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[87.253.79.154]) by smtp.gmail.com with ESMTPSA id v4sm1328444edy.54.2019.09.18.21.43.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Sep 2019 21:43:58 -0700 (PDT) Cc: mtk.manpages@gmail.com, linux-man@vger.kernel.org, jannh@google.com, oleg@redhat.com Subject: Re: [PATCH] clone.2: add CLONE_PIDFD entry To: Christian Brauner References: <20190511064908.21956-1-christian.brauner@ubuntu.com> <20190916074012.dpsfqfwcxh2pyyt7@wittgenstein> <7f115550-c7e6-c803-e47b-a37b7cdfb0a9@gmail.com> <20190918071415.gmxvovgiwgsi62tn@wittgenstein> <6a863c6a-3e61-f0b6-963e-a3545d9935d6@gmail.com> From: "Michael Kerrisk (man-pages)" Message-ID: <91e5e4fe-e09e-7f9b-1a53-0bc31c63ddce@gmail.com> Date: Thu, 19 Sep 2019 06:43:56 +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: <6a863c6a-3e61-f0b6-963e-a3545d9935d6@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-man-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Hello Christian, A tweak to one point from my mail of a few minutes ago... On 9/19/19 6:04 AM, Michael Kerrisk (man-pages) wrote: > A further question... We now have three ways of getting a > process file descriptor [*]: > > open() of /proc/PID > pidfd_open() > clone()/clone3() with CLONE_PIDFD > > I thought the FD was supposed to be equivalent in all three cases. > However, if I try (on kernel 5.3) poll() an FD returned by opening > /proc/PID, poll() tells me POLLNVAL for the FD. Is that difference > intentional? (I am guessing it is not.) Okay -- I made a misstep there, although my question remains. I got the POLLNVAL return because I had opened /proc/PID with the O_PATH flag. When I open /proc/PID with O_RDONLY, then poll() on the FD always returns immediately (i.e., before the target process has terminated), and the returned events are POLLIN+POLLOUT. Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/