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

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Message-Id: <20241031134815.81766b263e87f74a3e31471e@linux-foundation.org>
Date: Thu, 31 Oct 2024 13:48:15 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, 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

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?

--- 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;
_