On Sun, Jun 17, 2018 at 6:07 PM, Zvi Effron <zeffron@xxxxxxxxxxxxx> wrote:
> Hi XDPeople!
>
> In /include/uapi/linux/bpf.h, (in 4.18-rc1) the comment describing
> bpf_perf_event_output says:
>
> /*
> * Note that this helper is not restricted to tracing use cases
> * and can be used with programs attached to TC or XDP as well,
> * where it allows for passing data to user space listeners. Data
> * can be:
> *
> * * Only custom structs,
> * * Only the packet payload, or
> * * A combination of both.
> */
>
> This seems to imply that for both TC and XDP, the packet can be used
> for passing data. When I try this, the verifier rejects the program
> with "helper access to the packet is not allowed". Looking through the
> kernel it doesn't look like bpf_perf_output_event has been tagged with
> the appropriate metadata to allow it to access the packet structure,
> either for TC or for XDP. Neither bpf_skb_event_output_proto nor
The implementation is in net/core/filter.c
static const struct bpf_func_proto *
xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
{
switch (func_id) {
case BPF_FUNC_perf_event_output:
return &bpf_xdp_event_output_proto;
......
BPF_CALL_5(bpf_xdp_event_output, struct xdp_buff *, xdp, struct bpf_map *, map,
u64, flags, void *, meta, u64, meta_size)
{
u64 xdp_size = (flags & BPF_F_CTXLEN_MASK) >> 32;
if (unlikely(flags & ~(BPF_F_CTXLEN_MASK | BPF_F_INDEX_MASK)))
return -EINVAL;
if (unlikely(xdp_size > (unsigned long)(xdp->data_end - xdp->data)))
return -EFAULT;
return bpf_event_output(map, flags, meta, meta_size, xdp->data,
xdp_size, bpf_xdp_copy);
}
static const struct bpf_func_proto bpf_xdp_event_output_proto = {
.func = bpf_xdp_event_output,
.gpl_only = true,
.ret_type = RET_INTEGER,
.arg1_type = ARG_PTR_TO_CTX,
.arg2_type = ARG_CONST_MAP_PTR,
.arg3_type = ARG_ANYTHING,
.arg4_type = ARG_PTR_TO_MEM,
.arg5_type = ARG_CONST_SIZE_OR_ZERO,
};
Both tracing and networking uses the same func id
BPF_FUNC_perf_event_output, but depending on program type,
the implementation of the helper is different.
> bpf_xdp_event_output_proto have pkt_acess set to true. Is the
> documentation incorrect, should that metadata be updated to allow
> packet access, or is there something I'm missing?
>
> Thank you!
> --Zvi
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