selftests/ftrace: Choose target function for filter test from samples

Since the event-filter-function.tc expects the 'exit_mmap()' directly
calls 'kmem_cache_free()', this is vulnerable to code modifications.

Choose the target function for the filter test from the sample
event data so that it can keep test running correctly even if the caller
function name will be changed.

Link: https://lore.kernel.org/linux-trace-kernel/167919441260.1922645.18355804179347364057.stgit@mhiramat.roam.corp.google.com/

Link: https://lore.kernel.org/all/CA+G9fYtF-XEKi9YNGgR=Kf==7iRb2FrmEC7qtwAeQbfyah-UhA@mail.gmail.com/
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Fixes: 7f09d63 ("tracing/selftests: Add test for event filtering on function name")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

tools/testing/selftests/ftrace/test.d/filter/event-filter-function.tc

@@ -9,18 +9,33 @@ fail() { #msg
     exit_fail
 }
 
-echo "Test event filter function name"
+sample_events() {
+    echo > trace
+    echo 1 > events/kmem/kmem_cache_free/enable
+    echo 1 > tracing_on
+    ls > /dev/null
+    echo 0 > tracing_on
+    echo 0 > events/kmem/kmem_cache_free/enable
+}
+
 echo 0 > tracing_on
 echo 0 > events/enable
+
+echo "Get the most frequently calling function"
+sample_events
+
+target_func=`cut -d: -f3 trace | sed 's/call_site=\([^+]*\)+0x.*/\1/' | sort | uniq -c | sort | tail -n 1 | sed 's/^[ 0-9]*//'`
+if [ -z "$target_func" ]; then
+    exit_fail
+fi
 echo > trace
-echo 'call_site.function == exit_mmap' > events/kmem/kmem_cache_free/filter
-echo 1 > events/kmem/kmem_cache_free/enable
-echo 1 > tracing_on
-ls > /dev/null
-echo 0 > events/kmem/kmem_cache_free/enable
 
-hitcnt=`grep kmem_cache_free trace| grep exit_mmap | wc -l`
-misscnt=`grep kmem_cache_free trace| grep -v exit_mmap | wc -l`
+echo "Test event filter function name"
+echo "call_site.function == $target_func" > events/kmem/kmem_cache_free/filter
+sample_events
+
+hitcnt=`grep kmem_cache_free trace| grep $target_func | wc -l`
+misscnt=`grep kmem_cache_free trace| grep -v $target_func | wc -l`
 
 if [ $hitcnt -eq 0 ]; then
 	exit_fail
@@ -30,20 +45,14 @@ if [ $misscnt -gt 0 ]; then
 	exit_fail
 fi
 
-address=`grep ' exit_mmap$' /proc/kallsyms | cut -d' ' -f1`
+address=`grep " ${target_func}\$" /proc/kallsyms | cut -d' ' -f1`
 
 echo "Test event filter function address"
-echo 0 > tracing_on
-echo 0 > events/enable
-echo > trace
 echo "call_site.function == 0x$address" > events/kmem/kmem_cache_free/filter
-echo 1 > events/kmem/kmem_cache_free/enable
-echo 1 > tracing_on
-sleep 1
-echo 0 > events/kmem/kmem_cache_free/enable
+sample_events
 
-hitcnt=`grep kmem_cache_free trace| grep exit_mmap | wc -l`
-misscnt=`grep kmem_cache_free trace| grep -v exit_mmap | wc -l`
+hitcnt=`grep kmem_cache_free trace| grep $target_func | wc -l`
+misscnt=`grep kmem_cache_free trace| grep -v $target_func | wc -l`
 
 if [ $hitcnt -eq 0 ]; then
 	exit_fail