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([2001:a61:24d6:4e01:ef75:e978:47cd:1c50]) by smtp.gmail.com with ESMTPSA id z13sm3169830wrq.51.2019.09.24.14.00.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 24 Sep 2019 14:00:03 -0700 (PDT) Cc: mtk.manpages@gmail.com, Florian Weimer , Oleg Nesterov , Jann Horn , "Eric W. Biederman" , Daniel Colascione , Joel Fernandes , linux-man , Linux API , lkml Subject: Re: For review: pidfd_send_signal(2) manual page To: Christian Brauner References: <87pnjr9rth.fsf@mid.deneb.enyo.de> <20190923142325.jowzbnwjw7g7si7j@wittgenstein> <90dd38d5-34b3-b72f-8e5a-b51f944f22fb@gmail.com> <20190924195701.7pw2olbviieqsg5q@wittgenstein> From: "Michael Kerrisk (man-pages)" Message-ID: Date: Tue, 24 Sep 2019 23:00:03 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190924195701.7pw2olbviieqsg5q@wittgenstein> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-man-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Hello Christian, >>> If you're the parent of the process you can do this without CLONE_PIDFD: >>> pid = fork(); >>> pidfd = pidfd_open(); >>> ret = pidfd_send_signal(pidfd, 0, NULL, 0); >>> if (ret < 0 && errno == ESRCH) >>> /* pidfd refers to another, recycled process */ >> >> Although there is still the race between the fork() and the >> pidfd_open(), right? > > Actually no and my code is even too complex. > If you are the parent, and this is really a sequence that obeys the > ordering pidfd_open() before waiting: > > pid = fork(); > if (pid == 0) > exit(EXIT_SUCCESS); > pidfd = pidfd_open(pid, 0); > waitid(pid, ...); > > Then you are guaranteed that pidfd will refer to pid. No recycling can > happen since the process has not been waited upon yet (That is, D'oh! Yes, of course. > excluding special cases such as where you have a mainloop where a > callback reacts to a SIGCHLD event and waits on the child behind your > back and your next callback in the mainloop calls pidfd_open() while the > pid has been recycled etc.). > A race could only appear in sequences where waiting happens before > pidfd_open(): > > pid = fork(); > if (pid == 0) > exit(EXIT_SUCCESS); > waitid(pid, ...); > pidfd = pidfd_open(pid, 0); > > which honestly simply doesn't make any sense. So if you're the parent > and you combine fork() + pidfd_open() correctly things should be fine > without even having to verify via pidfd_send_signal() (I missed that in > my first mail.). Thanks for the additional detail. I added the following to the pidfd_open() page, to prevent people making the same thinko as me: The following code sequence can be used to obtain a file descrip‐ tor for the child of fork(2): pid = fork(); if (pid > 0) { /* If parent */ pidfd = pidfd_open(pid, 0); ... } Even if the child process has already terminated by the time of the pidfd_open() call, the returned file descriptor is guaranteed to refer to the child because the parent has not yet waited on the child (and therefore, the child's ID has not been recycled). Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/