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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Main segfaults importing `_pyrepl` with low value for `JUMP_BACKWARD_INITIAL_VALUE`","articleBody":"# Crash report\n\n### What happened?\n\nEdit: I'm seeing this crash even without patching `JUMP_BACKWARD_INITIAL_VALUE`. Will try to figure out a MRE for unpatched crash.\n\nIf we `#define JUMP_BACKWARD_INITIAL_VALUE 702` (or lower) in `Include/internal/pycore_backoff.h`, a debug JIT build will segfault when trying to run to the new REPL. Can also be triggered by:\n\n```\npython -m _pyrepl\n```\n\nUsing some other modules like `random` or `http.server` doesn't crash.\n\nAn even lower value like 344 will segfault earlier, during the build process.\n\nThis happens since https://github.com/python/cpython/pull/136307 landed in 377b787. \n\nThe diff below, which is part of that commit, is enough to cause the segfault together with defining `JUMP_BACKWARD_INITIAL_VALUE` to 702 or lower:\n```diff\ndiff --git a/Include/internal/pycore_global_objects_fini_generated.h b/Include/internal/pycore_global_objects_fini_generated.h\nindex 493377b4c25..5e7dda3a371 100644\n--- a/Include/internal/pycore_global_objects_fini_generated.h\n+++ b/Include/internal/pycore_global_objects_fini_generated.h\n@@ -1005,6 +1005,7 @@ _PyStaticObjects_CheckRefcnt(PyInterpreterState *interp) {\n     _PyStaticObject_CheckRefcnt((PyObject *)\u0026_Py_ID(imag));\n     _PyStaticObject_CheckRefcnt((PyObject *)\u0026_Py_ID(importlib));\n     _PyStaticObject_CheckRefcnt((PyObject *)\u0026_Py_ID(in_fd));\n+    _PyStaticObject_CheckRefcnt((PyObject *)\u0026_Py_ID(include_aliases));\n     _PyStaticObject_CheckRefcnt((PyObject *)\u0026_Py_ID(incoming));\n     _PyStaticObject_CheckRefcnt((PyObject *)\u0026_Py_ID(index));\n     _PyStaticObject_CheckRefcnt((PyObject *)\u0026_Py_ID(indexgroup));\ndiff --git a/Include/internal/pycore_global_strings.h b/Include/internal/pycore_global_strings.h\nindex 5dfea2f479d..6908cbf78f3 100644\n--- a/Include/internal/pycore_global_strings.h\n+++ b/Include/internal/pycore_global_strings.h\n@@ -496,6 +496,7 @@ struct _Py_global_strings {\n         STRUCT_FOR_ID(imag)\n         STRUCT_FOR_ID(importlib)\n         STRUCT_FOR_ID(in_fd)\n+        STRUCT_FOR_ID(include_aliases)\n         STRUCT_FOR_ID(incoming)\n         STRUCT_FOR_ID(index)\n         STRUCT_FOR_ID(indexgroup)\ndiff --git a/Include/internal/pycore_runtime_init_generated.h b/Include/internal/pycore_runtime_init_generated.h\nindex 85ced09d29d..da2ed7422c9 100644\n--- a/Include/internal/pycore_runtime_init_generated.h\n+++ b/Include/internal/pycore_runtime_init_generated.h\n@@ -1003,6 +1003,7 @@ extern \"C\" {\n     INIT_ID(imag), \\\n     INIT_ID(importlib), \\\n     INIT_ID(in_fd), \\\n+    INIT_ID(include_aliases), \\\n     INIT_ID(incoming), \\\n     INIT_ID(index), \\\n     INIT_ID(indexgroup), \\\ndiff --git a/Include/internal/pycore_unicodeobject_generated.h b/Include/internal/pycore_unicodeobject_generated.h\nindex 6018d98d156..b1f411945e7 100644\n--- a/Include/internal/pycore_unicodeobject_generated.h\n+++ b/Include/internal/pycore_unicodeobject_generated.h\n@@ -1772,6 +1772,10 @@ _PyUnicode_InitStaticStrings(PyInterpreterState *interp) {\n     _PyUnicode_InternStatic(interp, \u0026string);\n     assert(_PyUnicode_CheckConsistency(string, 1));\n     assert(PyUnicode_GET_LENGTH(string) != 1);\n+    string = \u0026_Py_ID(include_aliases);\n+    _PyUnicode_InternStatic(interp, \u0026string);\n+    assert(_PyUnicode_CheckConsistency(string, 1));\n+    assert(PyUnicode_GET_LENGTH(string) != 1);\n     string = \u0026_Py_ID(incoming);\n     _PyUnicode_InternStatic(interp, \u0026string);\n     assert(_PyUnicode_CheckConsistency(string, 1));\n```\n\nBacktrace:\n```\nProgram received signal SIGSEGV, Segmentation fault.\n_PyUnicode_Equal (str1=0x555555c88c18 \u003c_PyRuntime+96088\u003e, str2=0x555555c88c50 \u003c_PyRuntime+96144\u003e) at ./Include/object.h:815\n815         return ((flags \u0026 feature) != 0);\n\n#0  _PyUnicode_Equal (str1=0x555555c88c18 \u003c_PyRuntime+96088\u003e, str2=0x555555c88c50 \u003c_PyRuntime+96144\u003e) at ./Include/object.h:815\n#1  0x00007ffff731df15 in ?? ()\n#2  0x00007ffff764a751 in ?? ()\n#3  0x0000555555cc2230 in _PyRuntime ()\n#4  0x00007fffffffbfc0 in ?? ()\n#5  0x00007ffff7317017 in ?? ()\n#6  0x0000555556053f60 in ?? ()\n#7  0x00007ffff7e2a9c8 in ?? ()\n#8  0x00007ffff7317000 in ?? ()\n#9  0x00007ffff7e2a910 in ?? ()\n#10 0x0000555555cc2230 in _PyRuntime ()\n#11 0x00005555557cdcf9 in _PyEval_EvalFrameDefault (tstate=\u003coptimized out\u003e, frame=0x58, throwflag=1291845632)\n    at Python/generated_cases.c.h:7796\nBacktrace stopped: previous frame inner to this frame (corrupt stack?)\n```\n\nHere's the output from running `PYTHON_LLTRACE=4 ./python -m _pyrepl`:\n[lltrace_crash.txt](https://github.com/user-attachments/files/21497684/lltrace_crash.txt)\nAnd here for `PYTHON_OPT_DEBUG=4 ./python -m _pyrepl`:\n[opt_debug_crash.txt](https://github.com/user-attachments/files/21497685/opt_debug_crash.txt)\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 3.15.0a0 (heads/main-dirty:d7e12a362a2, Jul 29 2025, 18:24:07) [GCC 13.3.0]\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-137265\n* gh-139170\n* gh-139241\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/devdanzin","@type":"Person","name":"devdanzin"},"datePublished":"2025-07-29T21:49:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":16},"url":"https://github.com/137218/cpython/issues/137218"}