netdev - Re: [PATCH bpf-next 05/13] bpf: get better bpf_prog ksyms based on btf func type

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Date:   Wed, 17 Oct 2018 03:24:25 +0000
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Alexei Starovoitov <ast@...com>, Martin Lau <kafai@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 05/13] bpf: get better bpf_prog ksyms based on
 btf func type_id



On 10/16/18 10:59 AM, Alexei Starovoitov wrote:
> On Fri, Oct 12, 2018 at 11:54:42AM -0700, Yonghong Song wrote:
>> This patch added interface to load a program with the following
>> additional information:
>>     . prog_btf_fd
>>     . func_info and func_info_len
>> where func_info will provides function range and type_id
>> corresponding to each function.
>>
>> If verifier agrees with function range provided by the user,
>> the bpf_prog ksym for each function will use the func name
>> provided in the type_id, which is supposed to provide better
>> encoding as it is not limited by 16 bytes program name
>> limitation and this is better for bpf program which contains
>> multiple subprograms.
>>
>> The bpf_prog_info interface is also extended to
>> return btf_id and jited_func_types, so user spaces can
>> print out the function prototype for each jited function.
>>
>> Signed-off-by: Yonghong Song <yhs@...com>
> ...
>>   	BUILD_BUG_ON(sizeof("bpf_prog_") +
>>   		     sizeof(prog->tag) * 2 +
>> @@ -401,6 +403,13 @@ static void bpf_get_prog_name(const struct bpf_prog *prog, char *sym)
>>   
>>   	sym += snprintf(sym, KSYM_NAME_LEN, "bpf_prog_");
>>   	sym  = bin2hex(sym, prog->tag, sizeof(prog->tag));
>> +
>> +	if (prog->aux->btf) {
>> +		func_name = btf_get_name_by_id(prog->aux->btf, prog->aux->type_id);
>> +		snprintf(sym, (size_t)(end - sym), "_%s", func_name);
>> +		return;
> 
> Would it make sense to add a comment here that prog->aux->name is ignored
> when full btf name is available? (otherwise the same name will appear twice in ksym)

Will add a comment.

> 
>> +	}
>> +
>>   	if (prog->aux->name[0])
>>   		snprintf(sym, (size_t)(end - sym), "_%s", prog->aux->name);
> ...
>> +static int check_btf_func(struct bpf_prog *prog, struct bpf_verifier_env *env,
>> +			  union bpf_attr *attr)
>> +{
>> +	struct bpf_func_info *data;
>> +	int i, nfuncs, ret = 0;
>> +
>> +	if (!attr->func_info_len)
>> +		return 0;
>> +
>> +	nfuncs = attr->func_info_len / sizeof(struct bpf_func_info);
>> +	if (env->subprog_cnt != nfuncs) {
>> +		verbose(env, "number of funcs in func_info does not match verifier\n");
> 
> 'does not match verifier' is hard to make sense of.
> How about 'number of funcs in func_info doesn't match number of subprogs' ?

Sounds good to me.

> 
>> +		return -EINVAL;
>> +	}
>> +
>> +	data = kvmalloc(attr->func_info_len, GFP_KERNEL | __GFP_NOWARN);
>> +	if (!data) {
>> +		verbose(env, "no memory to allocate attr func_info\n");
> 
> I don't think we ever print such warnings for memory allocations.
> imo this can be removed, since enomem is enough.

Okay.

> 
>> +		return -ENOMEM;
>> +	}
>> +
>> +	if (copy_from_user(data, u64_to_user_ptr(attr->func_info),
>> +			   attr->func_info_len)) {
>> +		verbose(env, "memory copy error for attr func_info\n");
> 
> similar thing. kernel never warns about copy_from_user errors.

Okay.

> 
>> +		ret = -EFAULT;
>> +		goto cleanup;
>> +		}
>> +
>> +	for (i = 0; i < nfuncs; i++) {
>> +		if (env->subprog_info[i].start != data[i].insn_offset) {
>> +			verbose(env, "func_info subprog start (%d) does not match verifier (%d)\n",
>> +				env->subprog_info[i].start, data[i].insn_offset);
> 
> I think printing exact insn offset isn't going to be much help
> for regular user to debug it. If this happens, it's likely llvm issue.
> How about 'func_info BTF section doesn't match subprog layout in BPF program' ?

Okay.

>