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Howlett" , Suren Baghdasaryan , Vlastimil Babka , pedro.falcato@gmail.com, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 0/3] introduce PIDFD_SELF Message-ID: <1d19f18c-5a60-44b5-a96f-9d0e74f2b02c@lucifer.local> References: <87ttdxl9ch.fsf@oldenburg.str.redhat.com> <42df57ac-d89c-4111-a04d-290dd2197573@lucifer.local> <20240930-verbiegen-zinspolitik-cafb730c3c84@brauner> <20240930.141721-salted.birth.growing.forges-5Z29YNO700C@cyphar.com> <8b1b376b-3c4f-409a-b8e4-8faf3efecdc8@lucifer.local> <20241001-stapfen-darfst-5fe2a8b2c6ec@brauner> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241001-stapfen-darfst-5fe2a8b2c6ec@brauner> X-ClientProxiedBy: LO4P123CA0210.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:1a5::17) To SJ0PR10MB5613.namprd10.prod.outlook.com (2603:10b6:a03:3d0::5) Precedence: bulk X-Mailing-List: linux-api@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ0PR10MB5613:EE_|DM4PR10MB7526:EE_ X-MS-Office365-Filtering-Correlation-Id: b06d8358-d5c7-49fd-a911-08dce225cce0 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; 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In principle I guess users might use PIDFD_SELF by > > > accident but if we mirror the naming with /proc/{,thread-}self that > > > might not be that big of an issue? > > > > Lol, you know I wasn't even aware /proc/thread-self existed... > > Wait until you learn that /proc/$TID thread entries exist but aren't > shown when you do ls -al /proc, only when you explicitly access them. My God... You're right, that's crazy... :) > > > > > Yeah, that actually makes sense and is consistent, though obviously the > > concern is people will reflexively use PIDFD_SELF and end up with > > potentially confusing results. > > > > I will obviously be doing a manpage patch for this so we can spell it out > > there very clearly and also in the header - so I'd actually lean towards > > doing this myself. > > > > Christian, Florian? Thoughts? > > I think adding both would be potentially useful. How about: > > #define PIDFD_SELF -100 > #define PIDFD_THREAD_GROUP -200 Sure, makes sense to add both. > > This will make PIDFD_SELF mean current and PIDFD_THREAD_GROUP mean > current->pid_links[PIDTYPE_TGID]. I don't think we need to or should > mirror procfs in any way. pidfds are intended to be usable without > procfs at all. Yeah, I think it's important to ensure the _default_ choice, so in this case PIDFD_SELF clearly, is one that will be least surprising. The proc thing is sort of pleasing from an aesthetic point of view, but if you followed it you'd have to _clearly_ document PIDFD_THREAD_SELF as the default. Happy to go along with this. PIDFD_THREAD_GROUP is also clearer as it is distinct from PIDFD_SELF (doesn't reference 'self' at all). > > I want to leave one comment on a bit of quirkiness in the api when we > add this. I don't consider it a big deal but it should be pointed out. > > It can be useful to allow PIDFD_SELF or PIDFD_THREAD_GROUP to refer to > the calling thread even for pidfd_open() to avoid an additional getpid() > system call: > > (1) pidfd_open(PIDFD_SELF, PIDFD_THREAD) > (2) pidfd_open(PIDFD_SELF, 0) > Hm, this is a bit weird, as these are pid_t's and PIDFD_SELF and PIDFD_THREAD_GROUP are otherwise (pid)fd's. Being dummy values sort of allows us to put them into service here also, but it is just weird, we pass what is usually a pidfd to receive a pidfd, only this time it's an actually concrete one? I'm not sure I like this, even though as you say it avoids a getpid(). If we did this I'd prefer it to be a separate name, even if it has the same numeric value (I guess we also might want to check for anything that uses a negative pid_t to refer to an error or something else too). Perhaps PID_SELF and PID_THREAD_GROUP? > So if we allow this (Should we allow it?) then (1) is just redundant but > whathever. But (2) is at least worth discussing: Should we reject (2) on > the grounds of contradictory requests or allow it and document that it's > equivalent to pidfd_open(getpid(), PIDFD_THREAD)? It feels like the > latter would be ok. > > Similar for pidfd_send_signal(): > > // redundant but ok: > (1) pidfd_send_signal(PIDFD_SELF, PIDFD_SIGNAL_THREAD) > > // redundant but ok: > (2) pidfd_send_signal(PIDFD_THREAD_GROUP, PIDFD_SIGNAL_THREAD_GROUP) > > // weird way to spell pidfd_send_signal(PIDFD_THREAD_GROUP, 0) > (3) pidfd_send_signal(PIDFD_SELF, PIDFD_SIGNAL_THREAD_GROUP) > > // weird way to spell pidfd_send_signal(PIDFD_SELF, 0) > (4) pidfd_send_signal(PIDFD_THREAD_GROUP, PIDFD_SIGNAL_THREAD) > > I think all of this is ok but does anyone else have a strong opinion? I think it's fine to allow all 4 and we should get this behaviour by default (if we have no flags we use the f_flags as a hint, which will be set correctly). I think people might find contradictory ones, i.e. 3 and 4, strange, but it makes sense for the flags to override the pidfd (as they would for a non-sentinel pidfd) and it makes everything consistent vs. if you were not using a sentinel value. So yes I think that's fine.