Re: [PATCH bpf-next v1 1/8] bpf: Introduce pseudo_btf_id

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Hao Luo <haoluo@google.com>
Cc: netdev@vger.kernel.org, bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	Shuah Khan <shuah@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andriin@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Quentin Monnet <quentin@isovalent.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>, Andrey Ignatov <rdna@fb.com>,
	Jakub Sitnicki <jakub@cloudflare.com>
Subject: Re: [PATCH bpf-next v1 1/8] bpf: Introduce pseudo_btf_id
Date: Thu, 20 Aug 2020 14:53:27 -0700	[thread overview]
Message-ID: <20200820215327.jsvjbsvv6ts3x4wn@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20200819224030.1615203-2-haoluo@google.com>

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.