What is the role of LIST_POISON1 and LIST_POISON2?

[Vasu M]

On Fri, Mar 4, 2016 at 5:01 AM, Navy Cheng <navych at 126.com> wrote:

> On Fri, Mar 04, 2016 at 02:07:26AM -0500, Valdis.Kletnieks at vt.edu 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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