Re: [PATCH v3 11/21] exec: Add support for TARGET_TAGGED_ADDRESSES

[Peter Maydell]

On Fri, 15 Jan 2021 at 22:47, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> The AArch64 Linux ABI has always enabled TBI, but has historically
> required that pointer tags be removed before a syscall.  This has
> changed in the lead-up to ARMv8.5-MTE, in a way that affects the
> ABI generically and not specifically to MTE.
>
> This patch allows the target to indicate that (1) there are tags
> and (2) whether or not they should be taken into account at the
> syscall level.
>
> Adjust g2h, guest_addr_valid, and guest_range_valid to ignore
> pointer tags, similar to how TIF_TAGGED_ADDR alters __range_ok
> in the arm64 kernel source.
>
> The prctl syscall is not not yet updated, so this change by itself
> has no visible effect.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  include/exec/cpu_ldst.h | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/include/exec/cpu_ldst.h b/include/exec/cpu_ldst.h
> index e62f4fba00..1df9b93e59 100644
> --- a/include/exec/cpu_ldst.h
> +++ b/include/exec/cpu_ldst.h
> @@ -69,17 +69,31 @@ typedef uint64_t abi_ptr;
>  #define TARGET_ABI_FMT_ptr "%"PRIx64
>  #endif
>
> +static inline abi_ptr untagged_addr(abi_ptr x)
> +{
> +#ifdef TARGET_TAGGED_ADDRESSES
> +    if (current_cpu) {
> +        return cpu_untagged_addr(current_cpu, x);
> +    }
> +#endif
> +    return x;
> +}

The current_cpu global is a nasty hack and I don't like seeing
new usages of it. In particular, it's very difficult to
analyse in what places this will get called when current_cpu is
NULL and whether it's always OK to not clean the tag in that
situation.

thanks
-- PMM