netdev - Re: [Patch bpf-next v2 3/5] bpf: compute data

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Message-ID: <CAM_iQpU+X3iuxwDUK=+=Ux1EetkGYL6q59Wdsr4L1ppKxtfpOw@mail.gmail.com>
Date:   Fri, 12 Feb 2021 11:01:30 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Lorenz Bauer <lmb@...udflare.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        duanxiongchun@...edance.com,
        Dongdong Wang <wangdongdong.6@...edance.com>,
        jiang.wang@...edance.com, Cong Wang <cong.wang@...edance.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [Patch bpf-next v2 3/5] bpf: compute data_end dynamically with
 JIT code

On Fri, Feb 12, 2021 at 2:56 AM Lorenz Bauer <lmb@...udflare.com> wrote:
>
> On Wed, 10 Feb 2021 at 02:22, Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > From: Cong Wang <cong.wang@...edance.com>
> >
> > Currently, we compute ->data_end with a compile-time constant
> > offset of skb. But as Jakub pointed out, we can actually compute
> > it in eBPF JIT code at run-time, so that we can competely get
> > rid of ->data_end. This is similar to skb_shinfo(skb) computation
> > in bpf_convert_shinfo_access().
> >
> > Suggested-by: Jakub Sitnicki <jakub@...udflare.com>
> > Cc: John Fastabend <john.fastabend@...il.com>
> > Cc: Daniel Borkmann <daniel@...earbox.net>
> > Cc: Lorenz Bauer <lmb@...udflare.com>
> > Signed-off-by: Cong Wang <cong.wang@...edance.com>
>
> ...
>
> > @@ -9520,6 +9510,29 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
> >         return insn - insn_buf;
> >  }
> >
> > +static struct bpf_insn *bpf_convert_data_end_access(const struct bpf_insn *si,
> > +                                                   struct bpf_insn *insn)
>
> Is it worth adding a reference to this function in skb_headlen(),
> since we're basically open coding that function here?

I do not mind adding a comment for this.

>
> > +{
> > +       /* si->dst_reg = skb->data */
> > +       *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data),
> > +                             si->dst_reg, si->src_reg,
> > +                             offsetof(struct sk_buff, data));
> > +       /* AX = skb->len */
> > +       *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, len),
> > +                             BPF_REG_AX, si->src_reg,
> > +                             offsetof(struct sk_buff, len));
> > +       /* si->dst_reg = skb->data + skb->len */
> > +       *insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_AX);
> > +       /* AX = skb->data_len */
> > +       *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data_len),
> > +                             BPF_REG_AX, si->src_reg,
> > +                             offsetof(struct sk_buff, data_len));
> > +       /* si->dst_reg = skb->data + skb->len - skb->data_len */
> > +       *insn++ = BPF_ALU64_REG(BPF_SUB, si->dst_reg, BPF_REG_AX);
> > +
> > +       return insn;
> > +}
> > +
> >  static u32 sk_skb_convert_ctx_access(enum bpf_access_type type,
> >                                      const struct bpf_insn *si,
> >                                      struct bpf_insn *insn_buf,
> > @@ -9530,12 +9543,7 @@ static u32 sk_skb_convert_ctx_access(enum bpf_access_type type,
> >
> >         switch (si->off) {
> >         case offsetof(struct __sk_buff, data_end):
> > -               off  = si->off;
> > -               off -= offsetof(struct __sk_buff, data_end);
> > -               off += offsetof(struct sk_buff, cb);
> > -               off += offsetof(struct tcp_skb_cb, bpf.data_end);
> > -               *insn++ = BPF_LDX_MEM(BPF_SIZEOF(void *), si->dst_reg,
> > -                                     si->src_reg, off);
> > +               insn = bpf_convert_data_end_access(si, insn);
>
> This generates a new warning:
>
> ../net/core/filter.c: In function ‘sk_skb_convert_ctx_access’:
> ../net/core/filter.c:9542:6: warning: unused variable ‘off’ [-Wunused-variable]
>  9542 |  int off;
>       |      ^~~

Good catch!

Apparently neither my compiler nor kernel-test-bot's catches this.

Thanks.