Subject: Re: [PATCH v3 08/14] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0

From: Quentin Perret <[email-masked]>
Date: Thu, 19 Nov 2020 09:24:07 +0000

On Friday 13 Nov 2020 at 09:37:13 (+0000), Will Deacon wrote:
> When exec'ing a 32-bit task on a system with mismatched support for
> 32-bit EL0, try to ensure that it starts life on a CPU that can actually
> run it.
> 
> Signed-off-by: Will Deacon <[email-masked]>
> ---
>  arch/arm64/kernel/process.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 1540ab0fbf23..17b94007fed4 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -625,6 +625,16 @@ unsigned long arch_align_stack(unsigned long sp)
>  	return sp & ~0xf;
>  }
>  
> +static void adjust_compat_task_affinity(struct task_struct *p)
> +{
> +	const struct cpumask *mask = system_32bit_el0_cpumask();
> +
> +	if (restrict_cpus_allowed_ptr(p, mask))
> +		set_cpus_allowed_ptr(p, mask);

My understanding of this call to set_cpus_allowed_ptr() is that you're
mimicking the hotplug vs affinity case behaviour in some ways. That is,
if a task is pinned to a CPU and userspace hotplugs that CPU, then the
kernel will reset the affinity of the task to the remaining online CPUs.

I guess that is a sensible fallback path when userspace gives
contradictory commands to the kernel, but that most certainly deserves a
comment :)

> +
> +	set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
> +}
>  
>  /*
>   * Called from setup_new_exec() after (COMPAT_)SET_PERSONALITY.
>   */
> @@ -635,7 +645,7 @@ void arch_setup_new_exec(void)
>  	if (is_compat_task()) {
>  		mmflags = MMCF_AARCH32;
>  		if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
> -			set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
> +			adjust_compat_task_affinity(current);
>  	}
>  
>  	current->mm->context.flags = mmflags;
> -- 
> 2.29.2.299.gdc1121823c-goog
> 

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