Re: [PATCH 01/24] mm/swap: fix a potential undefined behavior issue

[Kairui Song]

Chris Li <chrisl@xxxxxxxxxx> 于2023年11月20日周一 11:36写道：
>
> Hi Kairui,
>
> On Sun, Nov 19, 2023 at 12:55 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, Nov 20, 2023 at 03:47:17AM +0800, Kairui Song wrote:
> > > From: Kairui Song <kasong@xxxxxxxxxxx>
> > >
> > > When folio is NULL, taking the address of its struct member is an
> > > undefined behavior, the UB is caused by applying -> operator
>
> I think dereferencing the NULL pointer is undefined behavior. There is
> no dereferencing here. It is just pointer arithmetic of NULL pointers,
> which is adding offset of page to the NULL pointer, you got NULL.
>
> > > won't lead to a real issue, still better to fix it, also makes the
> > > code less error-prone, when folio is NULL, page is also NULL,
> > > instead of a meanless offset value.
>
> I consider your reasoning is invalid. NULL pointer arithmetic should
> be legal. This patch is not needed.
>
> Chris

Hi, Chris and Matthew.

Thanks for the comments.

Right, it's just a language syntax level thing, since "->" have a
higher priority, so in the syntax level it is doing a member access
first, then take the address. By C definition  member access should
not happen if the object is invalid (NULL). Only a hypothesis problem
on paper...

This is indeed not needed since in reality it's just pointer
arithmetic. I'm OK dropping this.