linux-kernel - Re: [PATCH bpf-next v1 1/8] bpf: Introduce pseudo_btf

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Message-ID: <20200820215327.jsvjbsvv6ts3x4wn@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 20 Aug 2020 14:53:27 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hao Luo <haoluo@...gle.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Quentin Monnet <quentin@...valent.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Andrey Ignatov <rdna@...com>,
        Jakub Sitnicki <jakub@...udflare.com>
Subject: Re: [PATCH bpf-next v1 1/8] bpf: Introduce pseudo_btf_id

On Wed, Aug 19, 2020 at 03:40:23PM -0700, Hao Luo wrote:
> +
>  /* verify BPF_LD_IMM64 instruction */
>  static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn)
>  {
> @@ -7234,6 +7296,9 @@ static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn)
>  		return 0;
>  	}
>  
> +	if (insn->src_reg == BPF_PSEUDO_BTF_ID)
> +		return check_pseudo_btf_id(env, insn);
> +
>  	map = env->used_maps[aux->map_index];
>  	mark_reg_known_zero(env, regs, insn->dst_reg);
>  	regs[insn->dst_reg].map_ptr = map;
> @@ -9255,6 +9320,9 @@ static int replace_map_fd_with_map_ptr(struct bpf_verifier_env *env)
>  				/* valid generic load 64-bit imm */
>  				goto next_insn;
>  
> +			if (insn[0].src_reg == BPF_PSEUDO_BTF_ID)
> +				goto next_insn;
> +

Why did you choose to do it during main do_check() walk instead of this pre-pass ?
check_ld_imm() can be called multiple times for the same insn,
so it's faster and less surprising to do it during replace_map_fd_with_map_ptr().
BTF needs to be parsed first, of course.
You can either move check_btf_info() before replace_map_fd_with_map_ptr() or
move replace_map_fd_with_map_ptr() after check_btf_info().
The latter is probably cleaner.