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[2003:d8:2f2b:d900:2d94:8433:b532:3418]) by smtp.gmail.com with ESMTPSA id e1-20020adfe381000000b003140f47224csm14706551wrm.15.2023.08.14.08.37.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 14 Aug 2023 08:37:50 -0700 (PDT) Message-ID: <52192c2f-c7b1-9c07-7ca2-10fc6bd347b0@redhat.com> Date: Mon, 14 Aug 2023 17:37:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH] kernel/fork: stop playing lockless games for exe_file replacement To: Oleg Nesterov , Mateusz Guzik Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, brauner@kernel.org, ebiederm@xmission.com, akpm@linux-foundation.org, linux-mm@kvack.org, koct9i@gmail.com, dave@stgolabs.net References: <20230813123333.1705833-1-mjguzik@gmail.com> <20230814150530.GB17738@redhat.com> <20230814152038.GA2367@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20230814152038.GA2367@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: cj4srjemnhmahqkkmijqjc6b861m94no X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 3D3EE20029 X-Rspam-User: X-HE-Tag: 1692027473-850421 X-HE-Meta: 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 8Bifg3xG Lx70MGmgn86aostJP8sJkjMnJG8Q1ydGjDh9xGWUmDyTxQVvKC74sLdozKsZjxoORfqtX5ynOwV1Igsk8TYhKR3mUFLQ9dVZMUFtUMXEtiRZ27sOznxtvEw1ONHaAAWn03zKtifbOvPzkzD0IWRa5dW20poXZdwHgS3ZrjlZQNzpfeXNk/rPNGW4mOcdQmqOJpR2OxJ03zLJT4FvGJZ6wFeonM2wCx08HzI1bexXLgd9uQa8hm3KBYjDopwELmt+SQhcKWOEvFoU6VHBkroBPccvTKL5jhb5BNgLl+muvN5zMG+QbFStE1Bg4v13Fa3CzJtM0yFr1vrvtIKdBCwrk/PJnisSjuL8XxtxCi+7wL2R+Ozf1ihfyZWHSp8VFfDe0AiPK0p2KkfkS2RLz4ddJeu55ZZCWwIRUE/5gal8eoZL0WtI= 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: On 14.08.23 17:20, Oleg Nesterov wrote: > On 08/14, Oleg Nesterov wrote: >> >> On 08/13, Mateusz Guzik wrote: >>> >>> fe69d560b5bd ("kernel/fork: always deny write access to current MM >>> exe_file") added another lock trip to synchronize the state of exe_file >>> against fork, further defeating the point of xchg. >>> >>> As such I think the atomic here only adds complexity for no benefit. >>> >>> Just write-lock around the replacement. >> >> Well, I tend to agree but can't really comment because I forgot everything >> about these code paths. >> >> But I have to admit that I don't understand the code in replace_mm_exe_file() >> without this patch... >> >> old_exe_file = xchg(&mm->exe_file, new_exe_file); >> if (old_exe_file) { >> /* >> * Don't race with dup_mmap() getting the file and disallowing >> * write access while someone might open the file writable. >> */ >> mmap_read_lock(mm); >> allow_write_access(old_exe_file); >> fput(old_exe_file); >> mmap_read_unlock(mm); >> } >> >> Can someone please explain me which exactly race this mmap_read_lock() tries >> to avoid and how ? > > OK, I seem to understand... without mmap_read_lock() it is possible that > > - dup_mm_exe_file() sees mm->exe_file = old_exe_file > > - replace_mm_exe_file() does allow_write_access(old_exe_file) > > - another process does get_write_access(old_exe_file) > > - dup_mm_exe_file()->deny_write_access() fails > > Right? From what I recall, yes. -- Cheers, David / dhildenb