[PATCH v9 2/4] arm64: mte: change ASYNC and SYNC TCF settings into bitfields

[Will Deacon]

On Fri, Jul 02, 2021 at 12:41:08PM -0700, Peter Collingbourne wrote:
> Allow the user program to specify both ASYNC and SYNC TCF modes by
> repurposing the existing constants as bitfields. This will allow the
> kernel to select one of the modes on behalf of the user program. With
> this patch the kernel will always select async mode, but a subsequent
> patch will make this configurable.
> 
> Link: https://linux-review.googlesource.com/id/Icc5923c85a8ea284588cc399ae74fd19ec291230
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> ---
> v9:
> - make mte_update_sctlr_user static
> 
>  arch/arm64/include/asm/processor.h |  3 ++
>  arch/arm64/kernel/mte.c            | 70 ++++++++++++------------------
>  include/uapi/linux/prctl.h         | 11 ++---
>  3 files changed, 37 insertions(+), 47 deletions(-)

...

>  long set_mte_ctrl(struct task_struct *task, unsigned long arg)
>  {
> -	u64 sctlr = task->thread.sctlr_user & ~SCTLR_EL1_TCF0_MASK;
>  	u64 mte_ctrl = (~((arg & PR_MTE_TAG_MASK) >> PR_MTE_TAG_SHIFT) &
>  			SYS_GCR_EL1_EXCL_MASK) << MTE_CTRL_GCR_USER_EXCL_SHIFT;
>  
>  	if (!system_supports_mte())
>  		return 0;
>  
> -	switch (arg & PR_MTE_TCF_MASK) {
> -	case PR_MTE_TCF_NONE:
> -		sctlr |= SCTLR_EL1_TCF0_NONE;
> -		break;
> -	case PR_MTE_TCF_SYNC:
> -		sctlr |= SCTLR_EL1_TCF0_SYNC;
> -		break;
> -	case PR_MTE_TCF_ASYNC:
> -		sctlr |= SCTLR_EL1_TCF0_ASYNC;
> -		break;
> -	default:
> -		return -EINVAL;
> -	}
> +	if (arg & PR_MTE_TCF_ASYNC)
> +		mte_ctrl |= MTE_CTRL_TCF_ASYNC;
> +	if (arg & PR_MTE_TCF_SYNC)
> +		mte_ctrl |= MTE_CTRL_TCF_SYNC;
>  
> -	if (task != current) {
> -		task->thread.sctlr_user = sctlr;
> -		task->thread.mte_ctrl = mte_ctrl;
> -	} else {
> -		set_task_sctlr_el1(sctlr);
> -		set_gcr_el1_excl(mte_ctrl);
> +	task->thread.mte_ctrl = mte_ctrl;
> +	if (task == current) {
> +		mte_update_sctlr_user(task);

In conjunction with the next patch, what happens if we migrate at this
point? I worry that we can install a stale sctlr_user value.

> +		set_task_sctlr_el1(task->thread.sctlr_user);

Will