Re: [PATCH net-next v2] tcp: Dump bound-only sockets in inet_diag.

[Guillaume Nault <gnault@redhat.com>]

On Wed, Nov 29, 2023 at 04:39:55PM +0100, Eric Dumazet wrote:
> On Tue, Nov 28, 2023 at 11:18 PM Guillaume Nault <gnault@redhat.com> wrote:
> >
> > On Tue, Nov 28, 2023 at 11:14:28AM +0100, Eric Dumazet wrote:
> > > On Fri, Nov 24, 2023 at 12:11 AM Guillaume Nault <gnault@redhat.com> wrote:
> > > >
> > > > Walk the hashinfo->bhash2 table so that inet_diag can dump TCP sockets
> > > > that are bound but haven't yet called connect() or listen().
> > > >
> > > > This allows ss to dump bound-only TCP sockets, together with listening
> > > > sockets (as there's no specific state for bound-only sockets). This is
> > > > similar to the UDP behaviour for which bound-only sockets are already
> > > > dumped by ss -lu.
> > > >
> > > > The code is inspired by the ->lhash2 loop. However there's no manual
> > > > test of the source port, since this kind of filtering is already
> > > > handled by inet_diag_bc_sk(). Also, a maximum of 16 sockets are dumped
> > > > at a time, to avoid running with bh disabled for too long.
> > > >
> > > > No change is needed for ss. With an IPv4, an IPv6 and an IPv6-only
> > > > socket, bound respectively to 40000, 64000, 60000, the result is:
> > > >
> > > >   $ ss -lt
> > > >   State  Recv-Q Send-Q Local Address:Port  Peer Address:PortProcess
> > > >   UNCONN 0      0            0.0.0.0:40000      0.0.0.0:*
> > > >   UNCONN 0      0               [::]:60000         [::]:*
> > > >   UNCONN 0      0                  *:64000            *:*
> > >
> > >
> > > Hmm...   "ss -l" is supposed to only list listening sockets.
> > >
> > > So this change might confuse some users ?
> > >
> >
> > On the other hand I can't find a more sensible solution. The problem is
> > that "ss -l" sets both the TCPF_LISTEN and the TCPF_CLOSE flags. And
> > since we don't have a way to express "bound but not yet listening"
> > sockets, these sockets fall into the CLOSE category. So we're really
> > just returning what ss asked for.
> >
> > If we can't rely on TCPF_CLOSE, then I don't see what kind of filter we
> > could use to request a dump of these TCP sockets. Using "-a" doesn't
> > help as it just sets all the TCPF_* flags (appart from
> > TCPF_NEW_SYN_RECV). Adding a new option wouldn't help either as we
> > couldn't map it to any of the TCPF_* flags. In any case, we still need
> > to rely on TCPF_CLOSE.
> >
> > So maybe we can just improve the ss man page for "-l" and explain that
> > it also lists closed sockets, which includes the bound-only ones
> > (this is already true for non-TCP sockets anyway). We could also tell
> > the user to run "ss state listening" for getting listening sockets
> > exclusively (or we could implement a new option, like "-L", to make
> > that shorter if necessary).
> 
> This exists already : ss -t state LISTENING
> 
> >
> > What do you think?
> >
> 
> We might need a new bit in r->idiag_state (we have a lot of free bits
> there), different from
> the combination used by "ss -l"  which unfortunately used ( (1 <<
> SS_LISTEN) | (1 << SS_CLOSE) ) )
> 
> "ss -t state bound" (or ss -tB ???)  would then set this new bit ( 1
> << SS_BOUND) and the kernel would handle this pseudo state ?
> 
> (mapped to CLOSED and in bhash2)

I'll do that in v3. Thanks.

> diff --git a/include/net/tcp_states.h b/include/net/tcp_states.h
> index cc00118acca1b695a534bd73984b9d1f1794db25..97238c0f64aa6643cf492a856e8d67ddcca1a729
> 100644
> --- a/include/net/tcp_states.h
> +++ b/include/net/tcp_states.h
> @@ -22,6 +22,7 @@ enum {
>         TCP_LISTEN,
>         TCP_CLOSING,    /* Now a valid state */
>         TCP_NEW_SYN_RECV,
> +       TCP_BOUND,
> 
>         TCP_MAX_STATES  /* Leave at the end! */
>  };
> @@ -43,6 +44,7 @@ enum {
>         TCPF_LISTEN      = (1 << TCP_LISTEN),
>         TCPF_CLOSING     = (1 << TCP_CLOSING),
>         TCPF_NEW_SYN_RECV = (1 << TCP_NEW_SYN_RECV),
> +       TCPF_BOUND       = (1 << TCP_BOUND),
>  };
> 
>  #endif /* _LINUX_TCP_STATES_H */
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index a62423360a5681525496f8840bfe1d37ea3dc504..c3d22edb9e9563672356750153167884c92eae67
> 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -6570,6 +6570,7 @@ enum {
>         BPF_TCP_LISTEN,
>         BPF_TCP_CLOSING,        /* Now a valid state */
>         BPF_TCP_NEW_SYN_RECV,
> +       BPF_TCP_BOUND,
> 
>         BPF_TCP_MAX_STATES      /* Leave at the end! */
>  };
>