gh-91421: Use constant value check during runtime (GH-91422) (GH-91492)

The left-hand side expression of the if-check can be converted to a
constant by the compiler, but the addition on the right-hand side is
performed during runtime.

Move the addition from the right-hand side to the left-hand side by
turning it into a subtraction there. Since the values are known to
be large enough to not turn negative, this is a safe operation.

Prevents a very unlikely integer overflow on 32 bit systems.

Fixes GH-91421.
(cherry picked from commit 0859368335d470b9ff33fc53ed9a85ec2654b278)

Co-authored-by: Tobias Stoeckmann <stoeckmann@users.noreply.github.com>

diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-04-10-22-57-27.gh-issue-91421.dHhv6U.rst b/Misc/NEWS.d/next/Core and Builtins/2022-04-10-22-57-27.gh-issue-91421.dHhv6U.rst
new file mode 100644 (file)
index 0000000..898eb0d
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2022-04-10-22-57-27.gh-issue-91421.dHhv6U.rst
@@ -0,0 +1 @@
+Fix a potential integer overflow in _Py_DecodeUTF8Ex.

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 38a7b3c8b44b9b743b6125d16283f8499fc0afad..5ddde5054428a23fa93b67174cc9264109b36d01 100644 (file)
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -5373,7 +5373,7 @@ _Py_DecodeUTF8Ex(const char *s, Py_ssize_t size, wchar_t **wstr, size_t *wlen,
 
     /* Note: size will always be longer than the resulting Unicode
        character count */
-    if (PY_SSIZE_T_MAX / (Py_ssize_t)sizeof(wchar_t) < (size + 1)) {
+    if (PY_SSIZE_T_MAX / (Py_ssize_t)sizeof(wchar_t) - 1 < size) {
         return -1;
     }