[SOLVED] How can I work out the meaning of the return codes for BPF helper functions?

Issue

I am writing a BPF_PROG_TYPE_SOCKET_OPS program and I am seeing the following in /sys/kernel/debug/tracing/trace_pipe:

<...>-12586 [001] ....  6972.409111: 0: update err: -95

when I load it due to the following snippet:

ret = bpf_sock_map_update(ops, &sock_ops, &idx, BPF_ANY);
if (ret < 0) {
    bpf_debug("update err: %d\n", ret);
}

How can I work out what -95 means? When I look in https://elixir.bootlin.com/linux/latest/source/arch/alpha/include/uapi/asm/errno.h I see:

#define ENOCSI      95  /* No CSI structure available */

Is this the correct way to find the meaning of the error? And what is the meaning of the description if so?

Solution

The error was EOPNOTSUPP from here. This was caused by the socket not being 'full' when trying to add the socket (i.e. the TCP connection must be established).

Answered By - dippynark