René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`_set_str` missing `NULL` check after `PyUnicode_DecodeASCII` in `_csv.c`","articleBody":"# Crash report\n\n### What happened?\n\nIt's possible to segfault the interpreter on OOM by trying to write a CSV row.\n\n### Automated diagnosis:\n\nBug: On OOM, `PyUnicode_DecodeASCII` returns `NULL`, `_set_str` stores `NULL` in `*target` and returns 0 (success). Later `PyUnicode_GET_LENGTH(NULL)` segfaults.\n\nFile: Modules/_csv.c, line 319\n\nMRE:\n```python\nimport _testcapi, csv, io\n\n# Persistent failure ensures the allocation inside _set_str fails\nfor n in range(1, 30):\n    print(n)\n    _testcapi.set_nomemory(n, 0)\n    try:\n        w = csv.writer(io.StringIO())\n        w.writerow([\"a\", \"b\"])\n        _testcapi.remove_mem_hooks()\n        break\n    except MemoryError:\n        _testcapi.remove_mem_hooks()\n```\n\nBacktrace:\n```\nProgram received signal SIGSEGV, Segmentation fault.\ndialect_check_char (name=0x7ffff7fa39e5 \"delimiter\", c=44, dialect=dialect@entry=0x7ffff74dadb0, allowspace=true) at ./Modules/_csv.c:353\n353             PyUnicode_GET_LENGTH(dialect-\u003elineterminator), 1) \u003e= 0)\n\n#0  dialect_check_char (name=0x7ffff7fa39e5 \"delimiter\", c=44, dialect=dialect@entry=0x7ffff74dadb0, allowspace=true) at ./Modules/_csv.c:353\n#1  0x00007ffff7f9ee52 in dialect_new (type=0x555555f539b0, args=0x555555d62c40 \u003c_PyRuntime+135440\u003e, kwargs=0x0) at ./Modules/_csv.c:543\n#2  0x000055555576bd44 in type_call (self=0x555555f539b0, args=\u003coptimized out\u003e, kwds=\u003coptimized out\u003e) at Objects/typeobject.c:2441\n#3  0x00005555556a34b4 in _PyObject_MakeTpCall (tstate=0x555555d99c08 \u003c_PyRuntime+360664\u003e, callable=0x555555f539b0, args=0x0, nargs=0, keywords=0x0) at Objects/call.c:242\n#4  0x00007ffff7fa19d3 in _call_dialect (module_state=0x7ffff72d4cc0, dialect_inst=0x0, kwargs=0x0) at ./Modules/_csv.c:646\n#5  csv_writer (module=0x7ffff72b47a0, args=0x7ffff7477cf0, keyword_args=0x0) at ./Modules/_csv.c:1581\n#6  0x00005555557210e2 in cfunction_call (func=0x7ffff72cf170, args=0x7ffff7477cf0, kwargs=0x0) at Objects/methodobject.c:564\n#7  0x00005555556a34b4 in _PyObject_MakeTpCall (tstate=0x555555d99c08 \u003c_PyRuntime+360664\u003e, callable=0x7ffff72cf170, args=0x7fffffffb628, nargs=1, keywords=0x0) at Objects/call.c:242\n#8  0x000055555583f29d in _Py_VectorCall_StackRefSteal (callable=callable@entry=..., arguments=0x7ffff7fa7090, total_args=total_args@entry=1, kwnames=kwnames@entry=...) at Python/ceval.c:733\n#9  0x00005555558556d6 in _PyEval_EvalFrameDefault (tstate=tstate@entry=0x555555d99c08 \u003c_PyRuntime+360664\u003e, frame=\u003coptimized out\u003e, frame@entry=0x7ffff7fa7020, throwflag=throwflag@entry=0)\n    at Python/generated_cases.c.h:4170\n#10 0x000055555583f08b in _PyEval_EvalFrame (tstate=0x555555d99c08 \u003c_PyRuntime+360664\u003e, frame=0x7ffff7fa7020, throwflag=0) at ./Include/internal/pycore_ceval.h:118\n#11 _PyEval_Vector (tstate=tstate@entry=0x555555d99c08 \u003c_PyRuntime+360664\u003e, func=func@entry=0x7ffff7466690, locals=locals@entry=0x7ffff746a4b0, args=args@entry=0x0,\n    argcount=argcount@entry=0, kwnames=kwnames@entry=0x0) at Python/ceval.c:2130\n#12 0x000055555583ee1e in PyEval_EvalCode (co=co@entry=0x555555f396d0, globals=globals@entry=0x7ffff746a4b0, locals=locals@entry=0x7ffff746a4b0) at Python/ceval.c:686\n#13 0x00005555559c8f8e in run_eval_code_obj (tstate=0x555555d99c08 \u003c_PyRuntime+360664\u003e, co=co@entry=0x555555f396d0, globals=globals@entry=0x7ffff746a4b0, locals=locals@entry=0x7ffff746a4b0)\n    at Python/pythonrun.c:1368\n#14 0x00005555559c8adb in run_mod (mod=mod@entry=0x555555f40fa0, filename=filename@entry=0x7ffff74e8580, globals=globals@entry=0x7ffff746a4b0, locals=locals@entry=0x7ffff746a4b0,\n    flags=0x7fffffffc950, arena=arena@entry=0x7ffff74dad40, interactive_src=0x0, generate_new_source=0) at Python/pythonrun.c:1471\n```\n\n\nFound using [cpython-review-toolkit](https://github.com/devdanzin/cpython-review-toolkit) with Claude Opus 4.6, using the `/cpython-review-toolkit:explore Modules/_csv.c all deep` command.\n\n\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n### Output from running 'python -VV' on the command line:\n\nPython 3.15.0a7+ (heads/main:e0f7c1097e1, Mar 17 2026, 18:10:52) [Clang 21.1.2 (2ubuntu6)]\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-146113\n* gh-146130\n* gh-146131\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/devdanzin","@type":"Person","name":"devdanzin"},"datePublished":"2026-03-17T23:41:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/146093/cpython/issues/146093"}