netdev - Re: [PATCH v3 bpf-next 15/18] bpf: Support attaching tracing BPF program to other BPF programs

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Message-ID: <5a0caec2-c406-c2b5-b4a8-0412800a6e43@fb.com>
Date:   Fri, 8 Nov 2019 18:59:06 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Song Liu <songliubraving@...com>,
        Alexei Starovoitov <ast@...nel.org>
CC:     David Miller <davem@...emloft.net>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "x86@...nel.org" <x86@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 15/18] bpf: Support attaching tracing BPF
 program to other BPF programs

On 11/8/19 10:49 AM, Song Liu wrote:
> 
> 
>> On Nov 7, 2019, at 10:40 PM, Alexei Starovoitov <ast@...nel.org> wrote:
>>
>> Allow FENTRY/FEXIT BPF programs to attach to other BPF programs of any type
>> including their subprograms. This feature allows snooping on input and output
>> packets in XDP, TC programs including their return values. In order to do that
>> the verifier needs to track types not only of vmlinux, but types of other BPF
>> programs as well. The verifier also needs to translate uapi/linux/bpf.h types
>> used by networking programs into kernel internal BTF types used by FENTRY/FEXIT
>> BPF programs. In some cases LLVM optimizations can remove arguments from BPF
>> subprograms without adjusting BTF info that LLVM backend knows. When BTF info
>> disagrees with actual types that the verifiers sees the BPF trampoline has to
>> fallback to conservative and treat all arguments as u64. The FENTRY/FEXIT
>> program can still attach to such subprograms, but won't be able to recognize
>> pointer types like 'struct sk_buff *' into won't be able to pass them to
> 					^^^^^ these few words are confusing

yep. will fix.

>> bpf_skb_output() for dumping to user space.
>>
>> The BPF_PROG_LOAD command is extended with attach_prog_fd field. When it's set
>> to zero the attach_btf_id is one vmlinux BTF type ids. When attach_prog_fd
>> points to previously loaded BPF program the attach_btf_id is BTF type id of
>> main function or one of its subprograms.
>>
>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>>
> 
> [...]
> 
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index cd9a9395c4b5..f385c4043594 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -9390,13 +9390,17 @@ static void print_verification_stats(struct bpf_verifier_env *env)
>> static int check_attach_btf_id(struct bpf_verifier_env *env)
>> {
>> 	struct bpf_prog *prog = env->prog;
>> +	struct bpf_prog *tgt_prog = prog->aux->linked_prog;
>> 	u32 btf_id = prog->aux->attach_btf_id;
>> 	const char prefix[] = "btf_trace_";
>> 	struct bpf_trampoline *tr;
>> 	const struct btf_type *t;
>> +	int ret, subprog = -1, i;
>> +	bool conservative = true;
>> 	const char *tname;
>> +	struct btf *btf;
>> 	long addr;
>> -	int ret;
>> +	u64 key;
>>
>> 	if (prog->type != BPF_PROG_TYPE_TRACING)
>> 		return 0;
>> @@ -9405,19 +9409,42 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>> 		verbose(env, "Tracing programs must provide btf_id\n");
>> 		return -EINVAL;
>> 	}
>> -	t = btf_type_by_id(btf_vmlinux, btf_id);
>> +	btf = bpf_prog_get_target_btf(prog);
> 
> btf could be NULL here, so we need to check it?

yep. will fix.