Subject: Re: [PATCH bpf-next 4/4] bpf: allow any port in bpf_bind helper

From: [email-masked]
Date: Tue, 5 May 2020 09:02:05 -0700

On 05/04, Andrey Ignatov wrote:
> Stanislav Fomichev <[email-masked]> [Mon, 2020-05-04 10:34 -0700]:
> > We want to have a tighter control on what ports we bind to in
> > the BPF_CGROUP_INET{4,6}_CONNECT hooks even if it means
> > connect() becomes slightly more expensive. The expensive part
> > comes from the fact that we now need to call inet_csk_get_port()
> > that verifies that the port is not used and allocates an entry
> > in the hash table for it.

> FWIW: Initially that IP_BIND_ADDRESS_NO_PORT limitation came from the
> fact that on my specific use-case (mysql client making 200-500 connects
> per sec to mysql server) disabling the option was making application
> pretty much unusable (inet_csk_get_port was taking more time than mysql
> client connect timeout == 3sec).

> But I guess for some use-cases that call sys_connect not too often it
> makes sense.
Yeah, I don't think we plan to reach those QPS numbers.
But, for the record, did you try to bind to a random port in that
case? And did you bail out on error or did a couple of retries?

> > Since we can't rely on "snum || !bind_address_no_port" to prevent
> > us from calling POST_BIND hook anymore, let's add another bind flag
> > to indicate that the call site is BPF program.
> >
> > Cc: Andrey Ignatov <[email-masked]>
> > Signed-off-by: Stanislav Fomichev <[email-masked]>
> > ---
> >  include/net/inet_common.h                     |   2 +
> >  net/core/filter.c                             |   9 +-
> >  net/ipv4/af_inet.c                            |  10 +-
> >  net/ipv6/af_inet6.c                           |  12 +-
> >  .../bpf/prog_tests/connect_force_port.c       | 104 ++++++++++++++++++
> >  .../selftests/bpf/progs/connect_force_port4.c |  28 +++++
> >  .../selftests/bpf/progs/connect_force_port6.c |  28 +++++
> >  7 files changed, 177 insertions(+), 16 deletions(-)
> >  create mode 100644  
> tools/testing/selftests/bpf/prog_tests/connect_force_port.c
> >  create mode 100644  
> tools/testing/selftests/bpf/progs/connect_force_port4.c
> >  create mode 100644  
> tools/testing/selftests/bpf/progs/connect_force_port6.c

> Documentation in include/uapi/linux/bpf.h should be updated as well
> since now it states this:

>   *              **AF_INET6**). Looking for a free port to bind to can be
>   *              expensive, therefore binding to port is not permitted by  
> the
>   *              helper: *addr*\ **->sin_port** (or **sin6_port**,  
> respectively)
>   *              must be set to zero.

> IMO it's also worth to keep a note on performance implications of
> setting port to non zero.
Ah, thank you, will do!

> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index fa9ddab5dd1f..fc5161b9ff6a 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -4527,29 +4527,24 @@ BPF_CALL_3(bpf_bind, struct bpf_sock_addr_kern  
> *, ctx, struct sockaddr *, addr,
> >  	struct sock *sk = ctx->sk;
> >  	int err;
> >
> > -	/* Binding to port can be expensive so it's prohibited in the helper.
> > -	 * Only binding to IP is supported.
> > -	 */
> >  	err = -EINVAL;
> >  	if (addr_len < offsetofend(struct sockaddr, sa_family))
> >  		return err;
> >  	if (addr->sa_family == AF_INET) {
> >  		if (addr_len < sizeof(struct sockaddr_in))
> >  			return err;
> > -		if (((struct sockaddr_in *)addr)->sin_port != htons(0))
> > -			return err;
> >  		return __inet_bind(sk, addr, addr_len,
> > +				   BIND_FROM_BPF |
> >  				   BIND_FORCE_ADDRESS_NO_PORT);

> Should BIND_FORCE_ADDRESS_NO_PORT be passed only if port is zero?
> Passing non zero port and BIND_FORCE_ADDRESS_NO_PORT at the same time
> looks confusing (even though it works).
Makes sense, will remove it here, thx.