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

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 28 Nov 2022 12:22:20 +0100

On Wed, Nov 23, 2022 at 02:01 PM +08, Pengcheng Yang wrote:
> John Fastabend <[email-masked]> wrote:
>> 
>> Pengcheng Yang wrote:
>> > When redirecting, we use sk_msg_to_ingress() to get the BPF_F_INGRESS
>> > flag from the msg->flags. If apply_bytes is used and it is larger than
>> > the current data being processed, sk_psock_msg_verdict() will not be
>> > called when sendmsg() is called again. At this time, the msg->flags is 0,
>> > and we lost the BPF_F_INGRESS flag.
>> >
>> > So we need to save the BPF_F_INGRESS flag in sk_psock and assign it to
>> > msg->flags before redirection.
>> >
>> > Fixes: 8934ce2fd081 ("bpf: sockmap redirect ingress support")
>> > Signed-off-by: Pengcheng Yang <[email-masked]>
>> > ---
>> >  include/linux/skmsg.h | 1 +
>> >  net/core/skmsg.c      | 1 +
>> >  net/ipv4/tcp_bpf.c    | 1 +
>> >  net/tls/tls_sw.c      | 1 +
>> >  4 files changed, 4 insertions(+)
>> >
>> > diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
>> > index 48f4b64..e1d463f 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;
>> > +	u32				flags;
>> >  	struct sk_msg			*cork;
>> >  	struct sk_psock_progs		progs;
>> >  #if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
>> > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
>> > index 188f855..ab2f8f3 100644
>> > --- a/net/core/skmsg.c
>> > +++ b/net/core/skmsg.c
>> > @@ -888,6 +888,7 @@ int sk_psock_msg_verdict(struct sock *sk, struct sk_psock *psock,
>> >  		if (psock->sk_redir)
>> >  			sock_put(psock->sk_redir);
>> >  		psock->sk_redir = msg->sk_redir;
>> > +		psock->flags = msg->flags;
>> >  		if (!psock->sk_redir) {
>> >  			ret = __SK_DROP;
>> >  			goto out;
>> > diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
>> > index ef5de4f..1390d72 100644
>> > --- a/net/ipv4/tcp_bpf.c
>> > +++ b/net/ipv4/tcp_bpf.c
>> > @@ -323,6 +323,7 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
>> >  		break;
>> >  	case __SK_REDIRECT:
>> >  		sk_redir = psock->sk_redir;
>> > +		msg->flags = psock->flags;
>> >  		sk_msg_apply_bytes(psock, tosend);
>> >  		if (!psock->apply_bytes) {
>> >  			/* Clean up before releasing the sock lock. */
>>                  ^^^^^^^^^^^^^^^
>> In this block reposted here with the rest of the block
>> 
>> 
>> 		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,
+					    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);