On Fri, Feb 12, 2021 at 2:56 AM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote:
>
> On Wed, 10 Feb 2021 at 02:22, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> >
> > From: Cong Wang <cong.wang@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxx>
> > Cc: John Fastabend <john.fastabend@xxxxxxxxx>
> > Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > Cc: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> > Signed-off-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
>
> ...
>
> > @@ -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.
