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

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Date:   Fri, 18 Sep 2020 08:32:13 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Michael Ellerman <mpe@...erman.id.au>,
        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

On 9/17/20 9:37 PM, Michael Ellerman wrote:
> 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.
> 

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

thanks,
-- Shuah