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

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Message-ID: <ZVp2UE+MqkifJG4B@casper.infradead.org>
Date:   Sun, 19 Nov 2023 20:55:44 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Kairui Song <kasong@...cent.com>
Cc:     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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/24] mm/swap: fix a potential undefined behavior issue

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
> to a pointer not pointing to any object. Although in practice this
> 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.

Um, &folio->page is NULL if folio is NULL.  The offset of 'page' within
'folio' is 0.  By definition; and this will never change.