On Thu, Mar 03, 2022 at 11:55:55PM +0100, Daniel Borkmann wrote:
> On 3/3/22 9:43 PM, Martin KaFai Lau wrote:
> > On Thu, Mar 03, 2022 at 02:00:37PM +0100, Daniel Borkmann wrote:
> > > On 3/2/22 8:56 PM, Martin KaFai Lau wrote:
> > > > +static struct bpf_insn *bpf_convert_tstamp_write(const struct bpf_insn *si,
> > > > + struct bpf_insn *insn)
> > > > +{
> > > > + __u8 value_reg = si->src_reg;
> > > > + __u8 skb_reg = si->dst_reg;
> > > > +
> > > > +#ifdef CONFIG_NET_CLS_ACT
> > > > + __u8 tmp_reg = BPF_REG_AX;
> > > > +
> > > > + *insn++ = BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg, TC_AT_INGRESS_OFFSET);
> > > > + *insn++ = BPF_ALU32_IMM(BPF_AND, tmp_reg, TC_AT_INGRESS_MASK);
> > >
> > > Can't we get rid of tcf_bpf_act() and cls_bpf_classify() changes altogether by just doing:
> > >
> > > /* BPF_WRITE: __sk_buff->tstamp = a */
> > > skb->mono_delivery_time = !skb->tc_at_ingress && a;
> > > skb->tstamp = a;
> > It will then assume the bpf prog is writing a mono time.
> > Although mono should always be the case now, this assumption will be
> > an issue in the future if we need to support non-mono.
>
> Right, for that we should probably instrument verifier to track base based
> on ktime helper call once we get to that point.
The bpf prog does not necessary write a value which is obtained from a
bpf ktime helper. It could be a value based on the previous skb sent
from a cgroup.
>
> > > (Untested) pseudo code:
> > >
> > > // or see comment on common SKB_FLAGS_OFFSET define or such
> > > BUILD_BUG_ON(TC_AT_INGRESS_OFFSET != SKB_MONO_DELIVERY_TIME_OFFSET)
> > >
> > > BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg, SKB_MONO_DELIVERY_TIME_OFFSET)
> > > BPF_ALU32_IMM(BPF_OR, tmp_reg, SKB_MONO_DELIVERY_TIME_MASK)
> > > BPF_JMP32_IMM(BPF_JSET, tmp_reg, TC_AT_INGRESS_MASK, <clear>)
> > This can save a BPF_ALU32_IMM(BPF_AND). I will do that
> > together in the follow up. Thanks for the idea !
>
> Yeah the JSET comes in handy here.
>
> > > BPF_JMP32_REG(BPF_JGE, value_reg, tmp_reg, <store>)
> > > <clear>:
> > > BPF_ALU32_IMM(BPF_AND, tmp_reg, ~SKB_MONO_DELIVERY_TIME_MASK)
> > > <store>:
> > > BPF_STX_MEM(BPF_B, skb_reg, tmp_reg, SKB_MONO_DELIVERY_TIME_OFFSET)
> > > BPF_STX_MEM(BPF_DW, skb_reg, value_reg, offsetof(struct sk_buff, tstamp))
> > >
> > > (There's a small hack with the BPF_JGE for tmp_reg, so constant blinding for AX doesn't
> > > get into our way.)
> > >
> > > > + *insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg, 0, 3);
> > > > + /* Writing __sk_buff->tstamp at ingress as the (rcv) timestamp.
> > > > + * Clear the skb->mono_delivery_time.
> > > > + */
> > > > + *insn++ = BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg,
> > > > + SKB_MONO_DELIVERY_TIME_OFFSET);
> > > > + *insn++ = BPF_ALU32_IMM(BPF_AND, tmp_reg,
> > > > + ~SKB_MONO_DELIVERY_TIME_MASK);
> > > > + *insn++ = BPF_STX_MEM(BPF_B, skb_reg, tmp_reg,
> > > > + SKB_MONO_DELIVERY_TIME_OFFSET);
> > > > +#endif
> > > > +
> > > > + /* skb->tstamp = tstamp */
> > > > + *insn++ = BPF_STX_MEM(BPF_DW, skb_reg, value_reg,
> > > > + offsetof(struct sk_buff, tstamp));
> > > > + return insn;
> > > > +}
> > > > +
>
