gh-104016: Fixed off by 1 error in f string tokenizer (#104047)

Co-authored-by: sunmy2019 <59365878+sunmy2019@users.noreply.github.com>
Co-authored-by: Ken Jin <kenjin@python.org>
Co-authored-by: Pablo Galindo <pablogsal@gmail.com>

Lib/test/test_fstring.py

@@ -566,6 +566,22 @@ x = (
                             "f-string: expressions nested too deeply",
                             ['f"{1+2:{1+2:{1+1:{1}}}}"'])
         
+        def create_nested_fstring(n):
+            if n == 0:
+                return "1+1"
+            prev = create_nested_fstring(n-1)
+            return f'f"{{{prev}}}"'
+
+        self.assertAllRaise(SyntaxError,
+                            "too many nested f-strings",
+                            [create_nested_fstring(160)])
+    
+    def test_syntax_error_in_nested_fstring(self):
+        # See gh-104016 for more information on this crash
+        self.assertAllRaise(SyntaxError,
+                            "invalid syntax",
+                            ['f"{1 1:' + ('{f"1:' * 199)])
+    
     def test_double_braces(self):
         self.assertEqual(f'{{', '{')
         self.assertEqual(f'a{{', 'a{')

Parser/tokenizer.c

@@ -43,12 +43,12 @@
 #ifdef Py_DEBUG
 static inline tokenizer_mode* TOK_GET_MODE(struct tok_state* tok) {
     assert(tok->tok_mode_stack_index >= 0);
-    assert(tok->tok_mode_stack_index < MAXLEVEL);
+    assert(tok->tok_mode_stack_index < MAXFSTRINGLEVEL);
     return &(tok->tok_mode_stack[tok->tok_mode_stack_index]);
 }
 static inline tokenizer_mode* TOK_NEXT_MODE(struct tok_state* tok) {
     assert(tok->tok_mode_stack_index >= 0);
-    assert(tok->tok_mode_stack_index < MAXLEVEL);
+    assert(tok->tok_mode_stack_index + 1 < MAXFSTRINGLEVEL); 
     return &(tok->tok_mode_stack[++tok->tok_mode_stack_index]);
 }
 #else
@@ -2235,6 +2235,9 @@ tok_get_normal_mode(struct tok_state *tok, tokenizer_mode* current_tok, struct t
 
         p_start = tok->start;
         p_end = tok->cur;
+        if (tok->tok_mode_stack_index + 1 >= MAXFSTRINGLEVEL) {
+            return MAKE_TOKEN(syntaxerror(tok, "too many nested f-strings"));
+        }
         tokenizer_mode *the_current_tok = TOK_NEXT_MODE(tok);
         the_current_tok->kind = TOK_FSTRING_MODE;
         the_current_tok->f_string_quote = quote;

Parser/tokenizer.h

@@ -12,6 +12,7 @@ extern "C" {
 
 #define MAXINDENT 100       /* Max indentation level */
 #define MAXLEVEL 200        /* Max parentheses level */
+#define MAXFSTRINGLEVEL 150 /* Max f-string nesting level */
 
 enum decoding_state {
     STATE_INIT,
@@ -123,7 +124,7 @@ struct tok_state {
     enum interactive_underflow_t interactive_underflow;
     int report_warnings;
     // TODO: Factor this into its own thing
-    tokenizer_mode tok_mode_stack[MAXLEVEL];
+    tokenizer_mode tok_mode_stack[MAXFSTRINGLEVEL];
     int tok_mode_stack_index;
     int tok_report_warnings;
 #ifdef Py_DEBUG