Design for sk_lookup helper function in context of sk_lookup hook

Hello everyone,

Background discussion: 
https://lore.kernel.org/bpf/CAADnVQJnX-+9u--px_VnhrMTPB=O9Y0LH9T7RJbqzfLchbUFvg@mail.gmail.com/ (local)

In a previous e-mail I was explaining that sk_lookup BPF programs had no 
way to lookup existing sockets in kernel space. The sockets have to be 
provided by userspace, and the userspace has to find a way to get them 
and update them, which in some circumstances can be difficult. I'm 
working on a patch to make socket lookup available to BPF programs in 
the context of the sk_lookup hook.

There is also two helper function bpf_sk_lokup_tcp and bpf_sk_lookup_udp 
which exists but are not available in the context of sk_lookup hooks. 
Making them available in this context is not very difficult (just 
configure it in net/core/filter.c) but those helpers will call back BPF 
code as part of the socket lookup potentially causing an infinite loop. 
We need to find a way to perform socket lookup but disable the BPF hook 
while doing so.

Around all this, I have a few design questions :

Q1: How do we prevent socket lookup from triggering BPF sk_lookup 
causing a loop?

- 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?

For solution A above, the helper functions can be reused almose 
identically, just adding a flag or boolean argument to tell if we are in 
a sk_lookup program or not. In solution B is preferred, them perhaps it 
would make sense to expose the new raw lookup function created, and the 
BPF program would be responsible for falling back to INADDR_ANY if the 
specific socket is not found. It adds more power to the BPF program in 
this case but requires to create a new helper function.

I was going with Solution A abd identical function names, but as I am 
touching the code it seems that maybe solution B with a new helper 
function could be better. I'm open to ideas.

Thank you.

PS: please include me in replies if you are responding only to the 
netdev mailing list as I'm not part of it. I'm subscribed to bpf.