From c48f25a0ed8c507f427714af92ae4a845a2d520f Mon Sep 17 00:00:00 2001
From: Pablo Galindo <pablogsal@gmail.com>
Date: Fri, 12 Jun 2020 22:16:32 +0100
Subject: [PATCH 1/2] bpo-40958: Avoid buffer overflow in the parser when
 indexing the current line

---
 Parser/pegen.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/Parser/pegen.c b/Parser/pegen.c
index e29910bf86ed59..77dedf529bc7ec 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -141,19 +141,19 @@ _create_dummy_identifier(Parser *p)
 static inline Py_ssize_t
 byte_offset_to_character_offset(PyObject *line, int col_offset)
 {
-    const char *str = PyUnicode_AsUTF8(line);
+    Py_ssize_t linesize;
+    const char *str = PyUnicode_AsUTF8AndSize(line, &linesize);
     if (!str) {
         return 0;
     }
+    if (col_offset > linesize) {
+        col_offset = (int)linesize;
+    }
     PyObject *text = PyUnicode_DecodeUTF8(str, col_offset, "replace");
     if (!text) {
         return 0;
     }
     Py_ssize_t size = PyUnicode_GET_LENGTH(text);
-    str = PyUnicode_AsUTF8(text);
-    if (str != NULL && (int)strlen(str) == col_offset) {
-        size = strlen(str);
-    }
     Py_DECREF(text);
     return size;
 }
@@ -400,9 +400,6 @@ _PyPegen_raise_error_known_location(Parser *p, PyObject *errtype,
 
     if (!error_line) {
         Py_ssize_t size = p->tok->inp - p->tok->buf;
-        if (size && p->tok->buf[size-1] == '\n') {
-            size--;
-        }
         error_line = PyUnicode_DecodeUTF8(p->tok->buf, size, "replace");
         if (!error_line) {
             goto error;

From b854a0ffc7e7f62dfc53d0be45cda35c712643e6 Mon Sep 17 00:00:00 2001
From: Pablo Galindo <pablogsal@gmail.com>
Date: Fri, 12 Jun 2020 22:17:57 +0100
Subject: [PATCH 2/2] Add NEWS entry

---
 .../Core and Builtins/2020-06-12-22-17-51.bpo-40958.7O2Wh1.rst  | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2020-06-12-22-17-51.bpo-40958.7O2Wh1.rst

diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-06-12-22-17-51.bpo-40958.7O2Wh1.rst b/Misc/NEWS.d/next/Core and Builtins/2020-06-12-22-17-51.bpo-40958.7O2Wh1.rst
new file mode 100644
index 00000000000000..8e36897948f9b4
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-06-12-22-17-51.bpo-40958.7O2Wh1.rst	
@@ -0,0 +1,2 @@
+Fix a possible buffer overflow in the PEG parser when gathering information
+for emitting syntax errors. Patch by Pablo Galindo.