tracing: Add BUILD_BUG() to make sure stacktrace fits in strings

The max string length for a histogram variable is 256 bytes. The max depth
of a stacktrace is 16. With 8byte words, that's 16 * 8 = 128. Which can
easily fit in the string variable. The histogram stacktrace is being
stored in the string value (with the given max length), with the
assumption it will fit. To make sure that this is always the case (in the
case that the stack trace depth increases), add a BUILD_BUG_ON() to test
this.

Link: https://lore.kernel.org/linux-trace-kernel/20230214002418.0103b9e765d3e5c374d2aa7d@kernel.org/Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -3119,6 +3119,9 @@ static inline void __update_field_vars(struct tracing_map_elt *elt,
 	unsigned int i, j, var_idx;
 	u64 var_val;
 
+	/* Make sure stacktrace can fit in the string variable length */
+	BUILD_BUG_ON((HIST_STACKTRACE_DEPTH + 1) * sizeof(long) >= STR_VAR_LEN_MAX);
+
 	for (i = 0, j = field_var_str_start; i < n_field_vars; i++) {
 		struct field_var *field_var = field_vars[i];
 		struct hist_field *var = field_var->var;