linux-kernel - Re: [PATCH] selftests/harness: Flush stdout before forking

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Message-ID: <e24df908-c50d-59ef-563c-9db24c819248@linuxfoundation.org>
Date:   Thu, 17 Sep 2020 06:58:23 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Max Filippov <jcmvbkbc@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Kees Cook <keescook@...omium.org>, linux-kselftest@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] selftests/harness: Flush stdout before forking

On 9/16/20 10:53 PM, Max Filippov wrote:
> On Wed, Sep 16, 2020 at 9:16 PM Michael Ellerman <mpe@...erman.id.au> wrote:
>>
>> The test harness forks() a child to run each test. Both the parent and
>> the child print to stdout using libc functions. That can lead to
>> duplicated (or more) output if the libc buffers are not flushed before
>> forking.
>>
>> It's generally not seen when running programs directly, because stdout
>> will usually be line buffered when it's pointing to a terminal.
>>
>> This was noticed when running the seccomp_bpf test, eg:
>>
>>    $ ./seccomp_bpf | tee test.log
>>    $ grep -c "TAP version 13" test.log
>>    2
>>
>> But we only expect the TAP header to appear once.
>>
>> It can be exacerbated using stdbuf to increase the buffer size:
>>
>>    $ stdbuf -o 1MB ./seccomp_bpf > test.log
>>    $ grep -c "TAP version 13" test.log
>>    13
>>
>> The fix is simple, we just flush stdout & stderr before fork. Usually
>> stderr is unbuffered, but that can be changed, so flush it as well
>> just to be safe.
>>
>> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
>> ---
>>   tools/testing/selftests/kselftest_harness.h | 5 +++++
>>   1 file changed, 5 insertions(+)
> 
> Tested-by: Max Filippov <jcmvbkbc@...il.com>
> 

Thank you both. Applying to linux-kselftest fixes for 5.9-rc7

thanks,
-- Shuah