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

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Message-ID: <87o8m3oiv6.fsf@mpe.ellerman.id.au>
Date:   Fri, 18 Sep 2020 13:37:33 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Shuah Khan <skhan@...uxfoundation.org>,
        Max Filippov <jcmvbkbc@...il.com>
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

Shuah Khan <skhan@...uxfoundation.org> writes:
> 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

It can wait for v5.10 IMHO, but up to you.

cheers