On Wed, Feb 03, 2021 at 05:16 AM CET, Cong Wang wrote:
> From: Cong Wang <cong.w...@bytedance.com>
>
> Currently TCP_SKB_CB() is hard-coded in skmsg code, it certainly
> won't work for any other non-TCP protocols. We can move them to
> skb ext instead of playing with skb cb, which is harder to make
> correct.
>
> Of course, except ->data_end, which is used by
> sk_skb_convert_ctx_access() to adjust compile-time constant offset.
> Fortunately, we can reuse the anonymous union where the field
> 'tcp_tsorted_anchor' is and save/restore the overwritten part
> before/after a brief use.
>
> Cc: John Fastabend <john.fastab...@gmail.com>
> Cc: Daniel Borkmann <dan...@iogearbox.net>
> Cc: Jakub Sitnicki <ja...@cloudflare.com>
> Cc: Lorenz Bauer <l...@cloudflare.com>
> Signed-off-by: Cong Wang <cong.w...@bytedance.com>
> ---
> include/linux/skbuff.h | 4 ++++
> include/linux/skmsg.h | 45 ++++++++++++++++++++++++++++++++++++++++++
> include/net/tcp.h | 25 -----------------------
> net/Kconfig | 1 +
> net/core/filter.c | 3 +--
> net/core/skbuff.c | 7 +++++++
> net/core/skmsg.c | 44 ++++++++++++++++++++++++++++-------------
> net/core/sock_map.c | 12 +++++------
> 8 files changed, 94 insertions(+), 47 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 46f901adf1a8..12a28268233a 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -755,6 +755,7 @@ struct sk_buff {
> void (*destructor)(struct sk_buff *skb);
> };
> struct list_head tcp_tsorted_anchor;
> + void *data_end;
> };
I think we can avoid `data_end` by computing it in BPF with the help of
a scratch register. Similar to how we compute skb_shinfo(skb) in
bpf_convert_shinfo_access(). Something like:
static struct bpf_insn *bpf_convert_data_end_access(const struct bpf_insn *si,
struct bpf_insn *insn)
{
/* 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));
/* 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));
/* dst_reg = skb->data + skb->len - skb->data_len */
*insn++ = BPF_ALU64_REG(BPF_SUB, si->dst_reg, BPF_REG_AX);
return insn;
}
[...]
