linux-kernel - Re: [PATCH] radix-tree: get_slot_offset() returns invalid offset when parent is NULL

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Date:   Tue, 10 Oct 2017 13:53:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     mawilcox@...rosoft.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] radix-tree: get_slot_offset() returns invalid offset
 when parent is NULL

On Tue, 10 Oct 2017 10:52:01 +0800 Wei Yang <richard.weiyang@...il.com> wrote:

> When parent is NULL, get_slot_offset() returns almost the address of slot.
> This is an invalid value for offset.
> 
> One possible scenario happens on deleting #0 index, when it is the only one
> in tree.
> 
> Current behavior doesn't harm the system, because the offset will not be
> used when parent is NULL in the following procedure or parent is checked
> before get_slot_offset() called. While it is still not safe to return an
> invalid offset.
> 
> This patch returns 0 when parent is NULL in get_slot_offset().
> 

I'm confused.  If parent=NULL, get_slot_offset() will crash the kernel.
So why "Current behavior doesn't harm the system"?

> --- a/lib/radix-tree.c
> +++ b/lib/radix-tree.c
> @@ -119,7 +119,7 @@ bool is_sibling_entry(const struct radix_tree_node *parent, void *node)
>  static inline unsigned long
>  get_slot_offset(const struct radix_tree_node *parent, void __rcu **slot)
>  {
> -	return slot - parent->slots;
> +	return parent ? (slot - parent->slots):0;
>  }
>  
>  static unsigned int radix_tree_descend(const struct radix_tree_node *parent,