linux-kernel - Re: [PATCH 2/6] arm64/vdso: Zap vvar pages when switching to a time namespace

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Message-ID: <20200624151801.y27a5joml3mxeo4o@wittgenstein>
Date:   Wed, 24 Jun 2020 17:18:01 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH 2/6] arm64/vdso: Zap vvar pages when switching to a time
 namespace

On Wed, Jun 24, 2020 at 01:33:17AM -0700, Andrei Vagin wrote:
> The order of vvar pages depends on whether a task belongs to the root
> time namespace or not. In the root time namespace, a task doesn't have a
> per-namespace page. In a non-root namespace, the VVAR page which contains
> the system-wide VDSO data is replaced with a namespace specific page
> that contains clock offsets.
> 
> Whenever a task changes its namespace, the VVAR page tables are cleared
> and then they will be re-faulted with a corresponding layout.
> 
> A task can switch its time namespace only if its ->mm isn't shared with
> another task.
> 
> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>
> Reviewed-by: Dmitry Safonov <dima@...sta.com>
> Signed-off-by: Andrei Vagin <avagin@...il.com>
> ---
>  arch/arm64/kernel/vdso.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> index 7c4620451fa5..bdf492a17dff 100644
> --- a/arch/arm64/kernel/vdso.c
> +++ b/arch/arm64/kernel/vdso.c
> @@ -124,6 +124,37 @@ static int __vdso_init(enum vdso_abi abi)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_TIME_NS
> +/*
> + * The vvar mapping contains data for a specific time namespace, so when a task
> + * changes namespace we must unmap its vvar data for the old namespace.
> + * Subsequent faults will map in data for the new namespace.
> + *
> + * For more details see timens_setup_vdso_data().
> + */
> +int vdso_join_timens(struct task_struct *task, struct time_namespace *ns)
> +{
> +	struct mm_struct *mm = task->mm;
> +	struct vm_area_struct *vma;
> +
> +	mmap_read_lock(mm);

Perfect, thanks! I'll adapt my patches so that my change and this change
don't conflict and can go in together. Once they're landed we can simply
turn int vdso_join_timens() into void vdso_join_timens() everywhere.

Reviewed-by: Christian Brauner <christian.brauner@...ntu.com>

Thanks!
Christian