linux-kernel - Re: [PATCH v2 1/7] fs/proc/task_mmu.c: don't use task->mm in m_start() and show

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Date:	Wed, 6 Aug 2014 12:52:14 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	David Howells <dhowells@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <levinsasha928@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/7] fs/proc/task_mmu.c: don't use task->mm in
 m_start() and show_*map()

On Tue, Aug 05, 2014 at 09:46:44PM +0200, Oleg Nesterov wrote:
> get_gate_vma(priv->task->mm) looks ugly and wrong, task->mm can be
> NULL or it can changed by exec right after mm_access().
> 
> And in theory this race is not harmless, the task can exec and then
> later exit and free the new mm_struct. In this case get_task_mm(oldmm)
> can't help, get_gate_vma(task->mm) can read the freed/unmapped memory.
> 
> I think that priv->task should simply die and hold_task_mempolicy()
> logic can be simplified. tail_vma logic asks for cleanups too.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  fs/proc/task_mmu.c |    8 +++-----
>  1 files changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index cfa63ee..8a5271e 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -170,7 +170,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
>  		return mm;
>  	down_read(&mm->mmap_sem);
>  
> -	tail_vma = get_gate_vma(priv->task->mm);
> +	tail_vma = get_gate_vma(mm);
>  	priv->tail_vma = tail_vma;
>  	hold_task_mempolicy(priv);
>  	/* Start with last addr hint */
> @@ -351,12 +351,11 @@ static int show_map(struct seq_file *m, void *v, int is_pid)
>  {
>  	struct vm_area_struct *vma = v;
>  	struct proc_maps_private *priv = m->private;
> -	struct task_struct *task = priv->task;
>  
>  	show_map_vma(m, vma, is_pid);
>  
>  	if (m->count < m->size)  /* vma is copied successfully */
> -		m->version = (vma != get_gate_vma(task->mm))
> +		m->version = (vma != priv->tail_vma)
>  			? vma->vm_start : 0;

Drop excessive parenthesis while you're there? And in the next hunk.

Otherwise:

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

>  	return 0;
>  }
> @@ -584,7 +583,6 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
>  static int show_smap(struct seq_file *m, void *v, int is_pid)
>  {
>  	struct proc_maps_private *priv = m->private;
> -	struct task_struct *task = priv->task;
>  	struct vm_area_struct *vma = v;
>  	struct mem_size_stats mss;
>  	struct mm_walk smaps_walk = {
> @@ -639,7 +637,7 @@ static int show_smap(struct seq_file *m, void *v, int is_pid)
>  	show_smap_vma_flags(m, vma);
>  
>  	if (m->count < m->size)  /* vma is copied successfully */
> -		m->version = (vma != get_gate_vma(task->mm))
> +		m->version = (vma != priv->tail_vma)
>  			? vma->vm_start : 0;
>  	return 0;
>  }
> -- 
> 1.5.5.1
> 
> --
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-- 
 Kirill A. Shutemov
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