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Clang's new __builtin_dynamic_object_size can help detect more buffer overflows at runtime automatically. clang.llvm.org/docs/LanguageE

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This feature is something I thought would be a good idea and I filed an issue about it with GCC along with discussing it with Google for Clang, but we never took it anywhere: gcc.gnu.org/bugzilla/show_ It seems Apple ended up wanting the same thing but actually got it implemented.

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The __dynamic_object_size that I implemented for Bionic was something much different. It queries the hardened malloc implementation for sizes and uses fast paths for objects on the stack or in the executable. That was built with the old hardened malloc forked from OpenBSD malloc.

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11:02 PM · Feb 14, 2019Twitter Web Client

It fits into a far different performance profile and was only used for system calls, obviously not the C mem* and str* functions or even fread/fwrite. It's one of a dozen extensions that was implemented on top of OpenBSD malloc in the fork. The new malloc will also include this.

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The new implemented is going to be split into a function matching the old implementation (able to eventually provide fully accurate results taking into account offsets into objects and whether they're free) and a fast variant based on address checks leveraging size class regions.

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