On Fri, Mar 4, 2016 at 5:01 AM, Navy Cheng <navych@xxxxxxx> wrote:
On Fri, Mar 04, 2016 at 02:07:26AM -0500, Valdis.Kletnieks@xxxxxx wrote:
> 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
>
It's like when you find a pointer to 0xdeadbeef you will know that it is some uninitialized value which is more helpful in debugging. If its a NULL, it will be difficult to know if the pointer is uninitialized.
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