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Mon, 14 Aug 2023 17:58:20 +0200 (CEST) Date: Mon, 14 Aug 2023 17:58:17 +0200 From: Oleg Nesterov To: David Hildenbrand Cc: Mateusz Guzik , 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 Subject: Re: [PATCH] kernel/fork: stop playing lockless games for exe_file replacement Message-ID: <20230814155817.GC17738@redhat.com> References: <20230813123333.1705833-1-mjguzik@gmail.com> <20230814150530.GB17738@redhat.com> <20230814152038.GA2367@redhat.com> <52192c2f-c7b1-9c07-7ca2-10fc6bd347b0@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52192c2f-c7b1-9c07-7ca2-10fc6bd347b0@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 22EAF100009 X-Stat-Signature: dsbnbsi5woa8e7my6oagwbw7ry6oup5x X-HE-Tag: 1692028747-45048 X-HE-Meta: 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 03rfQAmW qkTlrRX3Gr3tEPFYu/MSGh5JuVVq++FsyaB5BFQ24M1h6WIomtYsAdYjwwxLLBQ0FLAiCfZt8qE7tAuRR1pCIgwZzDe500cbKaMrafAIAmP1vxrUlTEvZxi2de0a37Ug0QD/JISep/dTvlrmUwE+HAYJk1C8zibTSps1BSvNPhi6hFBK5mAn+7NOfRZZCXBi3wgFlwKVXwjqF570ZG+BreC+wQMTQWtdkJMuc9yhH8z8RXyHQf0M7qZt31QOTxybQrri38rOOGo3Oye4= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 08/14, David Hildenbrand wrote: > > >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. Thanks! but then... David, this all is subjective, feel free to ignore, but the current code doesn't look good to me, I mean the purpose of mmap_read_lock() is very unclear. To me something like if (old_exe_file) { /* * Ensure that if we race with dup_mm_exe_file() and it sees * mm->exe_file == old_exe_file deny_write_access(old_exe_file) * can't fail after we do allow_write_access() and another task * does get_write_access(old_exe_file). */ mmap_read_lock(mm); mmap_read_unlock(mm); allow_write_access(old_exe_file); fput(old_exe_file); } looks more understandable... But this patch from Mateusz looks even better to me. So, FWIW, Acked-by: Oleg Nesterov