netdev - Re: [PATCH bpf-next] libbpf: fix libbpf

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Date:   Tue, 5 Feb 2019 03:23:40 +0000
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Stanislav Fomichev <sdf@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next] libbpf: fix libbpf_print



On 2/4/19 5:51 PM, Alexei Starovoitov wrote:
> On Tue, Feb 05, 2019 at 12:37:29AM +0000, Yonghong Song wrote:
>>
>>
>> On 2/4/19 4:20 PM, Stanislav Fomichev wrote:
>>> With the recent print rework we now have the following problem:
>>> pr_{warning,info,debug} expand to __pr which calls libbpf_print.
>>> libbpf_print does va_start and calls __libbpf_pr with va_list argument.
>>> In __base_pr we again do va_start. Because the next argument is a
>>> va_list, we don't get correct pointer to the argument (and print noting
>>> in my case, I don't know why it doesn't crash tbh).
>>>
>>> Fix this by changing libbpf_print_fn_t signature to accept va_list and
>>> remove unneeded calls to va_start in the existing users.
>>>
>>> Alternatively, this can we solved by exporting __libbpf_pr and
>>> changing __pr macro to (and killing libbpf_print):
>>> {
>>> 	if (__libbpf_pr)
>>> 		__libbpf_pr(level, "libbpf: " fmt, ##__VA_ARGS__)
>>> }
>>>
>>> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
>>
>> It is my mistake. My early version did passed correctly and later
>> on I made some changes and did not test properly. Thanks for the fix!
>>
>> Acked-by: Yonghong Song <yhs@...com>
> 
> argh.
> Applied. Thanks for the fix.
> Yonghong, how was the earlier patch set tested?

Before the global function patch set, I have a global variable version,
which I tested and worked. Later on when changing to global
libbpf_print approach, I tested it through test_btf. That is why
I found a missing check for LIBBPF_DEBUG level in default __base_pr.
But I have to admit that I probably did not pay attention to contents 
somehow so I missed the garbled output.

> It sounds that nothing should have worked.
> How perf changes were tested?

I only tested compilation. The context is similar to a few other
bpf selftests programs and I assume with similar implementation, the
result should be similar. But really badly, they are all incorrect :-(