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Message-ID: <875z2i4qo5.fsf@cloudflare.com>
Date: Tue, 23 Feb 2021 18:52:58 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: Cong Wang <xiyou.wangcong@...il.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 08:27 PM CET, Cong Wang wrote:
> 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?
Based on what I've seen around, mask for sanitizing tagged pointers is
usually derived from the flag(s). For instance:
#define SKB_DST_NOREF 1UL
#define SKB_DST_PTRMASK ~(SKB_DST_NOREF)
#define SK_USER_DATA_NOCOPY 1UL
#define SK_USER_DATA_BPF 2UL /* Managed by BPF */
#define SK_USER_DATA_PTRMASK ~(SK_USER_DATA_NOCOPY | SK_USER_DATA_BPF)
Using ~(BPF_F_INGRESS) expression would be like substituting mask
definition.
[..]
>> > 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.
OK
>
>>
>> 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.
OK
>
>
>>
>> > +
>> > 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. ;)
OK, I think I follow you now.
Alternatively we could clear _skb_refdest after clone, but before
enqueuing the skb in ingress_skb. And only for when we're redirecting.
I believe that would be in sk_psock_skb_redirect, right before skb_queue_tail.
>
>>
>> > + 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.
>
OK
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