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

[Shuah Khan]

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@xxxxxxxxxxxxxx> 
> > 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@xxxxxxxxxxxxxx>
> > > ---
> > >   tools/testing/selftests/kselftest_harness.h | 5 +++++
> > >   1 file changed, 5 insertions(+)
> >
> > Tested-by: Max Filippov <jcmvbkbc@xxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxxxxx>

thanks,
-- Shuah