https://github.com/python/cpython/commit/7d54374f9c7d91e0ef90c4ad84baf10073cf1d8a
commit: 7d54374f9c7d91e0ef90c4ad84baf10073cf1d8a
branch: main
author: Cody Maloney <[email protected]>
committer: serhiy-storchaka <[email protected]>
date: 2025-11-12T11:57:05+02:00
summary:
gh-141311: Avoid assertion in BytesIO.readinto() (GH-141333)
Fix error in assertion which causes failure if pos is equal to PY_SSIZE_T_MAX.
Fix undefined behavior in read() and readinto() if pos is larger that the size
of the underlying buffer.
files:
A Misc/NEWS.d/next/Library/2025-11-09-18-55-13.gh-issue-141311.qZ3swc.rst
M Lib/test/test_io/test_memoryio.py
M Modules/_io/bytesio.c
diff --git a/Lib/test/test_io/test_memoryio.py
b/Lib/test/test_io/test_memoryio.py
index 63998a86c45b53..bb023735e21398 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -54,6 +54,12 @@ def testSeek(self):
self.assertEqual(buf[3:], bytesIo.read())
self.assertRaises(TypeError, bytesIo.seek, 0.0)
+ self.assertEqual(sys.maxsize, bytesIo.seek(sys.maxsize))
+ self.assertEqual(self.EOF, bytesIo.read(4))
+
+ self.assertEqual(sys.maxsize - 2, bytesIo.seek(sys.maxsize - 2))
+ self.assertEqual(self.EOF, bytesIo.read(4))
+
def testTell(self):
buf = self.buftype("1234567890")
bytesIo = self.ioclass(buf)
@@ -552,6 +558,14 @@ def test_relative_seek(self):
memio.seek(1, 1)
self.assertEqual(memio.read(), buf[1:])
+ def test_issue141311(self):
+ memio = self.ioclass()
+ # Seek allows PY_SSIZE_T_MAX, read should handle that.
+ # Past end of buffer read should always return 0 (EOF).
+ self.assertEqual(sys.maxsize, memio.seek(sys.maxsize))
+ buf = bytearray(2)
+ self.assertEqual(0, memio.readinto(buf))
+
def test_unicode(self):
memio = self.ioclass()
diff --git
a/Misc/NEWS.d/next/Library/2025-11-09-18-55-13.gh-issue-141311.qZ3swc.rst
b/Misc/NEWS.d/next/Library/2025-11-09-18-55-13.gh-issue-141311.qZ3swc.rst
new file mode 100644
index 00000000000000..bb425ce5df309d
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-09-18-55-13.gh-issue-141311.qZ3swc.rst
@@ -0,0 +1,2 @@
+Fix assertion failure in :func:`!io.BytesIO.readinto` and undefined behavior
+arising when read position is above capcity in :class:`io.BytesIO`.
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index d6bfb93177c9ee..96611823ab6b45 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -436,6 +436,13 @@ read_bytes_lock_held(bytesio *self, Py_ssize_t size)
return Py_NewRef(self->buf);
}
+ /* gh-141311: Avoid undefined behavior when self->pos (limit
PY_SSIZE_T_MAX)
+ is beyond the size of self->buf. Assert above validates size is always
in
+ bounds. When self->pos is out of bounds calling code sets size to 0. */
+ if (size == 0) {
+ return PyBytes_FromStringAndSize(NULL, 0);
+ }
+
output = PyBytes_AS_STRING(self->buf) + self->pos;
self->pos += size;
return PyBytes_FromStringAndSize(output, size);
@@ -609,11 +616,14 @@ _io_BytesIO_readinto_impl(bytesio *self, Py_buffer
*buffer)
n = self->string_size - self->pos;
if (len > n) {
len = n;
- if (len < 0)
- len = 0;
+ if (len < 0) {
+ /* gh-141311: Avoid undefined behavior when self->pos (limit
+ PY_SSIZE_T_MAX) points beyond the size of self->buf. */
+ return PyLong_FromSsize_t(0);
+ }
}
- assert(self->pos + len < PY_SSIZE_T_MAX);
+ assert(self->pos + len <= PY_SSIZE_T_MAX);
assert(len >= 0);
memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
self->pos += len;
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