[LARTC] LIST_FIND in netfilter_ipv4/listhelp.h

[LARTC] LIST_FIND in netfilter_ipv4/listhelp.h

[jeremie le-hen]

Hi folks,

I'm currently trying to make a patch on conntrack module. While reading some
code I saw the following in include/linux/netfilter_ipv4/listhelp.h : 

/* Works on circular linked list. */

#define LIST_FIND(head, cmpfn, type, args...)           \
({                                                      \
        const struct list_head *__i = (head);           \
                                                        \
        ASSERT_READ_LOCK(head);                         \
        do {                                            \
                __i = __i->next;                        \
                if (__i = (head)) {                    \
                        __i = NULL;                     \
                        break;                          \
                }                                       \
        } while (!cmpfn((const type)__i , ## args));    \
        (type)__i;                                      \
})


Unless I really drank too much yesterday evening, I'm pretty sure this
algorithm will fail if the list has only one element, returning NULL
immediately without even trying to compare the element.
On the other hand, it seems to work pretty well for months, since it is
widely used accross the NetFilter conntrack code. So where am I wrong ?

Thanks in advance.
Regards,
-- 
Jeremie LE HEN aka TtZ/TataZ                          jeremie.le-hen@epita.fr
                                                                 ttz@epita.fr
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Re: [LARTC] LIST_FIND in netfilter_ipv4/listhelp.h

[Thomas Graf]

Hi

* jeremie le-hen <20031006144458.GD24864@carpediem.epita.fr> 2003-10-06 16:44

> Unless I really drank too much yesterday evening, I'm pretty sure this
> algorithm will fail if the list has only one element, returning NULL
> immediately without even trying to compare the element.
> On the other hand, it seems to work pretty well for months, since it is
> widely used accross the NetFilter conntrack code. So where am I wrong ?

The code is correct.

Check include/linux/list.h, it's a cyclic list:
head->next points to first element
head->prev points to last element

the list iteration code will make it clear:
#define __list_for_each(pos, head) \
    for (pos = (head)->next; pos != (head); pos = pos->next)

Regards

-- 
Thomas GRAF <tgraf@suug.ch>
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