The syscall goes in a register but it does not have to appear literally right next to the `syscall` instruction in the binary. As TFA explains in the introduction, a syscall stub generally looks like

    mov eax,0x5
    syscall

However it doesn’t have to, `syscall` will work as long as `eax` is set no matter where it’s set, or where it’s set from. You could load it from an array or a computation for all `syscall` cares.

So as an attacker if you can get eax to a value you control (and probably a few other registries) then jump to the `syscall` instruction directly you have arbitrary syscall capabilities.

The point of this change is that the loader now records exact syscall stubs as “address X performs syscall S”, then on context switch the kernel validates if the syscall being performed matches what was recorded by the loader, and if not it aborts (I assume I didn’t actually check).

This means as long as your go binary uses a normal syscall stub it’ll be recognised by the loader and whitelisted, but if say a JIT constructs syscalls dynamically (instead of bouncing through libc or whatever) that will be rejected because the loader won’t have that (address, number) recorded.