pythongh-131505: Move len boundary assertions before using len. (python#131536)

naya451 · Pranjal095 · commit 946fce311adb · 2025-07-12T20:33:09.000+05:30
Move len boundary assertions before using len.
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
@@ -607,9 +607,9 @@ _io_BytesIO_readinto_impl(bytesio *self, Py_buffer *buffer)
             len = 0;
     }
 
-    memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
     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;
 
     return PyLong_FromSsize_t(len);

Original file line number	Diff line number	Diff line change
`@@ -607,9 +607,9 @@ _io_BytesIO_readinto_impl(bytesio self, Py_buffer buffer)`
`607`	`607`	`len = 0;`
`608`	`608`	`}`
`609`	`609`
`610`		`- memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);`
`611`	`610`	`assert(self->pos + len < PY_SSIZE_T_MAX);`
`612`	`611`	`assert(len >= 0);`
	`612`	`+ memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);`
`613`	`613`	`self->pos += len;`
`614`	`614`
`615`	`615`	`return PyLong_FromSsize_t(len);`