Capture xdp packets in an fentry BPF hook

* Capture xdp packets in an fentry BPF hook
@ 2020-02-19 14:38 Eelco Chaudron
  2020-02-19 20:36 ` Alexei Starovoitov
  0 siblings, 1 reply; 6+ messages in thread
From: Eelco Chaudron @ 2020-02-19 14:38 UTC (permalink / raw)
  To: Alexei Starovoitov
  Cc: Xdp, bpf, Toke Høiland-Jørgensen, Jesper Dangaard Brouer

Hi Alexei at al.,

I'm getting closer to finally have an xdpdump tool that uses the bpf 
fentry/fexit tracepoints, but I ran into a final hurdle...

To stuff the packet into a perf ring I'll need to use the 
bpf_perf_event_output(), but unfortunately, this is a program of trace 
type, and not XDP so the packet data is not added automatically :(

Secondly even trying to pass the actual packet data as a reference to 
bpf_perf_event_output() will not work as the verifier wants the data to 
be on the fp.

Even worse, the trace program gets the XDP info not thought the ctx, but 
trough the fentry/fexit input value, i.e.:

	SEC("fentry/func")
	int BPF_PROG(trace_on_entry, struct xdp_buff *xdp)...

	struct net_device {
	    int ifindex;
	} __attribute__((preserve_access_index));

	struct xdp_rxq_info {
	    struct net_device *dev;
	    __u32 queue_index;
	} __attribute__((preserve_access_index));

	struct xdp_buff {
	    void *data;
	    void *data_end;
	    void *data_meta;
	    void *data_hard_start;
	    unsigned long handle;
	    struct xdp_rxq_info *rxq;
	} __attribute__((preserve_access_index));

Hence even trying to copy in bytes to a local buffer is not allowed by 
the verifier, i.e. __u8 *data = (u8 *)(long)xdp->data;

Can you let me know how you envisioned a BPF entry hook to capture 
packets from XDP. Am I missing something, or is there something missing 
from the infrastructure?

Thanks,

Eelco

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