Hello everyone,
Background discussion:
https://lore.kernel.org/bpf/CAADnVQJnX-+9u--px_VnhrMTPB=O9Y0LH9T7RJbqzfLchbUFvg@xxxxxxxxxxxxxx/
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.
