tracing: Check keys for variable references in expressions too

There's an existing check for variable references in keys, but it
doesn't go far enough. It checks whether a key field is a variable
reference but doesn't check whether it's an expression containing
variable references, which can cause the same problems for callers.

Use the existing field_has_hist_vars() function rather than a direct
top-level flag check to catch all possible variable references.

Link: http://lkml.kernel.org/r/e8c[e-mail]

Cc: [e-mail]
Fixes: 067fe038e70f6 ("tracing: Add variable reference handling to hist triggers")
Reported-by: Vincent Bernat <[e-mail]>
Signed-off-by: Tom Zanussi <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

kernel/trace/trace_events_hist.c

···
 	C(NO_CLOSING_PAREN,	"No closing paren found"),		\
 	C(SUBSYS_NOT_FOUND,	"Missing subsystem"),			\
 	C(INVALID_SUBSYS_EVENT,	"Invalid subsystem or event name"),	\
-	C(INVALID_REF_KEY,	"Using variable references as keys not supported"), \
+	C(INVALID_REF_KEY,	"Using variable references in keys not supported"), \
 	C(VAR_NOT_FOUND,	"Couldn't find variable"),		\
 	C(FIELD_NOT_FOUND,	"Couldn't find field"),
 
···
 			goto out;
 		}
 
-		if (hist_field->flags & HIST_FIELD_FL_VAR_REF) {
+		if (field_has_hist_vars(hist_field, 0))	{
 			hist_err(tr, HIST_ERR_INVALID_REF_KEY, errpos(field_str));
 			destroy_hist_field(hist_field, 0);
 			ret = -EINVAL;