Re: AF_XDP sendto kick returning EPERM

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Fri, Apr 23, 2021 at 5:44 PM Srivats P <pstavirs@gmail.com> wrote:
>
> Hi,
>
> I'm using sendto() to kick tx in my AF_XDP program after I submit
> descriptors to the tx ring -
>
> ret = sendto(xsk_socket__fd(xsk_), NULL, 0, MSG_DONTWAIT, NULL, 0);
>
> However, I'm receiving EPERM as the return value every time. AFAIK
> this is not an expected return value. Since this is with i40e, I
> checked i40e_xsk_wakeup() - but that also doesn't return EPERM. I am
> running as root and I don't see any problems with creating the xsk,
> configuring umem etc.
>
> Also, no packets seem to go out either.
>
> # uname -a
> Linux Ostinato-1 5.11.15-1-default #1 SMP Fri Apr 16 16:47:34 UTC 2021
> (64fb5bf) x86_64 x86_64 x86_64 GNU/Linux
>
> I don't see the problem on another machine with i40e but older kernel 5.4 series
>
> Any suggestions on what to look for or how to proceed?

Weird. Have not seen this before. What is your command line for
xdpsock? Is it unmodified?

Using bpftrace, we can get the call stack of xsk_sendmsg. Somewhere in
this stack there must be an EPERM. You can run the same command on
your system, but use ftrace to see what a sendto call hits. Then see
where the code terminates.

mkarlsso@kurt:~/src/dna-linux$ sudo bpftrace -e 'kprobe:xsk_sendmsg {
@[kstack()] = count(); }'
Attaching 1 probe...
^C

@[
    xsk_sendmsg+1
    sock_sendmsg+94
    __sys_sendto+238
    __x64_sys_sendto+37
    do_syscall_64+51
    entry_SYSCALL_64_after_hwframe+68
]: 2244805


> Srivats