netdev - Re: [PATCH bpf-next v3] libbpf: fix probe code to return EPERM if encountered

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Date:   Tue, 12 May 2020 11:06:43 +0200
From:   "Eelco Chaudron" <echaudro@...hat.com>
To:     "Yonghong Song" <yhs@...com>
Cc:     bpf@...r.kernel.org, davem@...emloft.net, netdev@...r.kernel.org,
        ast@...nel.org, daniel@...earbox.net, kafai@...com,
        songliubraving@...com, andriin@...com, toke@...hat.com
Subject: Re: [PATCH bpf-next v3] libbpf: fix probe code to return EPERM if
 encountered



On 11 May 2020, at 22:43, Yonghong Song wrote:

> On 5/11/20 5:40 AM, Eelco Chaudron wrote:
>> When the probe code was failing for any reason ENOTSUP was returned, 
>> even
>> if this was due to no having enough lock space. This patch fixes this 
>> by
>> returning EPERM to the user application, so it can respond and 
>> increase
>> the RLIMIT_MEMLOCK size.
>>
>> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
>> ---
>> v3: Updated error message to be more specific as suggested by Andrii
>> v2: Split bpf_object__probe_name() in two functions as suggested by 
>> Andrii
>>
>>   tools/lib/bpf/libbpf.c |   31 ++++++++++++++++++++++++++-----
>>   1 file changed, 26 insertions(+), 5 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 8f480e29a6b0..ad3043c5db13 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -3149,7 +3149,7 @@ int bpf_map__resize(struct bpf_map *map, __u32 
>> max_entries)
>>   }
>>    static int
>> -bpf_object__probe_name(struct bpf_object *obj)
>> +bpf_object__probe_loading(struct bpf_object *obj)
>>   {
>>   	struct bpf_load_program_attr attr;
>>   	char *cp, errmsg[STRERR_BUFSIZE];
>> @@ -3170,14 +3170,34 @@ bpf_object__probe_name(struct bpf_object 
>> *obj)
>>   	ret = bpf_load_program_xattr(&attr, NULL, 0);
>>   	if (ret < 0) {
>>   		cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>> -		pr_warn("Error in %s():%s(%d). Couldn't load basic 'r0 = 0' BPF 
>> program.\n",
>> -			__func__, cp, errno);
>> +		pr_warn("Error in %s():%s(%d). Couldn't load trivial BPF "
>> +			"program. Make sure your kernel supports BPF "
>> +			"(CONFIG_BPF_SYSCALL=y) and/or that RLIMIT_MEMLOCK is "
>> +			"set to big enough value.\n", __func__, cp, errno);
>>   		return -errno;
>
> Just curious. Did "errno" always survive pr_warn() here? pr_warn() may 
> call user supplied print function which it outside libbpf control.
> Maybe should cache errno before calling pr_warn()?

I guess this issue has always been there, however, I sent out a v4 
fixing this case.

>>   	}
>>   	close(ret);
>>  -	/* now try the same program, but with the name */
>> +	return 0;
>> +}
>> +
>> +static int
>> +bpf_object__probe_name(struct bpf_object *obj)
>> +{
>> +	struct bpf_load_program_attr attr;
>> +	struct bpf_insn insns[] = {
>> +		BPF_MOV64_IMM(BPF_REG_0, 0),
>> +		BPF_EXIT_INSN(),
>> +	};
>> +	int ret;
>> +
>> +	/* make sure loading with name works */
>>  +	memset(&attr, 0, sizeof(attr));
>> +	attr.prog_type = BPF_PROG_TYPE_SOCKET_FILTER;
>> +	attr.insns = insns;
>> +	attr.insns_cnt = ARRAY_SIZE(insns);
>> +	attr.license = "GPL";
>>   	attr.name = "test";
>>   	ret = bpf_load_program_xattr(&attr, NULL, 0);
>>   	if (ret >= 0) {
>> @@ -5386,7 +5406,8 @@ int bpf_object__load_xattr(struct 
>> bpf_object_load_attr *attr)
>>    	obj->loaded = true;
>>  -	err = bpf_object__probe_caps(obj);
>> +	err = bpf_object__probe_loading(obj);
>> +	err = err ? : bpf_object__probe_caps(obj);
>>   	err = err ? : bpf_object__resolve_externs(obj, obj->kconfig);
>>   	err = err ? : bpf_object__sanitize_and_load_btf(obj);
>>   	err = err ? : bpf_object__sanitize_maps(obj);
>>