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gh-145792: Fix incorrect alloca allocation size in traceback.c #145814

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Merged
vstinner merged 4 commits into from
Mar 13, 2026
Merged

gh-145792: Fix incorrect alloca allocation size in traceback.c #145814

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6a20c42
gh-145792: Fix incorrect alloca allocation size in traceback.c
VanshAgarwal24036 Mar 11, 2026
e3149bb
Update test
VanshAgarwal24036 Mar 11, 2026
45e8e90
Update test
VanshAgarwal24036 Mar 11, 2026
d36912a
Address review comments
VanshAgarwal24036 Mar 12, 2026
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12 changes: 12 additions & 0 deletions Lib/test/test_traceback.py
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Original file line number Diff line number Diff line change
Expand Up @@ -625,6 +625,18 @@ def test_signatures(self):
str(inspect.signature(traceback.format_exception_only)),
'(exc, /, value=<implicit>, *, show_group=False, **kwargs)')

def test_traceback_deep_recursion_alloca(self):

def recurse(n):
if n == 0:
raise RuntimeError("boom")
return recurse(n - 1)
try:
recurse(50)
except RuntimeError as exc:
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@ZeroIntensity ZeroIntensity Mar 12, 2026

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Please remove this test.

tb = traceback.format_exception(exc)
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@vstinner vstinner Mar 12, 2026

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The test is unrelated to the fix, I suggest removing it. I tried to write a test using faulthandler.dump_c_stack() but I don't know how to create a long C stack. The Python recurse() reuses the same _PyEval_EvalFrameDefault frame for the 50 calls.

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@ZeroIntensity ZeroIntensity Mar 12, 2026

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I don't think there's a reliable test we can write for this at the moment. The code path in question is only triggered on compilers that don't support VLAs, which is likely only MSVC on our buildbots. But, since Windows doesn't support backtrace(), C stack dumps are disabled on MSVC anyway.

assert any("RuntimeError" in line for line in tb)


class PurePythonExceptionFormattingMixin:
def get_exception(self, callable, slice_start=0, slice_end=-1):
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-03-11-19-09-47.gh-issue-145792.X5KUhc.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix incorrect memory allocation in the VLA fallback macro in traceback.c
when using alloca(), preventing potential out-of-bounds access.
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@ZeroIntensity ZeroIntensity Mar 12, 2026

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This is too technical. We should say something like "Fix out-of-bounds access when invoking faulthandler on a CPython build compiled without support for VLAs."

2 changes: 1 addition & 1 deletion Python/traceback.c
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Expand Up @@ -41,7 +41,7 @@

#if defined(__STDC_NO_VLA__) && (__STDC_NO_VLA__ == 1)
/* Use alloca() for VLAs. */
# define VLA(type, name, size) type *name = alloca(size)
# define VLA(type, name, size) type *name = (type *)alloca(sizeof(type) * (size))
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@ZeroIntensity ZeroIntensity Mar 12, 2026

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Please remove the (type *) change. In C, void * pointers can be implicitly casted.

#elif !defined(__STDC_NO_VLA__) || (__STDC_NO_VLA__ == 0)
/* Use actual C VLAs.*/
# define VLA(type, name, size) type name[size]
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