CODEWITHSUNDEEP
linuxsocketsnetwork-programmingxdp-bpf
binaryBigInt
binaryBigInt

Reputation: 1704

AF-XDP: Is there a bug regarding small packets?

Is there a known (or maybe unknown) bug regarding the size of packets in the AF-XDP socket framework (+ libbpf)?

I am experiencing a strange packet loss for my application:

I added a bpf_printk statement in my XDP-Kernelprogram:

const int len = bpf_ntohs(iph->tot_len);
if(len < 400) {
    bpf_printk("FOUND PACKET LEN < 400: %d.\n", len);
}

This output is never observed via sudo cat /sys/kernel/debug/tracing/trace_pipe. So these small RTP-marker packets aren't even received by my kernel filter - no wonder why I don't receive them in userspace.

ethtool -S <if> shows me this number: rx_256_to_511_bytes_phy. This number is increasing in a similar rate as marker-packets should come in (about 30/s). So this means that my NIC does receive the packets but my XDP-program doesn't - why?

Any idea what could be the cause of this problem?

Upvotes: 1

Views: 397

Answers (1)

Que0Le
Que0Le

Reputation: 92

First, bpf_printk() doesn't always work for me. You may want to take a look at this snippet (kernel-space code):

// Nicer way to call bpf_trace_printk()
#define bpf_custom_printk(fmt, ...)                     \
        ({                                              \
            char ____fmt[] = fmt;                       \
            bpf_trace_printk(____fmt, sizeof(____fmt),  \
                    ##__VA_ARGS__);                     \
        })

// print:
bpf_custom_printk("This year is %d\n", 2020);
// output: sudo cat /sys/kernel/debug/tracing/trace_pipe

Second: May be the packet entered the other NIC queue. You may want to use vanilla code from xdp-tutorial and add the kernel tracing from the above snippet to print size of packet, then compile and run the example program with -q 1 for queue number 1 for example.

A way to get size of packet:

void *data_end = (void *)(long)ctx->data_end;
void *data = (void *)(long)ctx->data;
size_t size_pkt = data - data_end;
bpf_custom_printk("Packet size %d\n", size_pkt);

Upvotes: 0

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