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[209.85.220.65]) by mx.google.com with SMTPS id e18sor2240555vso.3.2019.03.20.12.29.43 for (Google Transport Security); Wed, 20 Mar 2019 12:29:43 -0700 (PDT) Received-SPF: pass (google.com: domain of dancol@google.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=lBL4jIOk; spf=pass (google.com: domain of dancol@google.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=dancol@google.com; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=HKDZVJLbci8RoVuRdoZreNJclj5oDrpcOZ3lLcIS9UU=; b=lBL4jIOkrEs3fwB/HbteEV20fxt6zSCyK8Sw2O3+wzERFwf15cqJpGY4Mi+5iej0Zu NHg9SskzswgeLim9HXHjEJRCl7ST8eFEaKKWzTAcVLRKg6qhbKjyLUZ6f132xHqmVM7p lTn1rzRQSjncaR+5FYB4A/WFEyeNPXjy72yMjkUKu2DpQ7nDsvTKpwzjWUIw2QNsbDMG s4BJdLnAon8HFvWqatmSY5sU9fwvUeOfiIYzg607jntJCJdbEHlON/C9M8W+e1sN76uX VZSwlf5WP1F1p4ZrbfiSGZ3xcO4E5T52xKHih4mlp6ybIuhV71X5Rc0duGsQF+K236LA yszQ== X-Google-Smtp-Source: APXvYqzZmYth0geZRwK8/cPaxo7MhfE1YtjjEraAL/oYZgwuyW0xlor+n6xIT1zPkSRABVAjWBYF3uVB+AKjBg5wf5k= X-Received: by 2002:a67:cc2:: with SMTP id 185mr6019174vsm.77.1553110182296; Wed, 20 Mar 2019 12:29:42 -0700 (PDT) MIME-Version: 1.0 References: <20190319221415.baov7x6zoz7hvsno@brauner.io> <20190319231020.tdcttojlbmx57gke@brauner.io> <20190320015249.GC129907@google.com> <20190320035953.mnhax3vd47ya4zzm@brauner.io> <4A06C5BB-9171-4E70-BE31-9574B4083A9F@joelfernandes.org> <20190320182649.spryp5uaeiaxijum@brauner.io> <20190320185156.7bq775vvtsxqlzfn@brauner.io> In-Reply-To: <20190320185156.7bq775vvtsxqlzfn@brauner.io> From: Daniel Colascione Date: Wed, 20 Mar 2019 12:29:31 -0700 Message-ID: Subject: Re: pidfd design To: Christian Brauner Cc: Joel Fernandes , Suren Baghdasaryan , Steven Rostedt , Sultan Alsawaf , Tim Murray , Michal Hocko , Greg Kroah-Hartman , =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= , Todd Kjos , Martijn Coenen , Ingo Molnar , Peter Zijlstra , LKML , "open list:ANDROID DRIVERS" , linux-mm , kernel-team , Oleg Nesterov , Andy Lutomirski , "Serge E. Hallyn" , Kees Cook Content-Type: text/plain; charset="UTF-8" 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 Wed, Mar 20, 2019 at 11:52 AM Christian Brauner wrote: > > On Wed, Mar 20, 2019 at 11:38:35AM -0700, Daniel Colascione wrote: > > On Wed, Mar 20, 2019 at 11:26 AM Christian Brauner wrote: > > > On Wed, Mar 20, 2019 at 07:33:51AM -0400, Joel Fernandes wrote: > > > > > > > > > > > > On March 20, 2019 3:02:32 AM EDT, Daniel Colascione wrote: > > > > >On Tue, Mar 19, 2019 at 8:59 PM Christian Brauner > > > > > wrote: > > > > >> > > > > >> On Tue, Mar 19, 2019 at 07:42:52PM -0700, Daniel Colascione wrote: > > > > >> > On Tue, Mar 19, 2019 at 6:52 PM Joel Fernandes > > > > > wrote: > > > > >> > > > > > > >> > > On Wed, Mar 20, 2019 at 12:10:23AM +0100, Christian Brauner > > > > >wrote: > > > > >> > > > On Tue, Mar 19, 2019 at 03:48:32PM -0700, Daniel Colascione > > > > >wrote: > > > > >> > > > > On Tue, Mar 19, 2019 at 3:14 PM Christian Brauner > > > > > wrote: > > > > >> > > > > > So I dislike the idea of allocating new inodes from the > > > > >procfs super > > > > >> > > > > > block. I would like to avoid pinning the whole pidfd > > > > >concept exclusively > > > > >> > > > > > to proc. The idea is that the pidfd API will be useable > > > > >through procfs > > > > >> > > > > > via open("/proc/") because that is what users expect > > > > >and really > > > > >> > > > > > wanted to have for a long time. So it makes sense to have > > > > >this working. > > > > >> > > > > > But it should really be useable without it. That's why > > > > >translate_pid() > > > > >> > > > > > and pidfd_clone() are on the table. What I'm saying is, > > > > >once the pidfd > > > > >> > > > > > api is "complete" you should be able to set CONFIG_PROCFS=N > > > > >- even > > > > >> > > > > > though that's crazy - and still be able to use pidfds. This > > > > >is also a > > > > >> > > > > > point akpm asked about when I did the pidfd_send_signal > > > > >work. > > > > >> > > > > > > > > >> > > > > I agree that you shouldn't need CONFIG_PROCFS=Y to use > > > > >pidfds. One > > > > >> > > > > crazy idea that I was discussing with Joel the other day is > > > > >to just > > > > >> > > > > make CONFIG_PROCFS=Y mandatory and provide a new > > > > >get_procfs_root() > > > > >> > > > > system call that returned, out of thin air and independent of > > > > >the > > > > >> > > > > mount table, a procfs root directory file descriptor for the > > > > >caller's > > > > >> > > > > PID namspace and suitable for use with openat(2). > > > > >> > > > > > > > >> > > > Even if this works I'm pretty sure that Al and a lot of others > > > > >will not > > > > >> > > > be happy about this. A syscall to get an fd to /proc? > > > > >> > > > > > >> > Why not? procfs provides access to a lot of core kernel > > > > >functionality. > > > > >> > Why should you need a mountpoint to get to it? > > > > >> > > > > > >> > > That's not going > > > > >> > > > to happen and I don't see the need for a separate syscall just > > > > >for that. > > > > >> > > > > > >> > We need a system call for the same reason we need a getrandom(2): > > > > >you > > > > >> > have to bootstrap somehow when you're in a minimal environment. > > > > >> > > > > > >> > > > (I do see the point of making CONFIG_PROCFS=y the default btw.) > > > > >> > > > > > >> > I'm not proposing that we make CONFIG_PROCFS=y the default. I'm > > > > >> > proposing that we *hardwire* it as the default and just declare > > > > >that > > > > >> > it's not possible to build a Linux kernel that doesn't include > > > > >procfs. > > > > >> > Why do we even have that button? > > > > >> > > > > > >> > > I think his point here was that he wanted a handle to procfs no > > > > >matter where > > > > >> > > it was mounted and then can later use openat on that. Agreed that > > > > >it may be > > > > >> > > unnecessary unless there is a usecase for it, and especially if > > > > >the /proc > > > > >> > > directory being the defacto mountpoint for procfs is a universal > > > > >convention. > > > > >> > > > > > >> > If it's a universal convention and, in practice, everyone needs > > > > >proc > > > > >> > mounted anyway, so what's the harm in hardwiring CONFIG_PROCFS=y? > > > > >If > > > > >> > we advertise /proc as not merely some kind of optional debug > > > > >interface > > > > >> > but *the* way certain kernel features are exposed --- and there's > > > > >> > nothing wrong with that --- then we should give programs access to > > > > >> > these core kernel features in a way that doesn't depend on > > > > >userspace > > > > >> > kernel configuration, and you do that by either providing a > > > > >> > procfs-root-getting system call or just hardwiring the "/proc/" > > > > >prefix > > > > >> > into VFS. > > > > >> > > > > > >> > > > Inode allocation from the procfs mount for the file descriptors > > > > >Joel > > > > >> > > > wants is not correct. Their not really procfs file descriptors > > > > >so this > > > > >> > > > is a nack. We can't just hook into proc that way. > > > > >> > > > > > > >> > > I was not particular about using procfs mount for the FDs but > > > > >that's the only > > > > >> > > way I knew how to do it until you pointed out anon_inode (my grep > > > > >skills > > > > >> > > missed that), so thank you! > > > > >> > > > > > > >> > > > > C'mon: /proc is used by everyone today and almost every > > > > >program breaks > > > > >> > > > > if it's not around. The string "/proc" is already de facto > > > > >kernel ABI. > > > > >> > > > > Let's just drop the pretense of /proc being optional and bake > > > > >it into > > > > >> > > > > the kernel proper, then give programs a way to get to /proc > > > > >that isn't > > > > >> > > > > tied to any particular mount configuration. This way, we > > > > >don't need a > > > > >> > > > > translate_pid(), since callers can just use procfs to do the > > > > >same > > > > >> > > > > thing. (That is, if I understand correctly what translate_pid > > > > >does.) > > > > >> > > > > > > > >> > > > I'm not sure what you think translate_pid() is doing since > > > > >you're not > > > > >> > > > saying what you think it does. > > > > >> > > > Examples from the old patchset: > > > > >> > > > translate_pid(pid, ns, -1) - get pid in our pid namespace > > > > >> > > > > > >> > Ah, it's a bit different from what I had in mind. It's fair to want > > > > >to > > > > >> > translate PIDs between namespaces, but the only way to make the > > > > >> > translate_pid under discussion robust is to have it accept and > > > > >produce > > > > >> > pidfds. (At that point, you might as well call it translate_pidfd.) > > > > >We > > > > >> > should not be adding new APIs to the kernel that accept numeric > > > > >PIDs: > > > > >> > > > > >> The traditional pid-based api is not going away. There are users that > > > > >> have the requirement to translate pids between namespaces and also > > > > >doing > > > > >> introspection on these namespaces independent of pidfds. We will not > > > > >> restrict the usefulness of this syscall by making it only work with > > > > >> pidfds. > > > > >> > > > > >> > it's not possible to use these APIs correctly except under very > > > > >> > limited circumstances --- mostly, talking about init or a parent > > > > >> > > > > >> The pid-based api is one of the most widely used apis of the kernel > > > > >and > > > > >> people have been using it quite successfully for a long time. Yes, > > > > >it's > > > > >> rac, but it's here to stay. > > > > >> > > > > >> > talking about its child. > > > > >> > > > > > >> > Really, we need a few related operations, and we shouldn't > > > > >necessarily > > > > >> > mingle them. > > > > >> > > > > >> Yes, we've established that previously. > > > > >> > > > > >> > > > > > >> > 1) Given a numeric PID, give me a pidfd: that works today: you just > > > > >> > open /proc/ > > > > >> > > > > >> Agreed. > > > > >> > > > > >> > > > > > >> > 2) Given a pidfd, give me a numeric PID: that works today: you just > > > > >> > openat(pidfd, "stat", O_RDONLY) and read the first token (which is > > > > >> > always the numeric PID). > > > > >> > > > > >> Agreed. > > > > >> > > > > >> > > > > > >> > 3) Given a pidfd, send a signal: that's what pidfd_send_signal > > > > >does, > > > > >> > and it's a good start on the rest of these operations. > > > > >> > > > > >> Agreed. > > > > >> > > > > >> > 5) Given a pidfd in NS1, get a pidfd in NS2. That's what > > > > >translate_pid > > > > >> > is for. My preferred signature for this routine is > > > > >translate_pid(int > > > > >> > pidfd, int nsfd) -> pidfd. We don't need two namespace arguments. > > > > >Why > > > > >> > not? Because the pidfd *already* names a single process, uniquely! > > > > >> > > > > >> Given that people are interested in pids we can't just always return > > > > >a > > > > >> pidfd. That would mean a user would need to do get the pidfd read > > > > >from > > > > >> /stat and then close the pidfd. If you do that for a 100 pids > > > > >or > > > > >> more you end up allocating and closing file descriptors constantly > > > > >for > > > > >> no reason. We can't just debate pids away. So it will also need to be > > > > >> able to yield pids e.g. through a flag argument. > > > > > > > > > >Sure, but that's still not a reason that we should care about pidfds > > > > >working separately from procfs.. > > > > > > That's unrelated to the point made in the above paragraph. > > > Please note, I said that the pidfd api should work when proc is not > > > available not that they can't be dirfds. > > > > What do you mean by "not available"? CONFIG_PROCFS=n? If pidfds > > I'm talking about the ability to clone processes and get fd handles on > them via pidfd_clone() or CLONE_NEWFD. I wouldn't call that situation "proc [not being] available". We need pidfd_clone to return a pidfd for atomicity reasons, not /proc availability reasons. Again, it doesn't make any sense to support this stuff when CONFIG_PROCFS=n, and CONFIG_PROCFS=n shouldn't even be a supported configuration. > > > translate_pid() should just return you a pidfd. Having it return a pidfd > > > and a status fd feels like stuffing too much functionality in there. If > > > you're fine with it I'll finish prototyping what I had in mind. As I > > > said in previous mails I'm already working on this. > > > > translate_pid also needs to *accept* pidfds, at least optionally. > > Unless you have a function from pidfd to pidfd, you race. > > You're misunderstanding. Again, I said in my previous mails it should > accept pidfds optionally as arguments, yes. But I don't want it to > return the status fds that you previously wanted pidfd_wait() to return. Agreed. There should be a different way to get these wait handle FDs. > I really want to see Joel's pidfd_wait() patchset and have more people > review the actual code. Sure. But it's also unpleasant to have people write code and then have to throw it away due to guessing incorrectly about unclear requirements.