netdev - Re: [PATCHv3 bpf-next 3/7] bpf: Add bpf_get_func

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Message-ID: <20210708021123.w4smo42jml57iowl@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 7 Jul 2021 19:11:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        kernel test robot <lkp@...el.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCHv3 bpf-next 3/7] bpf: Add bpf_get_func_ip helper for
 tracing programs

On Wed, Jul 07, 2021 at 11:47:47PM +0200, Jiri Olsa wrote:
>  
> +static bool allow_get_func_ip_tracing(struct bpf_verifier_env *env)
> +{
> +	return env->prog->jit_requested && IS_ENABLED(CONFIG_X86_64);

Why does it have to be gated by 'jited && x86_64' ?
It's gated by bpf trampoline and it's only implemented on x86_64 so far.
The trampoline has plenty of features. I would expect bpf trampoline
for arm64 to implement all of them. If not the func_ip would be just
one of the trampoline features that couldn't be implemented and at that
time we'd need a flag mask of a sort, but I'd rather push of feature
equivalence between trampoline implementations.

Then jited part also doesn't seem to be necessary.
The trampoline passed pointer to a stack in R1.
Interpreter should deal with BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8) insn
the same way and it should work, since trampoline prepared it.
What did I miss?

> +static int has_get_func_ip(struct bpf_verifier_env *env)
> +{
> +	enum bpf_attach_type eatype = env->prog->expected_attach_type;
> +	enum bpf_prog_type type = resolve_prog_type(env->prog);
> +	int func_id = BPF_FUNC_get_func_ip;
> +
> +	if (type == BPF_PROG_TYPE_TRACING) {
> +		if (eatype != BPF_TRACE_FENTRY && eatype != BPF_TRACE_FEXIT &&
> +		    eatype != BPF_MODIFY_RETURN) {
> +			verbose(env, "func %s#%d supported only for fentry/fexit/fmod_ret programs\n",
> +				func_id_name(func_id), func_id);
> +			return -ENOTSUPP;
> +		}
> +		if (!allow_get_func_ip_tracing(env)) {
> +			verbose(env, "func %s#%d for tracing programs supported only for JITed x86_64\n",
> +				func_id_name(func_id), func_id);
> +			return -ENOTSUPP;
> +		}
> +		return 0;
> +	}
> +
> +	verbose(env, "func %s#%d not supported for program type %d\n",
> +		func_id_name(func_id), func_id, type);
> +	return -ENOTSUPP;
> +}
> +
>  static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  			     int *insn_idx_p)
>  {
> @@ -6225,6 +6256,12 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
>  	if (func_id == BPF_FUNC_get_stackid || func_id == BPF_FUNC_get_stack)
>  		env->prog->call_get_stack = true;
>  
> +	if (func_id == BPF_FUNC_get_func_ip) {
> +		if (has_get_func_ip(env))
> +			return -ENOTSUPP;
> +		env->prog->call_get_func_ip = true;
> +	}
> +
>  	if (changes_data)
>  		clear_all_pkt_pointers(env);
>  	return 0;
> @@ -12369,6 +12406,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>  {
>  	struct bpf_prog *prog = env->prog;
>  	bool expect_blinding = bpf_jit_blinding_enabled(prog);
> +	enum bpf_prog_type prog_type = resolve_prog_type(prog);
>  	struct bpf_insn *insn = prog->insnsi;
>  	const struct bpf_func_proto *fn;
>  	const int insn_cnt = prog->len;
> @@ -12702,6 +12740,21 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>  			continue;
>  		}
>  
> +		/* Implement bpf_get_func_ip inline. */
> +		if (prog_type == BPF_PROG_TYPE_TRACING &&
> +		    insn->imm == BPF_FUNC_get_func_ip) {
> +			/* Load IP address from ctx - 8 */
> +			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +
> +			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
> +			if (!new_prog)
> +				return -ENOMEM;
> +
> +			env->prog = prog = new_prog;
> +			insn      = new_prog->insnsi + i + delta;
> +			continue;
> +		}
> +
>  patch_call_imm:
>  		fn = env->ops->get_func_proto(insn->imm, env->prog);
>  		/* all functions that have prototype and verifier allowed
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 64bd2d84367f..9edd3b1a00ad 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -948,6 +948,19 @@ const struct bpf_func_proto bpf_snprintf_btf_proto = {
>  	.arg5_type	= ARG_ANYTHING,
>  };
>  
> +BPF_CALL_1(bpf_get_func_ip_tracing, void *, ctx)
> +{
> +	/* Stub, the helper call is inlined in the program. */
> +	return 0;
> +}

may be add a WARN in here that it should never be executed ?
Or may be add an actual implementation:
 return ((u64 *)ctx)[-1];
and check that it works without inlining by the verifier?