From: Max Filippov <jcmvbkbc@gmail.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Kees Cook <keescook@chromium.org>,
linux-kselftest@vger.kernel.org,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] selftests/harness: Flush stdout before forking
Date: Wed, 16 Sep 2020 21:53:11 -0700 [thread overview]
Message-ID: <CAMo8BfJ5j4nG0z1Bk00J=3xPM++J68Hp2idJ-D5aHT84-vOzsQ@mail.gmail.com> (raw)
In-Reply-To: <20200917041519.3284582-1-mpe@ellerman.id.au>
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
next prev parent reply other threads:[~2020-09-17 4:53 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-17 4:15 [PATCH] selftests/harness: Flush stdout before forking Michael Ellerman
2020-09-17 4:53 ` Max Filippov [this message]
2020-09-17 12:58 ` Shuah Khan
2020-09-17 16:51 ` Shuah Khan
2020-09-17 22:06 ` Kees Cook
2020-09-18 3:37 ` Michael Ellerman
2020-09-18 14:32 ` Shuah Khan
2020-09-17 22:07 ` Kees Cook
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