netdev - Re: [RFC bpf-next 1/4] bpf: cgroup

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Date:   Wed, 16 Feb 2022 18:38:49 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org
Subject: Re: [RFC bpf-next 1/4] bpf: cgroup_sock lsm flavor

On Tue, Feb 15, 2022 at 04:12:38PM -0800, Stanislav Fomichev wrote:
>  {
> @@ -1767,14 +1769,23 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
>  
>  	/* arg1: lea rdi, [rbp - stack_size] */
>  	EMIT4(0x48, 0x8D, 0x7D, -stack_size);
> -	/* arg2: progs[i]->insnsi for interpreter */
> -	if (!p->jited)
> -		emit_mov_imm64(&prog, BPF_REG_2,
> -			       (long) p->insnsi >> 32,
> -			       (u32) (long) p->insnsi);
> -	/* call JITed bpf program or interpreter */
> -	if (emit_call(&prog, p->bpf_func, prog))
> -		return -EINVAL;
> +
> +	if (p->expected_attach_type == BPF_LSM_CGROUP_SOCK) {
> +		/* arg2: progs[i] */
> +		emit_mov_imm64(&prog, BPF_REG_2, (long) p >> 32, (u32) (long) p);
> +		if (emit_call(&prog, __cgroup_bpf_run_lsm_sock, prog))
> +			return -EINVAL;
> +	} else {
> +		/* arg2: progs[i]->insnsi for interpreter */
> +		if (!p->jited)
> +			emit_mov_imm64(&prog, BPF_REG_2,
> +				       (long) p->insnsi >> 32,
> +				       (u32) (long) p->insnsi);
> +
> +		/* call JITed bpf program or interpreter */
> +		if (emit_call(&prog, p->bpf_func, prog))
> +			return -EINVAL;

Overall I think it's a workable solution.
As far as mechanism I think it would be better
to allocate single dummy bpf_prog and use normal fmod_ret
registration mechanism instead of hacking arch trampoline bits.
Set dummy_bpf_prog->bpf_func = __cgroup_bpf_run_lsm_sock;
and keep as dummy_bpf_prog->jited = false;
>From p->insnsi pointer in arg2 it's easy to go back to struct bpf_prog.
Such dummy prog might even be statically defined like dummy_bpf_prog.
Or allocated dynamically once.
It can be added as fmod_ret to multiple trampolines.
Just gut the func_model check.

As far as api the attach should probably be to a cgroup+lsm_hook pair.
link_create.target_fd will be cgroup_fd.
At prog load time attach_btf_id should probably be one
of existing bpf_lsm_* hooks.
Feels wrong to duplicate the whole set into lsm_cgroup_sock set.

It's fine to have prog->expected_attach_type == BPF_LSM_CGROUP_SOCK
to disambiguate. Will we probably only have two:
BPF_LSM_CGROUP_SOCK and BPF_LSM_CGROUP_TASK ?

> +int __cgroup_bpf_run_lsm_sock(u64 *regs, const struct bpf_prog *prog)
> +{
> +	struct socket *sock = (void *)regs[BPF_REG_0];
> +	struct cgroup *cgrp;
> +	struct sock *sk;
> +
> +	sk = sock->sk;
> +	if (!sk)
> +		return 0;
> +
> +	cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
> +
> +	return BPF_PROG_RUN_ARRAY_CG(cgrp->bpf.effective[prog->aux->cgroup_atype],
> +				     regs, bpf_prog_run, 0);
> +}

Would it be fast enough?
We went through a bunch of optimization for normal cgroup and ended with:
        if (cgroup_bpf_enabled(CGROUP_INET_INGRESS) &&
            cgroup_bpf_sock_enabled(sk, CGROUP_INET_INGRESS))
Here the trampoline code plus call into __cgroup_bpf_run_lsm_sock
will be there for all cgroups.
Since cgroup specific check will be inside BPF_PROG_RUN_ARRAY_CG.
I suspect it's ok, since the link_create will be for few specific lsm hooks
which are typically not in the fast path.
Unlike traditional cgroup hook like ingress that is hot.

For BPF_LSM_CGROUP_TASK it will take cgroup from current instead of sock, right?
Args access should magically work. 'regs' above should be fine for
all lsm hooks.

The typical prog:
+SEC("lsm_cgroup_sock/socket_post_create")
+int BPF_PROG(socket_post_create, struct socket *sock, int family,
+            int type, int protocol, int kern)
looks good too.
Feel natural.
I guess they can be sleepable too?