gh-96678: Fix UB of null pointer arithmetic (GH-96782)

Automerge-Triggered-By: GH:pablogsal
(cherry picked from commit 81e36f350b75d2ed2668825f7df6e059b57f859c)

Co-authored-by: Matthias Görgens <matthias.goergens@gmail.com>

diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-09-13-12-06-46.gh-issue-96678.NqGFyb.rst b/Misc/NEWS.d/next/Core and Builtins/2022-09-13-12-06-46.gh-issue-96678.NqGFyb.rst
new file mode 100644 (file)
index 0000000..bdd33c8
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2022-09-13-12-06-46.gh-issue-96678.NqGFyb.rst
@@ -0,0 +1 @@
+Fix undefined behaviour in C code of null pointer arithmetic.

diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index 2f00d208fea9d8dd0122b4d607176701d61a5f82..0bbf1b1794e2c29b8455d3b1b597268b76c1beaf 100644 (file)
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -1530,7 +1530,7 @@ tok_get(struct tok_state *tok, const char **p_start, const char **p_end)
     } while (c == ' ' || c == '\t' || c == '\014');
 
     /* Set start of current token */
-    tok->start = tok->cur - 1;
+    tok->start = tok->cur == NULL ? NULL : tok->cur - 1;
 
     /* Skip comment, unless it's a type comment */
     if (c == '#') {