Add Shadow Stack support for a simple app

[mariasfiraiala]

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.pl on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): AArch64
• Platform(s): KVM
• Application(s): N/A

Additional configuration

You'll need to update the Makefile.uk of your app by adding this (let our app be helloworld for the sake of simplicity)

APPHELLOWORLD_CINCLUDES-y += -fsanitize=shadow-call-stack -ffixed-x18 -fno-exceptions
COMPFLAGS-y += -ffixed-x18 -fno-exceptions

Shadow Stack support comes with clang as a compiler, you'll have to build the app accordingly.

Build using

make CC=clang LD=~/toolchains/gcc-arm-11.2-2022.02-x86_64-aarch64-none-elf/bin/aarch64-none-elf-gcc OBJCOPY=~/toolchains/gcc-arm-11.2-2022.02-x86_64-aarch64-none-elf/bin/aarch64-none-elf-objcopy STRIP=~/toolchains/gcc-arm-11.2-2022.02-x86_64-aarch64-none-elf/bin/aarch64-none-elf-strip

You'll need the gcc cross-compiling toolchain installed. Get it from here.

Also, make sure to have erratum options disabled when using menuconfig (Architecture Selection -> Arm8 Compatible -> Workaround for [...] erratum).

What's more, you'll have to configure the Custom cross-compiler LLVM target too (Build Options -> Custom cross-compiler LLVM target); just write aarch64-none-elf and you should be good to go.

Description of changes

This PR aims to demonstrate how the Shadow Stack support will behave.

It brings minor changes in order to make possible the compilation with clang, as this security mechanism is supported by clang and gcc-12.

Further changes were made in boot.c in order to initialize the Shadow Stack.

Future changes will bring better performance by changing the memory allocator to a Unikraft based one and by integrating the constructor in the Unikraft constructor table.

For Proof of Concept and other information, refer this.