linux-kernel - Re: [PATCHv3 16/27] x86/vdso: Switch image on setns()/unshare()/clone()

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Date:   Thu, 25 Apr 2019 19:53:08 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jeff Dike <jdike@...toit.com>, Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        Linux API <linux-api@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCHv3 16/27] x86/vdso: Switch image on setns()/unshare()/clone()

On Thu, Apr 25, 2019 at 6:15 PM Dmitry Safonov <dima@...sta.com> wrote:
> As it has been discussed on timens RFC, adding a new conditional branch
> `if (inside_time_ns)` on VDSO for all processes is undesirable.
> It will add a penalty for everybody as branch predictor may mispredict
> the jump. Also there are instruction cache lines wasted on cmp/jmp.
>
> Those effects of introducing time namespace are very much unwanted
> having in mind how much work have been spent on micro-optimisation
> vdso code.
>
> Addressing those problems, there are two versions of VDSO's .so:
> for host tasks (without any penalty) and for processes inside of time
> namespace with clk_to_ns() that subtracts offsets from host's time.
>
> Whenever a user does setns()/unshare() or clone() with CLONE_TIMENS,
> change VDSO image in mm and zap existing VVAR/VDSO page tables.
> They will be re-faulted with corresponding image and VVAR offsets.
[...]
> +#ifdef CONFIG_TIME_NS
> +int vdso_join_timens(struct task_struct *task, bool inside_ns)

The parameter "inside_ns" is never used, right?

> +{
> +       struct mm_struct *mm = task->mm;
> +       struct vm_area_struct *vma;
> +
> +       if (down_write_killable(&mm->mmap_sem))
> +               return -EINTR;
> +
> +       for (vma = mm->mmap; vma; vma = vma->vm_next) {
> +               unsigned long size = vma->vm_end - vma->vm_start;
> +
> +               if (vma_is_special_mapping(vma, &vvar_mapping))
> +                       zap_page_range(vma, vma->vm_start, size);
> +               if (vma_is_special_mapping(vma, &vdso_mapping))
> +                       zap_page_range(vma, vma->vm_start, size);

Nit: This could be rewritten as:

if (vma_is_special_mapping(vma, &vvar_mapping) ||
    vma_is_special_mapping(vma, &vdso_mapping))
        zap_page_range(vma, vma->vm_start, size);

> +       }
> +
> +       up_write(&mm->mmap_sem);
> +       return 0;
> +}
[...]