Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 17 Sep 2020 15:07:35 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
        jcmvbkbc@gmail.com
Subject: Re: [PATCH] selftests/harness: Flush stdout before forking
Message-ID: <202009171506.7147F47FE@keescook>
References: <20200917041519.3284582-1-mpe@ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200917041519.3284582-1-mpe@ellerman.id.au>
Precedence: bulk

On Thu, Sep 17, 2020 at 02:15:19PM +1000, Michael Ellerman 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

Oh thank you for tracking this down! I thought seccomp wasn't killing a
child or something, and hadn't found it yet. :)

Acked-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook