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(+)
diff --git a/tools/testing/selftests/kselftest_harness.h b/tools/testing/selftests/kselftest_harness.h index 4f78e4805633..f19804df244c 100644 --- a/tools/testing/selftests/kselftest_harness.h +++ b/tools/testing/selftests/kselftest_harness.h @@ -971,6 +971,11 @@ void __run_test(struct __fixture_metadata *f,
ksft_print_msg(" RUN %s%s%s.%s ...\n", f->name, variant->name[0] ? "." : "", variant->name, t->name); + + /* Make sure output buffers are flushed before fork */ + fflush(stdout); + fflush(stderr); + t->pid = fork(); if (t->pid < 0) { ksft_print_msg("ERROR SPAWNING TEST CHILD\n");