Re: [PATCH] man/man7/socket.7: fix documentation for SO_ATTACH_REUSEPORT_EBPF

[Vincent Bernat <vincent@bernat.ch>]

On 2025-11-03 17:31, Craig Gallek wrote:

>      > On Mon, Oct 27, 2025 at 02:17:00PM +0100, Vincent Bernat wrote:
>      >> When using eBPF, the program should not return an index, but a
>      >> decision (SK_PASS/SK_DROP). The socket should be selected using the
>      >> sk_select_reuseport helper.
>      >>
>      >> Cc: Craig Gallek <kraig@google.com <mailto:kraig@google.com>>
>      >> Signed-off-by: Vincent Bernat <vincent@bernat.ch
>     <mailto:vincent@bernat.ch>>
>      >
>      > Would you mind adding a Fixes: tag for the commit that introduced the
>      > incorrect text?
> 
>     This is 1fa871f5378e (2016-03-01; "socket.7: Document some BPF-related
>     socket options")
>      > Also, do you have a guess of why the wrong text was written?  It
>     would
>      > be useful to have an idea of that.
> 
>     Not a clue, but maybe let's wait when Craig comes back from vacation so
>     he could say if there was a change or not and tell if I am correct.
> 
> 
> It's been a while since I've used this code, but it looks like the 
> sk_select_reuseport helper was added a few years later by Martin KaFai 
> Lau in 2dbb9b9e6df6.  The execution of the bpf program (run_bpf_filter 
> in sock_reuseport.c) still uses the program's 'returned' value in both 
> the cbpf and ebpf cases, but it looks like Martin's change tightened up 
> the 'type' of eBPF program that can be used for this purpose and 
> included this helper function to allow for an explicit validation 
> instead of just having the program return a raw integer value.
> 
> So I think this is just a case of the documentation not keeping up with 
> the code.  I'm less familiar with the PASS/DROP syntax, but I'm guessing 
> that came along after the initial implementation of these reuseport BPF 
> programs as well.
> 
> In any event, my quick read of the code lines up with your proposed 
> documentation change.  It might be worth having Martin verify as well, 
> but this looks good to me.

OK, so, two things are possible and the kernel selects depending on the 
program type (BPF_PROG_TYPE_SOCKET_FILTER or BPF_PROG_TYPE_SK_REUSEPORT).

if (prog->type == BPF_PROG_TYPE_SK_REUSEPORT)
     sk2 = bpf_run_sk_reuseport(reuse, sk, prog, skb, NULL, hash);
else
     sk2 = run_bpf_filter(reuse, socks, prog, skb, hdr_len);

So, I can update the documentation for that, as I have missed it. 
Waiting for Martin feedback in case there is something else.