Re: [PATCH v3 12/21] linux-user/aarch64: Implement PR_TAGGED_ADDR_ENABLE

[Peter Maydell]

On Fri, 15 Jan 2021 at 22:47, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> This is the prctl bit that controls whether syscalls accept tagged
> addresses.  See Documentation/arm64/tagged-address-abi.rst in the
> linux kernel.

> +#ifdef TARGET_TAGGED_ADDRESSES
> +/**
> + * cpu_untagged_addr:
> + * @cs: CPU context
> + * @x: tagged address
> + *
> + * Remove any address tag from @x.  This is explicitly related to the
> + * linux syscall TIF_TAGGED_ADDR setting, not TBI in general.
> + *
> + * There should be a better place to put this, but we need this in
> + * include/exec/cpu_ldst.h, and not some place linux-user specific.
> + */
> +static inline target_ulong cpu_untagged_addr(CPUState *cs, target_ulong x)
> +{
> +    ARMCPU *cpu = ARM_CPU(cs);
> +    return x & cpu->env.untagged_addr_mask;
> +}
> +#endif

Forgot to mention: this only does the right thing on addresses
in the lower half of the address space. I guess that's mostly
OK for our purposes? It probably means that if a guest program
deliberately dereferences a bad address in the top half of the
address space we'll report the wrong (ie different to what a real
kernel reports) address value to it in the SEGV signal handler.

The kernel's "untagged_addr()" implementation:
https://elixir.bootlin.com/linux/latest/source/arch/arm64/include/asm/memory.h#L203
slightly confusingly does "untag the addr if it's in the userspace
half, leave the tag bits alone if in the kernel half".

thanks
-- PMM