selftests/ftrace: Differentiate bash and dash in dynevent_limitations.tc

bash and dash evaluate variables differently.
dash will evaluate '\\' every time it is read whereas bash does not.

  TEST_STRING="$TEST_STRING \\$i"
  echo $TEST_STRING

With i=123
On bash, that will print "\123"
but on dash, that will print the escape sequence of \123 as the \ will be
interpreted again in the echo.

The dynevent_limitations.tc test created a very large list of arguments to
test the maximum number of arguments to pass to the dynamic events file.
It had a loop of:

   TEST_STRING=$1
   # Acceptable
   for i in `seq 1 $MAX_ARGS`; do
     TEST_STRING="$TEST_STRING \\$i"
   done
   echo "$TEST_STRING" >> dynamic_events

This worked fine on bash, but when run on dash it failed.

This was due to dash interpreting the "\\$i" twice. Once when it was
assigned to TEST_STRING and a second time with the echo $TEST_STRING.

bash does not process the backslash more than the first time.

To solve this, assign a double backslash to a variable "bs" and then echo
it to "ts". If "ts" changes, it is dash, if not, it is bash. Then update
"bs" accordingly, and use that to assign TEST_STRING.

Now this could possibly just check if "$BASH" is defined or not, but this
is testing if the issue exists and not just which shell is being used.

Link: https://lore.kernel.org/r/20250414210900.4de5e8b9@gandalf.local.home
Fixes: 581a7b26ab ("selftests/ftrace: Add dynamic events argument limitation test case")
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/all/350786cc-9e40-4396-ab95-4f10d69122fb@sirena.org.uk/
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/ftrace/test.d/dynevent/dynevent_limitations.tc

@@ -7,11 +7,32 @@
 MAX_ARGS=128
 EXCEED_ARGS=$((MAX_ARGS + 1))
 
+# bash and dash evaluate variables differently.
+# dash will evaluate '\\' every time it is read whereas bash does not.
+#
+#   TEST_STRING="$TEST_STRING \\$i"
+#   echo $TEST_STRING
+#
+# With i=123
+# On bash, that will print "\123"
+# but on dash, that will print the escape sequence of \123 as the \ will
+# be interpreted again in the echo.
+#
+# Set a variable "bs" to save a double backslash, then echo that
+# to "ts" to see if $ts changed or not. If it changed, it's dash,
+# if not, it's bash, and then bs can equal a single backslash.
+bs='\\'
+ts=`echo $bs`
+if [ "$ts" = '\\' ]; then
+  # this is bash
+  bs='\'
+fi
+
 check_max_args() { # event_header
   TEST_STRING=$1
   # Acceptable
   for i in `seq 1 $MAX_ARGS`; do
-    TEST_STRING="$TEST_STRING \\$i"
+    TEST_STRING="$TEST_STRING $bs$i"
   done
   echo "$TEST_STRING" >> dynamic_events
   echo > dynamic_events