Motivation for this change

The Scudo allocator is a good compromise between security, performance and compatibility. The GrapheneOS allocator which is currently the only "secure" allocator option in NixOS is incompatible with Nix itself (due to coping badly with tons of small allocs) and has a fairly big overhead.

Scudo doesn't go as far in terms of error checking as the Graphene allocator (Graphene is closer to ASAN/MSAN). But being able to run Nix is fairly important :-) It's around a 20% CPU time increase on some test nix evaluations I've done compared to libc malloc.

See:

• https://llvm.org/docs/ScudoHardenedAllocator.html
• https://android-developers.googleblog.com/2019/05/queue-hardening-enhancements.html which mentions Android Q planning to use Scudo for some security critical components.

@joachifm

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.