linux-kernel - Re: vdso_join

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Date:   Thu, 11 Jun 2020 20:17:12 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Christian Brauner <christian.brauner@...ntu.com>,
        Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: vdso_join_timens() question

Hi Christian,

On 6/11/20 12:02 PM, Christian Brauner wrote:
> Hey,
> 
> I'm about to finish a patch to add CLONE_NEWTIME support to setns().
> Since setns() now allows to attach to a multiple namespaces at the same
> time I've also reworked it to be atomic (already upstream). Either all
> namespaces are switched or no namespace is switched. All namespaces
> basically now have a commit mode after which installation should ideally
> not fail anymore. That could work for CLONE_NEWTIME too, I think. The
> only blocker to this is vdso_join_timens() which can fail due to
> mmap_write_lock_killable().
> 
> Is it possible to change this to mmap_write_lock()? So sm like:
> 
> diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
> index ea7c1f0b79df..5c5b4cc61fce 100644
> --- a/arch/x86/entry/vdso/vma.c
> +++ b/arch/x86/entry/vdso/vma.c
> @@ -144,8 +144,7 @@ int vdso_join_timens(struct task_struct *task, struct time_namespace *ns)
>         struct mm_struct *mm = task->mm;
>         struct vm_area_struct *vma;
> 
> -       if (mmap_write_lock_killable(mm))
> -               return -EINTR;
> +       mmap_write_lock(mm);
> 
>         for (vma = mm->mmap; vma; vma = vma->vm_next) {
>                 unsigned long size = vma->vm_end - vma->vm_start;

I think, it should be fine.

I'm thinking if it actually could be downgraded to mmap_read_lock()..
Probably, write lock was being over-cautious.

> vdso_join_timens() is called in two places. Once during fork() and once
> during timens_install(). I would only need the mmap_write_lock() change
> for the latter. So alternatively we could have:
> 
> __vdso_join_timens_unlocked()
> 
> and then have/expose:
> 
> vdso_join_timens_fork()
> {
>         if (mmap_write_lock_killable(mm))
>                 return -EINTR;
> 	__vdso_join_timens_unlocked()
> 	mmap_write_unlock(mm);
> }
> 
> and 
> 
> vdso_join_timens_install()
> {
>         mmap_write_lock(mm);
> 	__vdso_join_timens_unlocked()
> 	mmap_write_unlock(mm);
> }

I think it's not needed. On fork() it's called on creation of new timens:

:	if (nsproxy->time_ns == nsproxy->time_ns_for_children)
:		return 0;

So the vdso_join_timens() is called on setns() or on creation of new
namespace, which both are quite heavy operations themselves (in sense of
locking).

Thanks,
          Dmitry