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b=sB8jmsaenxaQSZtTnEZhkNerXZATf+rVxpKOLUwpsTswT10G+JSMumZkBKND6uKv0 PXj9CITTz6fNPvlwmaPvBOYfg7L2ZfgGG1hGr1+MFmeWHXTPQT5SXQ327vjIKsMeve wPHhq+QzhxuyZpolzmrMhp3uxO083zodSWQt4DfYZXieNO/6YSEEAHuZgaUPX8I5Kz +v5AxoXgGNeQLCwJN7GH/jJWR65nBMUiq8qn9FVRcqZrQO3KTj64QiIPIMZUmnlXll WuBPQxxxYnrUUi5z90LNPZXyj7RIM3iXzVFeNoEToMQpOindvc9cLa0czfSPcnYIxe Yl0/WoJEyyd0A== Received: by mail-pj1-f48.google.com with SMTP id 98e67ed59e1d1-2809a824bbbso2607732a91.3 for ; Sun, 19 Nov 2023 19:35:54 -0800 (PST) X-Gm-Message-State: AOJu0YwW4jNmy0ew23/sg+pL8qgEUtWjcScCr9KoSbvS2SP9qWzHCbyA IBcQ98adANtCsUlgh0WKSjIqkQAN5jpRS5QfQzn51g== X-Google-Smtp-Source: AGHT+IFUvXLlhgsD7jLkOF1XQH8/mI72vbQ7j6Zc0xfO3seezj0akSl7BHHfUOPtPPKAo3HgVCPtqg4HDTyzUC2s1UQ= X-Received: by 2002:a17:90a:191a:b0:27d:c95:b0ad with SMTP id 26-20020a17090a191a00b0027d0c95b0admr4539183pjg.21.1700451353958; Sun, 19 Nov 2023 19:35:53 -0800 (PST) MIME-Version: 1.0 References: <20231119194740.94101-1-ryncsn@gmail.com> <20231119194740.94101-2-ryncsn@gmail.com> In-Reply-To: From: Chris Li Date: Sun, 19 Nov 2023 19:35:42 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 01/24] mm/swap: fix a potential undefined behavior issue To: Matthew Wilcox Cc: Kairui Song , linux-mm , Andrew Morton , "Huang, Ying" , David Hildenbrand , Hugh Dickins , Johannes Weiner , Michal Hocko , LKML Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 8FABF40008 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: 3kypt1n89ind7xn9c789aosqbu6bscdo X-HE-Tag: 1700451358-316538 X-HE-Meta: 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 +nTg2Z0f LLoD0Jads4g+QpX+lIEUjZI+wI/5HmbhFBS/wDI9KBygGb+fwWM3T93t4UQVp+RlERRACafzfEFGX6NaLvXy8zfVBKJe7fpdzxy17ENzFx+V8s4utLmXuQ8kbidyKIeDR31FL96bEpPq8BHvr3hi2gUiP1JrzGnXvYvXhJA2BZEem/u16yOWoIW7ZMCOkm2u+yB31e0Atl2zs614NvPia1g7sUu1CnTZMJzBHxnsHIuxq4fmUt3wKXxMUxc4C3kJgZhgRNTlTSUQth0p0GcR+l1jXKwtV8gOcjjxiO8K/uDt6qwLzW9uyl+QeSh0fw1owxozjLRPK/KzCNGM= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Kairui, On Sun, Nov 19, 2023 at 12:55=E2=80=AFPM Matthew Wilcox wrote: > > On Mon, Nov 20, 2023 at 03:47:17AM +0800, Kairui Song wrote: > > From: Kairui Song > > > > 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