[PATCH v3 3/4] tracing: Only report the stacktrace modifier on a real field

get_hist_field_flags() returns "stacktrace" for any field carrying
HIST_FIELD_FL_STACKTRACE, including the common_stacktrace pseudo-field,
which has no ftrace_event_field behind it. parse_field() no longer takes
the modifier there:

	if (stack_modifier &&
	    (!field || field->filter_type != FILTER_STACKTRACE)) {
		hist_err(tr, HIST_ERR_BAD_FIELD_MODIFIER, errpos(field_str));

so expr_field_str() renders "common_stacktrace.stacktrace", a spelling
that cannot be parsed back.

Report the modifier only when there is a field to report it for.

hist_field_print(), the other caller of get_hist_field_flags(), excludes
HIST_FIELD_FL_STACKTRACE before it calls, so this is confined to
expr_field_str(), whose only key renderer arrives in the next patch.

Fixes: a5e70ba87ca8 ("tracing: Fix memory corruption from the histogram stacktrace modifier")
Signed-off-by: Donggeun Yoo <redacted>
Assisted-by: Claude:claude-fable-5
---
 kernel/trace/trace_events_hist.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 91a550411e8a..fdd097abb0d6 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c

@@ -1725,7 +1725,7 @@ static const char *get_hist_field_flags(struct hist_field *hist_field)
 		flags_str = "percent";
 	else if (hist_field->flags & HIST_FIELD_FL_GRAPH)
 		flags_str = "graph";
-	else if (hist_field->flags & HIST_FIELD_FL_STACKTRACE)
+	else if (hist_field->flags & HIST_FIELD_FL_STACKTRACE && hist_field->field)
 		flags_str = "stacktrace";
 
 	return flags_str;

-- 
2.53.0