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Date: Mon, 22 Feb 2021 11:27:04 -0800
From: Cong Wang <xiyou.wangcong@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>
Cc: Linux Kernel Network Developers <netdev@...r.kernel.org>,
bpf <bpf@...r.kernel.org>, duanxiongchun@...edance.com,
Dongdong Wang <wangdongdong.6@...edance.com>,
Jiang Wang <jiang.wang@...edance.com>,
Cong Wang <cong.wang@...edance.com>,
Daniel Borkmann <daniel@...earbox.net>,
Lorenz Bauer <lmb@...udflare.com>,
John Fastabend <john.fastabend@...il.com>
Subject: Re: [Patch bpf-next v6 4/8] skmsg: move sk_redir from TCP_SKB_CB to skb
On Mon, Feb 22, 2021 at 4:20 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> On Sat, Feb 20, 2021 at 06:29 AM CET, Cong Wang wrote:
> > From: Cong Wang <cong.wang@...edance.com>
> >
> > Currently TCP_SKB_CB() is hard-coded in skmsg code, it certainly
> > does not work for any other non-TCP protocols. We can move them to
> > skb ext, but it introduces a memory allocation on fast path.
> >
> > Fortunately, we only need to a word-size to store all the information,
> > because the flags actually only contains 1 bit so can be just packed
> > into the lowest bit of the "pointer", which is stored as unsigned
> > long.
> >
> > Inside struct sk_buff, '_skb_refdst' can be reused because skb dst is
> > no longer needed after ->sk_data_ready() so we can just drop it.
> >
> > Cc: Daniel Borkmann <daniel@...earbox.net>
> > Cc: Jakub Sitnicki <jakub@...udflare.com>
> > Cc: Lorenz Bauer <lmb@...udflare.com>
> > Acked-by: John Fastabend <john.fastabend@...il.com>
> > Signed-off-by: Cong Wang <cong.wang@...edance.com>
> > ---
>
> LGTM. I have some questions (below) that would help me confirm if I
> understand the changes, and what could be improved, if anything.
>
> Acked-by: Jakub Sitnicki <jakub@...udflare.com>
>
> > include/linux/skbuff.h | 3 +++
> > include/linux/skmsg.h | 35 +++++++++++++++++++++++++++++++++++
> > include/net/tcp.h | 19 -------------------
> > net/core/skmsg.c | 32 ++++++++++++++++++++------------
> > net/core/sock_map.c | 8 ++------
> > 5 files changed, 60 insertions(+), 37 deletions(-)
> >
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 6d0a33d1c0db..bd84f799c952 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -755,6 +755,9 @@ struct sk_buff {
> > void (*destructor)(struct sk_buff *skb);
> > };
> > struct list_head tcp_tsorted_anchor;
> > +#ifdef CONFIG_NET_SOCK_MSG
> > + unsigned long _sk_redir;
> > +#endif
> > };
> >
> > #if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
> > diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> > index e3bb712af257..fc234d507fd7 100644
> > --- a/include/linux/skmsg.h
> > +++ b/include/linux/skmsg.h
> > @@ -459,4 +459,39 @@ static inline bool sk_psock_strp_enabled(struct sk_psock *psock)
> > return false;
> > return !!psock->saved_data_ready;
> > }
> > +
> > +#if IS_ENABLED(CONFIG_NET_SOCK_MSG)
> > +static inline bool skb_bpf_ingress(const struct sk_buff *skb)
> > +{
> > + unsigned long sk_redir = skb->_sk_redir;
> > +
> > + return sk_redir & BPF_F_INGRESS;
> > +}
> > +
> > +static inline void skb_bpf_set_ingress(struct sk_buff *skb)
> > +{
> > + skb->_sk_redir |= BPF_F_INGRESS;
> > +}
> > +
> > +static inline void skb_bpf_set_redir(struct sk_buff *skb, struct sock *sk_redir,
> > + bool ingress)
> > +{
> > + skb->_sk_redir = (unsigned long)sk_redir;
> > + if (ingress)
> > + skb->_sk_redir |= BPF_F_INGRESS;
> > +}
> > +
> > +static inline struct sock *skb_bpf_redirect_fetch(const struct sk_buff *skb)
> > +{
> > + unsigned long sk_redir = skb->_sk_redir;
> > +
> > + sk_redir &= ~0x1UL;
>
> We're using the enum when setting the bit flag, but a hardcoded constant
> when masking it. ~BPF_F_INGRESS would be more consistent here.
Well, here we need a mask, not a bit, but we don't have a mask yet,
hence I just use hard-coded 0x1. Does #define BPF_F_MASK 0x1UL
look any better?
>
> > + return (struct sock *)sk_redir;
> > +}
> > +
> > +static inline void skb_bpf_redirect_clear(struct sk_buff *skb)
> > +{
> > + skb->_sk_redir = 0;
> > +}
> > +#endif /* CONFIG_NET_SOCK_MSG */
> > #endif /* _LINUX_SKMSG_H */
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index 947ef5da6867..075de26f449d 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -883,30 +883,11 @@ struct tcp_skb_cb {
> > struct inet6_skb_parm h6;
> > #endif
> > } header; /* For incoming skbs */
> > - struct {
> > - __u32 flags;
> > - struct sock *sk_redir;
> > - } bpf;
> > };
> > };
> >
> > #define TCP_SKB_CB(__skb) ((struct tcp_skb_cb *)&((__skb)->cb[0]))
> >
> > -static inline bool tcp_skb_bpf_ingress(const struct sk_buff *skb)
> > -{
> > - return TCP_SKB_CB(skb)->bpf.flags & BPF_F_INGRESS;
> > -}
> > -
> > -static inline struct sock *tcp_skb_bpf_redirect_fetch(struct sk_buff *skb)
> > -{
> > - return TCP_SKB_CB(skb)->bpf.sk_redir;
> > -}
> > -
> > -static inline void tcp_skb_bpf_redirect_clear(struct sk_buff *skb)
> > -{
> > - TCP_SKB_CB(skb)->bpf.sk_redir = NULL;
> > -}
> > -
> > extern const struct inet_connection_sock_af_ops ipv4_specific;
> >
> > #if IS_ENABLED(CONFIG_IPV6)
> > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > index 2d8bbb3fd87c..05b5af09ff42 100644
> > --- a/net/core/skmsg.c
> > +++ b/net/core/skmsg.c
> > @@ -494,6 +494,8 @@ static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb
> > static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb,
> > u32 off, u32 len, bool ingress)
> > {
> > + skb_bpf_redirect_clear(skb);
>
> This is called to avoid leaking state in skb->_skb_refdst. Correct?
This is to teach kfree_skb() not to consider it as a valid _skb_refdst.
>
> I'm wondering why we're doing it every time sk_psock_handle_skb() gets
> invoked from the do/while loop in sk_psock_backlog(), instead of doing
> it once after reading ingress flag with skb_bpf_ingress()?
It should also work, I don't see much difference here, as we almost
always process a full skb, that is, ret == skb->len.
>
> > +
> > if (!ingress) {
> > if (!sock_writeable(psock->sk))
> > return -EAGAIN;
> > @@ -525,7 +527,7 @@ static void sk_psock_backlog(struct work_struct *work)
> > len = skb->len;
> > off = 0;
> > start:
> > - ingress = tcp_skb_bpf_ingress(skb);
> > + ingress = skb_bpf_ingress(skb);
> > do {
> > ret = -EIO;
> > if (likely(psock->sk->sk_socket))
> > @@ -631,7 +633,12 @@ void __sk_psock_purge_ingress_msg(struct sk_psock *psock)
> >
> > static void sk_psock_zap_ingress(struct sk_psock *psock)
> > {
> > - __skb_queue_purge(&psock->ingress_skb);
> > + struct sk_buff *skb;
> > +
> > + while ((skb = __skb_dequeue(&psock->ingress_skb)) != NULL) {
> > + skb_bpf_redirect_clear(skb);
>
> I believe we clone the skb before enqueuing it psock->ingress_skb.
> Clone happens either in sk_psock_verdict_recv() or in __strp_recv().
> There are not other users holding a ref, so clearing the redirect seems
> unneeded. Unless I'm missing something?
Yes, skb dst is also cloned:
980 static void __copy_skb_header(struct sk_buff *new, const struct
sk_buff *old)
981 {
982 new->tstamp = old->tstamp;
983 /* We do not copy old->sk */
984 new->dev = old->dev;
985 memcpy(new->cb, old->cb, sizeof(old->cb));
986 skb_dst_copy(new, old);
Also, if without this, dst_release() would complain again. I was not smart
enough to add it in the beginning, dst_release() taught me this lesson. ;)
>
> > + kfree_skb(skb);
> > + }
> > __sk_psock_purge_ingress_msg(psock);
> > }
> >
> > @@ -752,7 +759,7 @@ static void sk_psock_skb_redirect(struct sk_buff *skb)
> > struct sk_psock *psock_other;
> > struct sock *sk_other;
> >
> > - sk_other = tcp_skb_bpf_redirect_fetch(skb);
> > + sk_other = skb_bpf_redirect_fetch(skb);
> > /* This error is a buggy BPF program, it returned a redirect
> > * return code, but then didn't set a redirect interface.
> > */
> > @@ -802,9 +809,10 @@ int sk_psock_tls_strp_read(struct sk_psock *psock, struct sk_buff *skb)
> > * TLS context.
> > */
> > skb->sk = psock->sk;
> > - tcp_skb_bpf_redirect_clear(skb);
> > + skb_dst_drop(skb);
> > + skb_bpf_redirect_clear(skb);
>
> After skb_dst_drop(), skb->_skb_refdst is clear. So it seems the
> redirect_clear() is not needed. But I'm guessing it is being invoked
> to communicate the intention?
Technically true, but I prefer to call them explicitly, not to rely on the
fact skb->_skb_refdst shares the same storage with skb->_sk_redir,
which would also require some comments to explain.
Thanks.
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