py/obj: Add new type flag to indicate subscr accepts slice-on-stack. #17736
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Codecov ReportAll modified and coverable lines are covered by tests ✅
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## master #17736 +/- ##
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Coverage 98.38% 98.38%
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This was referenced Jul 22, 2025
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This makes sense and seems worth spending a flag bit on. |
projectgus
approved these changes
Jul 23, 2025
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Ah, thanks! Now fixed. |
The recently merged 5e9189d now allows temporary slices to be allocated on the C stack, which is much better than allocating them on the GC heap. Unfortunately there are cases where the C-allocated slice can escape and be retained as an object, which leads to crashes (because that object points to the C stack which now has other values on it). The fix here is to add a new `MP_TYPE_FLAG_SUBSCR_ALLOWS_STACK_SLICE`. Native types should set this flag if their subscr method is guaranteed not to hold on to a reference of the slice object. Fixes issue micropython#17733 (see also micropython#17723). Signed-off-by: Damien George <damien@micropython.org>
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Summary
The recently merged PR #12518 allowed temporary slices to be allocated on the C stack, which is much better than allocating them on the GC heap.
Unfortunately there are cases where the C-allocated slice can escape and be retained as an object, which leads to crashes (because that object points to the C stack which now has other values on it).
See #17723 and #17733.
The fix here is to add a new
MP_TYPE_FLAG_SUBSCR_ALLOWS_STACK_SLICE. Native types should set this flag if theirsubscrmethod is guaranteed not to hold on to a reference of the slice object.Testing
The existing tests pass. A new test is added to test the bugs reported in #17723 and #17733.
Trade-offs and Alternatives
The function
mp_obj_is_native_type()actually tests a type flag, so we replace the test of one flag with another flag. Thus the fix here should be no change in code size and no change in performance.We do use up another
MP_TYPE_FLAG_xxxbit, but there are still 5 left.Eventually more native types (eg tuple, list) can have this flag set, but I'll leave that for a follow-up PR.