linux-kernel - Re: [PATCH v2] vma: Detect infinite loop in vma tree

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Message-ID: <nitzqi2v37ojb2ron7bjwushyxwjw7yq753z42un7eotzs3vpm@6kr5xblggaz6>
Date: Thu, 31 Oct 2024 17:21:25 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2] vma: Detect infinite loop in vma tree

* Andrew Morton <akpm@...ux-foundation.org> [241031 16:48]:
> On Thu, 31 Oct 2024 15:36:08 -0400 "Liam R. Howlett" <Liam.Howlett@...cle.com> wrote:
> 
> > From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> > 
> > There have been no reported infinite loops in the tree, but checking the
> > detection of an infinite loop during validation is simple enough.  Add
> > the detection to the validate_mm() function so that error reports are
> > clear and don't just report stalls.
> > 
> > This does not protect against internal maple tree issues, but it does
> > detect too many vmas being returned from the tree.
> > 
> > The variance of +10 is to allow for the debugging output to be more useful for
> > nearly correct counts.  In the event of more than 10 over the map_count, the
> > count will be set to -1 for easier identification of a potential infinite loop.
> > 
> > Note that the mmap lock is held to ensure a consistent tree state during the
> > validation process.
> > 
> > ...
> >
> > +++ b/mm/vma.c
> > @@ -615,7 +615,10 @@ void validate_mm(struct mm_struct *mm)
> >  			anon_vma_unlock_read(anon_vma);
> >  		}
> >  #endif
> > -		i++;
> > +		if (++i > mm->map_count + 10) {
> > +			i = -1;
> > +			break;
> > +		}
> >  	}
> >  	if (i != mm->map_count) {
> >  		pr_emerg("map_count %d vma iterator %d\n", mm->map_count, i);
> 
> It might be helpful to tell readers what's going on here?

Sounds good.  I guess I should have waited longer for a v2.

> 
> --- a/mm/vma.c~vma-detect-infinite-loop-in-vma-tree-fix
> +++ a/mm/vma.c
> @@ -615,6 +615,7 @@ void validate_mm(struct mm_struct *mm)
>  			anon_vma_unlock_read(anon_vma);
>  		}
>  #endif
> +		/* Check for a infinite loop */
>  		if (++i > mm->map_count + 10) {
>  			i = -1;
>  			break;
> _
>