gh-91051: fix segfault when using all 8 type watchers #107853
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@Yhg1s I think this fix should be backported to 3.12. |
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itamaro
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Aug 11, 2023
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Closing and re-opening to retrigger CLA checks. Sorry for the noise. |
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Thanks @carljm for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12. |
miss-islington
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Aug 11, 2023
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GH-107876 is a backport of this pull request to the 3.12 branch. |
Yhg1s
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Aug 16, 2023
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The type watcher implementation adds
char tp_watched;to thePyTypeObjectstruct, and then inPyType_Modifiedit assignsint bits = type->tp_watched;and then treatsbitsas a bitset, expecting that only the low 8 bits should ever be set.On platforms where
charis signed (e.g. x86), if the top bit intp_watchedis set, the cast toint bitswill sign-extend with 1s, and we will thus have a lot of high bits set inbitsthat are not supposed to ever be set.On x86 in a debug build, without the fix in this PR, the new test hits the assertion in
PyType_Modifiedthat only bit positions< TYPE_MAX_WATCHERSshould be set, and aborts.The fix is simple:
tp_watchedshould be defined asunsigned charso that it is zero-extended, not sign-extended.Note that the test is carefully designed to not assume there are no type watchers already active. It doesn't attempt to register a fixed number of type watchers, instead it just registers type watchers until it reaches
TYPE_MAX_WATCHERS - 1, the last available slot and the one that can trigger this bug (since it will set the highest bit intp_watchedwhen watching a type.)