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selftests/harness: Flush stdout before forking

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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>
Tested-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
This commit is contained in:
Michael Ellerman 2020-09-17 14:15:19 +10:00 committed by Shuah Khan
parent 5c1e4f7e9e
commit c8bd596f93
1 changed files with 5 additions and 0 deletions
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5
tools/testing/selftests/kselftest_harness.h
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@ -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");