just the ingress bit,
>    we should store just that. It's not about size. Less state passed
>    around is easier to reason about. See patch below.
> 
> 3. Alternatively, I'd turn psock->sk_redir into a tagged pointer, like
>    skb->_sk_redir is. This way all state (pointer & flags) is bundled
>    and managed together. It would be a bigger change. Would have to be
>    split out from this patch set.
> 
> --8<--
> 
> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> index 70d6cb94e580..84f787416a54 100644
> --- a/include/linux/skmsg.h
> +++ b/include/linux/skmsg.h
> @@ -82,6 +82,7 @@ struct sk_psock {
>  	u32				apply_bytes;
>  	u32				cork_bytes;
>  	u32				eval;
> +	bool				redir_ingress; /* undefined if sk_redir is null */
>  	struct sk_msg			*cork;
>  	struct sk_psock_progs		progs;
>  #if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 14d45661a84d..5b70b241ce71 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -2291,8 +2291,8 @@ int tcp_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore);
>  void tcp_bpf_clone(const struct sock *sk, struct sock *newsk);
>  #endif /* CONFIG_BPF_SYSCALL */
> 
> -int tcp_bpf_sendmsg_redir(struct sock *sk, struct sk_msg *msg, u32 bytes,
> -			  int flags);
> +int tcp_bpf_sendmsg_redir(struct sock *sk, bool ingress,
> +			  struct sk_msg *msg, u32 bytes, int flags);
>  #endif /* CONFIG_NET_SOCK_MSG */
> 
>  #if !defined(CONFIG_BPF_SYSCALL) || !defined(CONFIG_NET_SOCK_MSG)
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index e6b9ced3eda8..53d0251788aa 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -886,13 +886,16 @@ int sk_psock_msg_verdict(struct sock *sk, struct sk_psock *psock,
>  	ret = sk_psock_map_verd(ret, msg->sk_redir);
>  	psock->apply_bytes = msg->apply_bytes;
>  	if (ret == __SK_REDIRECT) {
> -		if (psock->sk_redir)
> +		if (psock->sk_redir) {
>  			sock_put(psock->sk_redir);
> -		psock->sk_redir = msg->sk_redir;
> -		if (!psock->sk_redir) {
> +			psock->sk_redir = NULL;
> +		}
> +		if (!msg->sk_redir) {
>  			ret = __SK_DROP;
>  			goto out;
>  		}
> +		psock->redir_ingress = sk_msg_to_ingress(msg);
> +		psock->sk_redir = msg->sk_redir;
>  		sock_hold(psock->sk_redir);
>  	}
>  out:
> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> index cf9c3e8f7ccb..490b359dc814 100644
> --- a/net/ipv4/tcp_bpf.c
> +++ b/net/ipv4/tcp_bpf.c
> @@ -131,10 +131,9 @@ static int tcp_bpf_push_locked(struct sock *sk, struct sk_msg *msg,
>  	return ret;
>  }
> 
> -int tcp_bpf_sendmsg_redir(struct sock *sk, struct sk_msg *msg,
> -			  u32 bytes, int flags)
> +int tcp_bpf_sendmsg_redir(struct sock *sk, bool ingress,
> +			  struct sk_msg *msg, u32 bytes, int flags)
>  {
> -	bool ingress = sk_msg_to_ingress(msg);
>  	struct sk_psock *psock = sk_psock_get(sk);
>  	int ret;
> 
> @@ -337,7 +336,8 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
>  		release_sock(sk);
> 
>  		origsize = msg->sg.size;
> -		ret = tcp_bpf_sendmsg_redir(sk_redir, msg, tosend, flags);
> +		ret = tcp_bpf_sendmsg_redir(sk_redir, psock->redir_ingress,
                                       ^^^^^^^
Thanks for such detailed advice.
Here it looks like we need to pre-save the redir_ingress before
setting psock->sk_redir to NULL and release_sock.

> +					    msg, tosend, flags);
>  		sent = origsize - msg->sg.size;
> 
>  		if (eval == __SK_REDIRECT)
> diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
> index 264cf367e265..b22d97610b9a 100644
> --- a/net/tls/tls_sw.c
> +++ b/net/tls/tls_sw.c
> @@ -846,7 +846,8 @@ static int bpf_exec_tx_verdict(struct sk_msg *msg, struct sock *sk,
>  		sk_msg_return_zero(sk, msg, send);
>  		msg->sg.size -= send;
>  		release_sock(sk);
> -		err = tcp_bpf_sendmsg_redir(sk_redir, &msg_redir, send, flags);
> +		err = tcp_bpf_sendmsg_redir(sk_redir, psock->redir_ingress,
> +					    &msg_redir, send, flags);
>  		lock_sock(sk);
>  		if (err < 0) {
>  			*copied -= sk_msg_free_nocharge(sk, &msg_redir);