What is the role of LIST_POISON1 and LIST_POISON2?

[Valdis.Kletnieks at vt.edu]

On Fri, 04 Mar 2016 13:02:02 +0800, Navy Cheng said:
> Hi,
>
> When I read the code of list_del(), I find LIST_POISON1 and LIST_POISON2:
>
>     static inline void list_del(struct list_head *entry)
>     {
> 	__list_del(entry->prev, entry->next);
> 	entry->next = LIST_POISON1;
> 	entry->prev = LIST_POISON2;
>     }
>
> Why not set entry->next and entry->prev to NULL ?

To more easily detect different classes of list corruption, use-after-free, and
other programming errors.  If ->next and ->prev are NULL, it may be the result
of following a bad pointer.  If  they're equal to POISON 1 and 2, you're almost
certainly looking at a once-valid pointer that is a use-after-free situation.
It's easy to end up pointing at a zeroed page.  The chances of pointing at
some random data that happens to be POISON 1/2 is much lower.

See the code in lib/list_debug.c

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