tracing: Fix possible out of bounds memory access when parsing enums

The code that replaces the enum names with the enum values in the
tracepoints' format files could possible miss the end of string nul
character. This was caused by processing things like backslashes, quotes
and other tokens. After processing the tokens, a check for the nul
character needed to be done before continuing the loop, because the loop
incremented the pointer before doing the check, which could bypass the nul
character.

Link: http://lkml.kernel.org/r/552E661D.5060502@oracle.com

Reported-by: Sasha Levin <sasha.levin@oracle.com> # via KASan
Tested-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Fixes: 0c564a538aa9 "tracing: Add TRACE_DEFINE_ENUM() macro to map enums to their values"
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events.c

@@ -1760,6 +1760,8 @@ static void update_event_printk(struct ftrace_event_call *call,
 				ptr++;
 				/* Check for alpha chars like ULL */
 			} while (isalnum(*ptr));
+			if (!*ptr)
+				break;
 			/*
 			 * A number must have some kind of delimiter after
 			 * it, and we can ignore that too.
@@ -1786,12 +1788,16 @@ static void update_event_printk(struct ftrace_event_call *call,
 			do {
 				ptr++;
 			} while (isalnum(*ptr) || *ptr == '_');
+			if (!*ptr)
+				break;
 			/*
 			 * If what comes after this variable is a '.' or
 			 * '->' then we can continue to ignore that string.
 			 */
 			if (*ptr == '.' || (ptr[0] == '-' && ptr[1] == '>')) {
 				ptr += *ptr == '.' ? 1 : 2;
+				if (!*ptr)
+					break;
 				goto skip_more;
 			}
 			/*