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