From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Bergmann Subject: Re: [PATCH v4 2/5] pid: Add PIDFD_IOCTL_GETFD to fetch file descriptors from processes Date: Sat, 21 Dec 2019 13:53:32 +0000 Message-ID: References: <20191218235459.GA17271@ircssh-2.c.rugged-nimbus-611.internal> <20191219103525.yqb5f4pbd2dvztkb@wittgenstein> <20191220043510.r5h6wvsp2p5glyjv@yavin.dot.cyphar.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <20191220043510.r5h6wvsp2p5glyjv@yavin.dot.cyphar.com> Sender: linux-kernel-owner@vger.kernel.org To: Aleksa Sarai Cc: Sargun Dhillon , Christian Brauner , Oleg Nesterov , Florian Weimer , "linux-kernel@vger.kernel.org" , Linux Containers , Linux API , Linux FS-devel Mailing List , Tycho Andersen , Jann Horn , Andy Lutomirski , Al Viro , Gian-Carlo Pascutto , =?UTF-8?Q?Emilio_Cobos_=C3=81lvarez?= , Jed Davis List-Id: linux-api@vger.kernel.org On Fri, Dec 20, 2019 at 4:35 AM Aleksa Sarai wrote: > > On 2019-12-19, Sargun Dhillon wrote: > > On Thu, Dec 19, 2019 at 2:35 AM Christian Brauner > > wrote: > > > I guess this is the remaining question we should settle, i.e. what do we > > > prefer. > > > I still think that adding a new syscall for this seems a bit rich. On > > > the other hand it seems that a lot more people agree that using a > > > dedicated syscall instead of an ioctl is the correct way; especially > > > when it touches core kernel functionality. I mean that was one of the > > > takeaways from the pidfd API ioctl-vs-syscall discussion. > > > > > > A syscall is nicer especially for core-kernel code like this. > > > So I guess the only way to find out is to try the syscall approach and > > > either get yelled and switch to an ioctl() or have it accepted. > > > > > > What does everyone else think? Arnd, still in favor of a syscall I take > > > it. Oleg, you had suggested a syscall too, right? Florian, any > > > thoughts/worries on/about this from the glibc side? > > > > > > Christian > > > > My feelings towards this are that syscalls might pose a problem if we > > ever want to extend this API. Of course we can have a reserved > > "flags" field, and populate it later, but what if we turn out to need > > a proper struct? I already know we're going to want to add one > > around cgroup metadata (net_cls), and likely we'll want to add > > a "steal" flag as well. As Arnd mentioned earlier, this is trivial to > > fix in a traditional ioctl environment, as ioctls are "cheap". How > > do we feel about potentially adding a pidfd_getfd2? Or are we > > confident that reserved flags will save us? > > If we end up making this a syscall, then we can re-use the > copy_struct_from_user() API to make it both extensible and compatible in > both directions. I wasn't aware that this was frowned upon for ioctls > (sorry for the extra work) but there are several syscalls which use this > model for extendability (clone3, openat2, sched_setattr, > perf_events_open) so there shouldn't be any such complaints for a > syscall which is extensible. I would still not do it for syscalls, although for other reasons: - in an ioctl, it's better to come up with a new command code if you have a larger structure - in a system call, it's best to pass all arguments as individual registers, the only time we use indirect data structures is when there are more than six arguments. Arnd