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Date:   Thu, 17 Sep 2020 15:06:39 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Shuah Khan <skhan@linuxfoundation.org>
Cc:     Max Filippov <jcmvbkbc@gmail.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linux-kselftest@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] selftests/harness: Flush stdout before forking
Message-ID: <202009171506.535C5678@keescook>
References: <20200917041519.3284582-1-mpe@ellerman.id.au>
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 <e24df908-c50d-59ef-563c-9db24c819248@linuxfoundation.org>
 <99f67c84-4dcd-278a-e7b8-e19054818ecf@linuxfoundation.org>
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On Thu, Sep 17, 2020 at 10:51:32AM -0600, Shuah Khan wrote:
> On 9/17/20 6:58 AM, Shuah Khan wrote:
> > On 9/16/20 10:53 PM, Max Filippov wrote:
> > > 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>
> > > 
> > 
> > Thank you both. Applying to linux-kselftest fixes for 5.9-rc7
> > 
> 
> Kees,
> 
> I haven't pulled this in yet. If you want to take this through seccomp
> tree for dependencies:
> 
> Acked-by: Shuah Khan <skhan@linuxfoundation.org>

You can have it -- it's a global fix and more than the seccomp selftest
is likely affected. :)

-- 
Kees Cook