Re: [PATCH] selftests/livepatch: fix and refactor new dmesg message code

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On Wed, 2023-12-20 at 10:11 -0500, Joe Lawrence wrote:
> The livepatching kselftests rely on comparing expected vs. observed
> dmesg output.  After each test, new dmesg entries are determined by
> the
> 'comm' utility comparing a saved, pre-test copy of dmesg to post-test
> dmesg output.
> 
> Alexander reports that the 'comm --nocheck-order -13' invocation used
> by
> the tests can be confused when dmesg entry timestamps vary in
> magnitude
> (ie, "[   98.820331]" vs. "[  100.031067]"), in which case,
> additional
> messages are reported as new.  The unexpected entries then spoil the
> test results.
> 
> Instead of relying on 'comm' or 'diff' to determine new testing dmesg
> entries, refactor the code:
> 
>   - pre-test  : log a unique canary dmesg entry
>   - test      : run tests, log messages
>   - post-test : filter dmesg starting from pre-test message
> 
> Reported-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
> Closes:
> https://lore.kernel.org/live-patching/ZYAimyPYhxVA9wKg@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>

I liked the solution. As I don't speak awk I had to do some manual
testing to understand that the syntax you used "prints everything after
the $last_dmesg_msg message on dmesg". Either way, it's better then
using 'comm' utility. I tested on my x86_64 VM, and the tests passed as
expected.

LGTM, so

Tested-by: Marcos Paulo de Souza <mpdesouza@xxxxxxxx>
Reviewed-by: Marcos Paulo de Souza <mpdesouza@xxxxxxxx> 

> ---
>  .../testing/selftests/livepatch/functions.sh  | 37 +++++++++--------
> --
>  1 file changed, 17 insertions(+), 20 deletions(-)
> 
> diff --git a/tools/testing/selftests/livepatch/functions.sh
> b/tools/testing/selftests/livepatch/functions.sh
> index c8416c54b463..b1fd7362c2fe 100644
> --- a/tools/testing/selftests/livepatch/functions.sh
> +++ b/tools/testing/selftests/livepatch/functions.sh
> @@ -42,17 +42,6 @@ function die() {
>  	exit 1
>  }
>  
> -# save existing dmesg so we can detect new content
> -function save_dmesg() {
> -	SAVED_DMESG=$(mktemp --tmpdir -t klp-dmesg-XXXXXX)
> -	dmesg > "$SAVED_DMESG"
> -}
> -
> -# cleanup temporary dmesg file from save_dmesg()
> -function cleanup_dmesg_file() {
> -	rm -f "$SAVED_DMESG"
> -}
> -
>  function push_config() {
>  	DYNAMIC_DEBUG=$(grep '^kernel/livepatch'
> /sys/kernel/debug/dynamic_debug/control | \
>  			awk -F'[: ]' '{print "file " $1 " line " $2
> " " $4}')
> @@ -99,7 +88,6 @@ function set_ftrace_enabled() {
>  
>  function cleanup() {
>  	pop_config
> -	cleanup_dmesg_file
>  }
>  
>  # setup_config - save the current config and set a script exit trap
> that
> @@ -280,7 +268,15 @@ function set_pre_patch_ret {
>  function start_test {
>  	local test="$1"
>  
> -	save_dmesg
> +	# Dump something unique into the dmesg log, then stash the
> entry
> +	# in LAST_DMESG.  The check_result() function will use it to
> +	# find new kernel messages since the test started.
> +	local last_dmesg_msg="livepatch kselftest timestamp: $(date
> --rfc-3339=ns)"
> +	log "$last_dmesg_msg"
> +	loop_until 'dmesg | grep -q "$last_dmesg_msg"' ||
> +		die "buffer busy? can't find canary dmesg message:
> $last_dmesg_msg"
> +	LAST_DMESG=$(dmesg | grep "$last_dmesg_msg")
> +
>  	echo -n "TEST: $test ... "
>  	log "===== TEST: $test ====="
>  }
> @@ -291,23 +287,24 @@ function check_result {
>  	local expect="$*"
>  	local result
>  
> -	# Note: when comparing dmesg output, the kernel log
> timestamps
> -	# help differentiate repeated testing runs.  Remove them
> with a
> -	# post-comparison sed filter.
> -
> -	result=$(dmesg | comm --nocheck-order -13 "$SAVED_DMESG" - |
> \
> +	# Test results include any new dmesg entry since LAST_DMESG,
> then:
> +	# - include lines matching keywords
> +	# - exclude lines matching keywords
> +	# - filter out dmesg timestamp prefixes
> +	result=$(dmesg | awk -v last_dmesg="$LAST_DMESG" 'p; $0 ==
> last_dmesg { p=1 }' | \
>  		 grep -e 'livepatch:' -e 'test_klp' | \
>  		 grep -v '\(tainting\|taints\) kernel' | \
>  		 sed 's/^\[[ 0-9.]*\] //')
>  
>  	if [[ "$expect" == "$result" ]] ; then
>  		echo "ok"
> +	elif [[ "$result" == "" ]] ; then
> +		echo -e "not ok\n\nbuffer overrun? can't find canary
> dmesg entry: $LAST_DMESG\n"
> +		die "livepatch kselftest(s) failed"
>  	else
>  		echo -e "not ok\n\n$(diff -upr --label expected --
> label result <(echo "$expect") <(echo "$result"))\n"
>  		die "livepatch kselftest(s) failed"
>  	fi
> -
> -	cleanup_dmesg_file
>  }
>  
>  # check_sysfs_rights(modname, rel_path, expected_rights) - check
> sysfs






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