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Subject: Re: [PATCH] selftests/harness: Flush stdout before forking
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Max Filippov <jcmvbkbc@gmail.com>
Cc:     Kees Cook <keescook@chromium.org>, linux-kselftest@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>
References: <20200917041519.3284582-1-mpe@ellerman.id.au>
 <CAMo8BfJ5j4nG0z1Bk00J=3xPM++J68Hp2idJ-D5aHT84-vOzsQ@mail.gmail.com>
 <e24df908-c50d-59ef-563c-9db24c819248@linuxfoundation.org>
 <87o8m3oiv6.fsf@mpe.ellerman.id.au>
From:   Shuah Khan <skhan@linuxfoundation.org>
Message-ID: <b610e625-159d-cdf8-373e-47f0b56412a3@linuxfoundation.org>
Date:   Fri, 18 Sep 2020 08:32:13 -0600
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On 9/17/20 9:37 PM, Michael Ellerman wrote:
> Shuah Khan <skhan@linuxfoundation.org> writes:
>> On 9/16/20 10:53 PM, Max Filippov wrote:
>>> On Wed, Sep 16, 2020 at 9:16 PM Michael Ellerman <mpe@ellerman.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@ellerman.id.au>
>>>> ---
>>>>    tools/testing/selftests/kselftest_harness.h | 5 +++++
>>>>    1 file changed, 5 insertions(+)
>>>
>>> Tested-by: Max Filippov <jcmvbkbc@gmail.com>
>>
>> Thank you both. Applying to linux-kselftest fixes for 5.9-rc7
> 
> It can wait for v5.10 IMHO, but up to you.
> 

Good to know. I will send this for 5.10-rc1 then.

thanks,
-- Shuah