RE: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

David Laight
2022-03-03T04:58:34Z

From: Xiaomeng Tong
> Sent: 03 March 2022 02:27
>
> On Wed, 2 Mar 2022 14:04:06 +0000, David Laight
> <David.Laight@ACULAB.COM> wrote:
> > I think that it would be better to make any alternate loop macro
> > just set the variable to NULL on the loop exit.
> > That is easier to code for and the compiler might be persuaded to
> > not redo the test.
>
> No, that would lead to a NULL dereference.

Why, it would make it b ethe same as the 'easy to use':
for (item = head; item; item = item->next) {
...
if (...)
break;
...
}
if (!item)
return;

> The problem is the mis-use of iterator outside the loop on exit, and
> the iterator will be the HEAD's container_of pointer which pointers
> to a type-confused struct. Sidenote: The *mis-use* here refers to
> mistakely access to other members of the struct, instead of the
> list_head member which acutally is the valid HEAD.

The problem is that the HEAD's container_of pointer should never
be calculated at all.
This is what is fundamentally broken about the current definition.

> IOW, you would dereference a (NULL + offset_of_member) address here.

Where?

> Please remind me if i missed something, thanks.
>
> Can you share your "alternative definitions" details? thanks!

The loop should probably use as extra variable that points
to the 'list node' in the next structure.
Something like:
for (xxx *iter = head->next;
iter == &head ? ((item = NULL),0) : ((item = list_item(iter),1));
iter = item->member->next) {
...
With a bit of casting you can use 'item' to hold 'iter'.

>
> > OTOH there may be alternative definitions that can be used to get
> > the compiler (or other compiler-like tools) to detect broken code.
> > Even if the definition can't possibly generate a working kerrnel.
>
> The "list_for_each_entry_inside(pos, type, head, member)" way makes
> the iterator invisiable outside the loop, and would be catched by
> compiler if use-after-loop things happened.

It is also a compete PITA for anything doing a search.

David

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