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?
> +{
> + /* 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;
| ^~~
--
Lorenz Bauer | Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
www.cloudflare.com
