[New-bugs-announce] [issue41204] Use of unitialized variable `fields` along error path in code generated from asdl_c.py

[Brad Larsen]

New submission from Brad Larsen <brad at bradfordlarsen.com>:

In commit b1cc6ba73 from earlier today, an error-handling path can now read an uninitialized variable.

https://github.com/python/cpython/commit/b1cc6ba73a51d5cc3aeb113b5e7378fb50a0e20a#diff-fa7f27df4c8df1055048e78340f904c4R695-R697

In particular, asdl_c.py is used to generate C source, and when building that code with Clang 10, there is the attached warning.

Likely fix: initialize `fields` to `NULL`. Also, perhaps a CI loop that has `-Werror=sometimes-uninitialized` would help detect these.

Compiler warning:

Python/Python-ast.c:1147:9: warning: variable 'fields' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
    if (state == NULL) {
        ^~~~~~~~~~~~~
Python/Python-ast.c:1210:16: note: uninitialized use occurs here
    Py_XDECREF(fields);
               ^~~~~~
./Include/object.h:520:51: note: expanded from macro 'Py_XDECREF'
#define Py_XDECREF(op) _Py_XDECREF(_PyObject_CAST(op))
                                                  ^~
./Include/object.h:112:41: note: expanded from macro '_PyObject_CAST'
#define _PyObject_CAST(op) ((PyObject*)(op))
                                        ^~
Python/Python-ast.c:1147:5: note: remove the 'if' if its condition is always false
    if (state == NULL) {
    ^~~~~~~~~~~~~~~~~~~~
Python/Python-ast.c:1145:35: note: initialize the variable 'fields' to silence this warning
    PyObject *key, *value, *fields;
                                  ^
                                   = NULL
1 warning generated.

----------
components: Interpreter Core
messages: 372963
nosy: blarsen, vstinner
priority: normal
severity: normal
status: open
title: Use of unitialized variable `fields` along error path in code generated from asdl_c.py
type: compile error
versions: Python 3.10

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41204>
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