Messages in this thread | | | From | Alexei Starovoitov <> | Date | Mon, 29 Nov 2021 17:42:57 -0800 | Subject | Re: [PATCH bpf-next] bpf: allow readonly direct path access for skfilter |
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On Tue, Nov 23, 2021 at 12:56 PM Maciej Żenczykowski <zenczykowski@gmail.com> wrote: > > From: Maciej Żenczykowski <maze@google.com> > > skfilter bpf programs can read the packet directly via llvm.bpf.load.byte/ > /half/word which are 8/16/32-bit primitive bpf instructions and thus > behave basically as well as DPA reads. But there is no 64-bit equivalent, > due to the support for the equivalent 64-bit bpf opcode never having been > added (unclear why, there was a patch posted). > DPA uses a slightly different mechanism, so doesn't suffer this limitation. > > Using 64-bit reads, 128-bit ipv6 address comparisons can be done in just > 2 steps, instead of the 4 steps needed with llvm.bpf.word.
llvm.bpf.word is a pseudo instruction. It's actually a function call for classic bpf. See bpf_gen_ld_abs. We used to have ugly special cases for them in JITs, but then got rid of it. Don't use them if performance is a requirement.
> This should hopefully allow simpler (less instructions, and possibly less > logic and maybe even less jumps) programs. Less jumps may also mean vastly > faster bpf verifier times (it can be exponential in the number of jumps...). > > This can be particularly important when trying to do something like scan > a netlink message for a pattern (2000 iteration loop) to decide whether > a message should be dropped, or delivered to userspace (thus waking it up). > > I'm requiring CAP_NET_ADMIN because I'm not sure of the security > implications... > > Tested: only build tested > Signed-off-by: Maciej Żenczykowski <maze@google.com> > --- > kernel/bpf/verifier.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 331b170d9fcc..0c2e25fb9844 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -3258,6 +3258,11 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env, > enum bpf_prog_type prog_type = resolve_prog_type(env->prog); > > switch (prog_type) { > + case BPF_PROG_TYPE_SOCKET_FILTER: > + if (meta || !capable(CAP_NET_ADMIN)) > + return false;
probably needs CAP_BPF too.
Other than that I think it's fine.
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