netdev - Re: [PATCH bpf-next v2 1/5] bpf: Support injecting chain calls into BPF programs on load

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Date:   Sat, 5 Oct 2019 20:39:45 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Marek Majkowski <marek@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Alan Maguire <alan.maguire@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 1/5] bpf: Support injecting chain calls into
 BPF programs on load

On Sat, 05 Oct 2019 12:29:14 +0200, Toke Høiland-Jørgensen wrote:
> >> +static int bpf_inject_chain_calls(struct bpf_verifier_env *env)
> >> +{
> >> +	struct bpf_prog *prog = env->prog;
> >> +	struct bpf_insn *insn = prog->insnsi;
> >> +	int i, cnt, delta = 0, ret = -ENOMEM;
> >> +	const int insn_cnt = prog->len;
> >> +	struct bpf_array *prog_array;
> >> +	struct bpf_prog *new_prog;
> >> +	size_t array_size;
> >> +
> >> +	struct bpf_insn call_next[] = {
> >> +		BPF_LD_IMM64(BPF_REG_2, 0),
> >> +		/* Save real return value for later */
> >> +		BPF_MOV64_REG(BPF_REG_6, BPF_REG_0),
> >> +		/* First try tail call with index ret+1 */
> >> +		BPF_MOV64_REG(BPF_REG_3, BPF_REG_0),  
> >
> > Don't we need to check against the max here, and spectre-proofing
> > here?  
> 
> No, I don't think so. This is just setting up the arguments for the
> BPF_TAIL_CALL instruction below. The JIT will do its thing with that and
> emit the range check and the retpoline stuff...

Sorry, wrong CPU bug, I meant Meltdown :)

https://elixir.bootlin.com/linux/v5.4-rc1/source/kernel/bpf/verifier.c#L9029

> >> +		BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, 1),
> >> +		BPF_RAW_INSN(BPF_JMP | BPF_TAIL_CALL, 0, 0, 0, 0),
> >> +		/* If that doesn't work, try with index 0 (wildcard) */
> >> +		BPF_MOV64_IMM(BPF_REG_3, 0),
> >> +		BPF_RAW_INSN(BPF_JMP | BPF_TAIL_CALL, 0, 0, 0, 0),
> >> +		/* Restore saved return value and exit */
> >> +		BPF_MOV64_REG(BPF_REG_0, BPF_REG_6),
> >> +		BPF_EXIT_INSN()
> >> +	};