❯ cat bad.py
0000
00000000000

cpython on  null_bytes [!?] via 🐍 v3.9.1 took 2s
❯ ./python bad.py
=================================================================
==29963==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6250000168ff at pc 0x55eb401ba788 bp 0x7fff8d1d60d0 sp 0x7fff8d1d60c0
READ of size 1 at 0x6250000168ff thread T0
    #0 0x55eb401ba787 in tok_readline_raw Parser/tokenizer.c:817
    #1 0x55eb401bb4b8 in tok_underflow_file Parser/tokenizer.c:947
    #2 0x55eb401bbc97 in tok_nextc Parser/tokenizer.c:1031
    #3 0x55eb401bd1c7 in tok_get Parser/tokenizer.c:1213
    #4 0x55eb401c1286 in PyTokenizer_Get Parser/tokenizer.c:1872
    #5 0x55eb402b09b2 in _PyPegen_fill_token Parser/pegen.c:633
    #6 0x55eb402b2578 in _PyPegen_expect_token Parser/pegen.c:832
    #7 0x55eb4030032d in _tmp_15_rule Parser/parser.c:19552
    #8 0x55eb402b2485 in _PyPegen_lookahead Parser/pegen.c:823
    #9 0x55eb402bf986 in compound_stmt_rule Parser/parser.c:2008
    #10 0x55eb402bd4fd in statement_rule Parser/parser.c:1365
    #11 0x55eb402ff7d2 in _loop1_11_rule Parser/parser.c:19322
    #12 0x55eb402bd342 in statements_rule Parser/parser.c:1324
    #13 0x55eb402bbd2f in file_rule Parser/parser.c:853
    #14 0x55eb40320c20 in _PyPegen_parse Parser/parser.c:29381
    #15 0x55eb402b4e71 in _PyPegen_run_parser Parser/pegen.c:1232
    #16 0x55eb402b5587 in _PyPegen_run_parser_from_file_pointer Parser/pegen.c:1316
    #17 0x55eb401b61d3 in _PyParser_ASTFromFile Parser/peg_api.c:26
    #18 0x55eb40051d55 in pyrun_file Python/pythonrun.c:1125
    #19 0x55eb4004f35a in _PyRun_SimpleFileObject Python/pythonrun.c:457
    #20 0x55eb4004e2bb in _PyRun_AnyFileObject Python/pythonrun.c:91
    #21 0x55eb3fdf17a2 in pymain_run_file_obj Modules/main.c:353
    #22 0x55eb3fdf1961 in pymain_run_file Modules/main.c:372
    #23 0x55eb3fdf2bc6 in pymain_run_python Modules/main.c:587
    #24 0x55eb3fdf2f07 in Py_RunMain Modules/main.c:666
    #25 0x55eb3fdf3282 in pymain_main Modules/main.c:696
    #26 0x55eb3fdf361c in Py_BytesMain Modules/main.c:720
    #27 0x55eb3fdef709 in main Programs/python.c:15
    #28 0x7efd0aa47bf6 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21bf6)
    #29 0x55eb3fdef609 in _start (/home/pablogsal/github/cpython/python+0x14f609)

cpython on  null_bytes [!?] via 🐍 v3.9.1 took 2s
❯ ./python bad.py

echo|PYTHONMALLOC=malloc valgrind ./python Tools/scripts/reindent.py 

diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index d18fffa..9ea6af9 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -815,6 +815,7 @@ struct tok_state *
         }
         tok->inp = strchr(tok->inp, '\0');
     } while (tok->inp[-1] != '\n');
+    tok->end = tok->buf + strlen(tok->buf);
     return 1;
 }
 

diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index d18fffa..d8ccb44 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -963,10 +963,14 @@ struct tok_state *
         if (tok->lineno > 2) {
             tok->decoding_state = STATE_NORMAL;
         }
-        else if (!check_coding_spec(tok->cur, tok->end - tok->cur,
-                                    tok, fp_setreadl))
-        {
-            return 0;
+        else {
+            Py_ssize_t size = tok->end - tok->cur;
+            Py_ssize_t len = strnlen(tok->cur, size);
+            size = Py_MIN(size, len);
+            if (!check_coding_spec(tok->cur, size, tok, fp_setreadl))
+            {
+                return 0;
+            }
         }
     }
     /* The default encoding is UTF-8, so make sure we don't have any