Update ASan information regarding --without-pymalloc flag #1609
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As discussed with @encukou on the CPython Core sprint on EuroPython 2025. We initially thought that the `--without-pymalloc` flag is needed due to the fact pymalloc must hit the begining of page when determining if the memory to be freed comes from pymalloc or was allocated by the system malloc. In other words, we thought, that ASan would crash CPython during free of big objects (allocated by system malloc). It may be that this was the case in the past, but it is not the case anymore as the `address_in_range` function used by pymalloc is annotated to be skipped from the ASan instrumentation. This code can be seen here: https://github.com/python/cpython/blob/acefb978dcb5dd554e3c49a3015ee5c2ad6bfda1/Objects/obmalloc.c#L2096-L2110 While the annotation macro is defined here: https://github.com/python/cpython/blob/acefb978dcb5dd554e3c49a3015ee5c2ad6bfda1/Include/pyport.h#L582-L598 And the corresponding attribute is documented in: * for gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-no_005fsanitize_005faddress-function-attribute * for clang: https://clang.llvm.org/docs/AttributeReference.html#no-sanitize-address-no-address-safety-analysis
encukou
reviewed
Jul 19, 2025
encukou
reviewed
Jul 19, 2025
encukou
reviewed
Jul 19, 2025
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I have tested the CPython latest main build with both ASan and pymalloc enabled and it seems to work just fine. I did run the
python -m testsuite which didn't uncover any ASan crashes (though, it detected some memory leaks, which I believe are irrelevant here).I have discussed ASan and this flag with @encukou on the CPython Core sprint on EuroPython 2025. We initially thought that the
--without-pymallocflag is needed for ASan builds due to the fact pymalloc must hit the begining of page when determining if the memory to be freed comes from pymalloc or was allocated by the system malloc. In other words, we thought, that ASan would crash CPython during free of big objects (allocated by system malloc). It may be that this was the case in the past, but it is not the case anymore as theaddress_in_rangefunction used by pymalloc is annotated to be skipped from the ASan instrumentation.This code can be seen here:
https://github.com/python/cpython/blob/acefb978dcb5dd554e3c49a3015ee5c2ad6bfda1/Objects/obmalloc.c#L2096-L2110
While the annotation macro is defined here:
https://github.com/python/cpython/blob/acefb978dcb5dd554e3c49a3015ee5c2ad6bfda1/Include/pyport.h#L582-L598
And the corresponding attribute is documented in: