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");
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
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
thanks, -- Shuah
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
thanks, -- Shuah
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. :)
Shuah Khan skhan@linuxfoundation.org writes:
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
It can wait for v5.10 IMHO, but up to you.
cheers
On 9/17/20 9:37 PM, Michael Ellerman wrote:
Shuah Khan skhan@linuxfoundation.org writes:
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 2But 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 13The 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
It can wait for v5.10 IMHO, but up to you.
Good to know. I will send this for 5.10-rc1 then.
thanks, -- Shuah
On Thu, Sep 17, 2020 at 02:15:19PM +1000, Michael Ellerman 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
Oh thank you for tracking this down! I thought seccomp wasn't killing a child or something, and hadn't found it yet. :)
Acked-by: Kees Cook keescook@chromium.org
linux-kselftest-mirror@lists.linaro.org
-
Kees Cook -
Max Filippov -
Michael Ellerman -
Shuah Khan