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TIL: The most popular 64-bit architecture in the world has a small address space. Kills any hope of never reusing malloc addresses.

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Daniel Micay

@DanielMicay

Mar 22, 2021

Replying to @DanielMicay @pcwalton and 2 others

On arm64, you typically only have a 39-bit address space although it can be 48-bit like x86_64. The kernel takes half so it's really 38-bit or 47-bit. 38-bit is already far too small to use hardened_malloc with the normal configuration. 47-bit is 128 TiB which isn't limitless.

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Can enable 4-level page tables in the Linux kernel for 48-bit address space. In practice, nearly everyone is using 3-level page tables. I made a thread about this here: twitter.com/DanielMicay/st ARMv8.3 has major pressure against large address space via pointer authentication.

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Daniel Micay

@DanielMicay

Mar 16, 2021

Linux on 64-bit ARM is most commonly used with 4k pages and 3-level page tables. This results in a 39-bit address space rather than the full 64-bit address space. It's possible to use 4-level page tables and then you get a 48-bit address space like you typically have on x86_64.

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If you don't control the kernel or if you don't want to give up high entropy pointer authentication, you're pretty much stuck with 39-bit address space on arm64 Linux. You can see the choices here: github.com/torvalds/linux Highlighted line is a nearly universal configuration.

github.com

linux/Kconfig at 79cc2ed5a716544621b11a3f90550e5c7d314306 · torvalds/linux

Linux kernel source tree. Contribute to torvalds/linux development by creating an account on GitHub.

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and

Nearly everyone uses 3-level page tables + 4k pages for Linux giving 39-bit address space. Pointer authentication is just as greedy for bits if you use larger pages or deeper page tables so I don't think we'll really see deployment of larger address spaces for consumer devices.

10:06 PM · Mar 22, 2021Twitter Web App

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and

We just shipped 48-bit address space for GrapheneOS: grapheneos.org/releases#2021. That's because the devices are still ARMv8.2 though... When the non-Apple world moves to ARMv8.4 we'll have to go back to 39-bit address space if we don't want to cripple pointer authentication. :(

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and

Related fact: the standard Android Runtime (ART) on 64-bit still largely uses a 32-bit address space for data. Puts all the managed memory in the lower 4G of address space in order to use 32-bit pointers directly rather than paying the cost of 64-bit pointers or 32-bit offsets.

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