Le 19/03/2021 à 17:55, Martin KaFai Lau a écrit :
On Wed, Mar 17, 2021 at 10:04:18AM +0100, Shanti Lombard née Bouchez-Mongardé wrote:
Q1: How do we prevent socket lookup from triggering BPF sk_lookup causing a
loop?
The bpf_sk_lookup_(tcp|udp) will be called from the BPF_PROG_TYPE_SK_LOOKUP program?
Yes, the idea is to allow the BPF program to redirect incoming
connections for 0.0.0.0:1234 to a specific IP address such as
127.0.12.34:1234 or any other combinaison with a binding not done based
on a predefined socket file descriptor but based on a listening IP
address for a socket.
See linked discussion in the original message
- Solution A: We add a flag to the existing inet_lookup exported function
(and similarly for inet6, udp4 and udp6). The INET_LOOKUP_SKIP_BPF_SK_LOOKUP
flag, when set, would prevent BPF sk_lookup from happening. It also requires
modifying every location making use of those functions.
- Solution B: We export a new symbol in inet_hashtables, a wrapper around
static function inet_lhash2_lookup for inet4 and similar functions for inet6
and udp4/6. Looking up specific IP/port and if not found looking up for
INADDR_ANY could be done in the helper function in net/core/filters.c or in
the BPF program.
Q2: Should we reuse the bpf_sk_lokup_tcp and bpf_sk_lookup_udp helper
functions or create new ones?
If the args passing to the bpf_sk_lookup_(tcp|udp) is the same,
it makes sense to reuse the same BPF_FUNC_sk_lookup_*.
The actual helper implementation could be different though.
Look at bpf_xdp_sk_lookup_tcp_proto and bpf_sk_lookup_tcp_proto.
I was thinking that perhaps a different helper method which would take
IPPROTO_TCP or IPPROTO_UDP parameter would be better suited. it would
avoid BPF code such as :
switch(ctx->protocol){
case IPPROTO_TCP:
sk = bpf_sk_lookup_tcp(ctx, &tuple, tuple_size, -1, 0);
break;
case IPPROTO_UDP:
sk = bpf_sk_lookup_udp(ctx, &tuple, tuple_size, -1, 0);
break;
default:
return SK_PASS;
}
But then there is the limit of 5 arguments, isn't it, so perhaps the
_tcp/_udp functions are not such a bad idea after all.
I saw already that the same helper functions could be given different
implementations. And if there is no way to have more than 5 arguments
then this is probably better to reuse the same helper function name and
signature.
Thank you
