On Fri, Mar 4, 2016 at 5:01 AM, Navy Cheng [off-list ref] wrote:

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

quoted

On Fri, 04 Mar 2016 13:02:02 +0800, Navy Cheng said:

quoted

Hi,

When I read the code of list_del(), I find LIST_POISON1 and

LIST_POISON2:

quoted

quoted

    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

quoted

other programming errors.  If ->next and ->prev are NULL, it may be the

result

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of following a bad pointer.  If  they're equal to POISON 1 and 2, you're

almost

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certainly looking at a once-valid pointer that is a use-after-free

situation.

quoted

It's easy to end up pointing at a zeroed page.  The chances of pointing

at

quoted

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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