Re: [PATCH 1/2] selftests/lkdtm: Don't clear dmesg when running tests

[Joe Lawrence <joe.lawrence@redhat.com>]

On 6/22/20 4:51 AM, Naresh Kamboju wrote:
> On Fri, 8 May 2020 at 12:23, Michael Ellerman <mpe@ellerman.id.au> wrote:
>>
>> It is Very Rude to clear dmesg in test scripts. That's because the
>> script may be part of a larger test run, and clearing dmesg
>> potentially destroys the output of other tests.
>>
>> We can avoid using dmesg -c by saving the content of dmesg before the
>> test, and then using diff to compare that to the dmesg afterward,
>> producing a log with just the added lines.
>>
>> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
>> ---
>>   tools/testing/selftests/lkdtm/run.sh | 14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/tools/testing/selftests/lkdtm/run.sh b/tools/testing/selftests/lkdtm/run.sh
>> index dadf819148a4..0b409e187c7b 100755
>> --- a/tools/testing/selftests/lkdtm/run.sh
>> +++ b/tools/testing/selftests/lkdtm/run.sh
>> @@ -59,23 +59,25 @@ if [ -z "$expect" ]; then
>>          expect="call trace:"
>>   fi
>>
>> -# Clear out dmesg for output reporting
>> -dmesg -c >/dev/null
>> -
>>   # Prepare log for report checking
>> -LOG=$(mktemp --tmpdir -t lkdtm-XXXXXX)
>> +LOG=$(mktemp --tmpdir -t lkdtm-log-XXXXXX)
>> +DMESG=$(mktemp --tmpdir -t lkdtm-dmesg-XXXXXX)
>>   cleanup() {
>> -       rm -f "$LOG"
>> +       rm -f "$LOG" "$DMESG"
>>   }
>>   trap cleanup EXIT
>>
>> +# Save existing dmesg so we can detect new content below
>> +dmesg > "$DMESG"
>> +
>>   # Most shells yell about signals and we're expecting the "cat" process
>>   # to usually be killed by the kernel. So we have to run it in a sub-shell
>>   # and silence errors.
>>   ($SHELL -c 'cat <(echo '"$test"') >'"$TRIGGER" 2>/dev/null) || true
>>
>>   # Record and dump the results
>> -dmesg -c >"$LOG"
>> +dmesg | diff --changed-group-format='%>' --unchanged-group-format='' "$DMESG" - > "$LOG" || true
> 
> We are facing problems with the diff `=%>` part of the option.
> This report is from the OpenEmbedded environment.
> We have the same problem from livepatch_testcases.
> 
> # selftests lkdtm BUG.sh
> lkdtm: BUG.sh_ #
> # diff unrecognized option '--changed-group-format=%>'
> unrecognized: option_'--changed-group-format=%>' #
> # BusyBox v1.27.2 (2020-03-30 164108 UTC) multi-call binary.
> v1.27.2: (2020-03-30_164108 #
> #
> : _ #
> # Usage diff [-abBdiNqrTstw] [-L LABEL] [-S FILE] [-U LINES] FILE1 FILE2
> diff: [-abBdiNqrTstw]_[-L #
> # BUG missing 'kernel BUG at' [FAIL]
> 
> Full test output log,
> https://qa-reports.linaro.org/lkft/linux-next-oe/build/next-20200621/testrun/2850083/suite/kselftest/test/lkdtm_BUG.sh/log
> 

D'oh!  Using diff's changed/unchanged group format was a nice trick to 
easily fetch the new kernel log messages.

I can't think of any simple alternative off the top of my head, so 
here's a kludgy tested-once awk script:

  SAVED_DMESG="$(dmesg | tail -n1)"
  ... tests ...
  NEW_DMESG=$(dmesg | awk -v last="$SAVED_DMESG" 'p; $0 == last{p=1}')

I think timestamps should make each log line unique, but this probably 
won't handle kernel log buffer overflow.

Maybe it would be easier to log a known unique test delimiter msg and 
then fetch all new messages after that?

-- Joe