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Hallyn" , Kees Cook Subject: Re: pidfd design Message-ID: <20190320182649.spryp5uaeiaxijum@brauner.io> References: <20190318002949.mqknisgt7cmjmt7n@brauner.io> <20190318235052.GA65315@google.com> <20190319221415.baov7x6zoz7hvsno@brauner.io> <20190319231020.tdcttojlbmx57gke@brauner.io> <20190320015249.GC129907@google.com> <20190320035953.mnhax3vd47ya4zzm@brauner.io> <4A06C5BB-9171-4E70-BE31-9574B4083A9F@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <4A06C5BB-9171-4E70-BE31-9574B4083A9F@joelfernandes.org> User-Agent: NeoMutt/20180716 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 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. > > Agreed. I can't imagine pidfd being anything but a proc pid directory handle. So I am confused what Christian meant. Pidfd *is* a procfs directory fid always. That's what I gathered from his pidfd_send_signal patch but let me know if I'm way off in the woods. (K9 Mail still hasn't learned to wrap lines at 80 it seems. :)) Again, I never said that pidfds should be a directory handle. (Though I would like to point out that one of the original ideas I discussed at LPC was to have something like this to get regular file descriptors instead of dirfds: https://gist.github.com/brauner/59eec91550c5624c9999eaebd95a70df) > > For my next revision, I am thinking of adding the flag argument Christian mentioned to make translate_pid return an anon_inode FD which can be used for death status, given a . Since it is thought that translate_pid can be made to return a pid FD, I think it is ok to have it return a pid status FD for the purposes of the death status as well. 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. Would you be ok with prototyping the pidfd_wait() syscall you had in mind? Especially the wait_fd part that you want to have I would like to see how that is supposed to work, e.g. who is allowed to wait on the process and how notifications will work for non-parent processes and so on. I feel we won't get anywhere by talking in the abstrace and other people are far more likely to review/comment once there's actual code. Christian