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

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Message-ID: <CAMgjq7CO_9tXfwd2at-tT+O_JU-NGoG=9OWcqwkc9M3iL1Ga1A@mail.gmail.com>
Date:   Mon, 20 Nov 2023 19:14:59 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     Chris Li <chrisl@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/24] mm/swap: fix a potential undefined behavior issue

Chris Li <chrisl@...nel.org> 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@...radead.org> wrote:
> >
> > On Mon, Nov 20, 2023 at 03:47:17AM +0800, Kairui Song wrote:
> > > From: Kairui Song <kasong@...cent.com>
> > >
> > > 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.