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From:   Max Filippov <jcmvbkbc@gmail.com>
Date:   Wed, 16 Sep 2020 21:53:11 -0700
Message-ID: <CAMo8BfJ5j4nG0z1Bk00J=3xPM++J68Hp2idJ-D5aHT84-vOzsQ@mail.gmail.com>
Subject: Re: [PATCH] selftests/harness: Flush stdout before forking
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Kees Cook <keescook@chromium.org>, linux-kselftest@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
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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>

-- 
Thanks.
-- Max