linux-kernel - Re: [PATCH v4 04/14] arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs

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Date:   Fri, 27 Nov 2020 13:12:17 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH v4 04/14] arm64: Kill 32-bit applications scheduled on
 64-bit-only CPUs

On 11/24/20 15:50, Will Deacon wrote:
> Scheduling a 32-bit application on a 64-bit-only CPU is a bad idea.
> 
> Ensure that 32-bit applications always take the slow-path when returning
> to userspace on a system with mismatched support at EL0, so that we can
> avoid trying to run on a 64-bit-only CPU and force a SIGKILL instead.
> 
> Signed-off-by: Will Deacon <will@...nel.org>
> ---

nit: We drop this patch at the end. Can't we avoid it altogether instead?

[...]

> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index a8184cad8890..bcb6ca2d9a7c 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -911,6 +911,19 @@ static void do_signal(struct pt_regs *regs)
>  	restore_saved_sigmask();
>  }
>  
> +static bool cpu_affinity_invalid(struct pt_regs *regs)
> +{
> +	if (!compat_user_mode(regs))
> +		return false;

Silly question. Is there an advantage of using compat_user_mode() vs
is_compat_task()? I see the latter used in the file although struct pt_regs
*regs is passed to the functions calling it.

Nothing's wrong with it, just curious.

Thanks

--
Qais Yousef

> +
> +	/*
> +	 * We're preemptible, but a reschedule will cause us to check the
> +	 * affinity again.
> +	 */
> +	return !cpumask_test_cpu(raw_smp_processor_id(),
> +				 system_32bit_el0_cpumask());
> +}