perf expr: Prevent normalize() from reading into undefined memory in the expression lexer

The current implementation does not account for a trailing backslash
followed by a null-byte.

If a null-byte is encountered following a backslash, normalize() will
continue reading (and potentially writing) into garbage memory ignoring
the EOS null-byte.

Signed-off-by: Sohom Datta <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: sohomdatta1

tools/perf/util/expr.l

@@ -42,8 +42,11 @@ static char *normalize(char *str, int runtime)
 	char *dst = str;
 
 	while (*str) {
-		if (*str == '\\')
+		if (*str == '\\') {
 			*dst++ = *++str;
+			if (!*str)
+				break;
+		}
 		else if (*str == '?') {
 			char *paramval;
 			int i = 0;