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Date: Wed, 27 May 2020 18:49:21 -0700 From: Kees Cook <keescook@...omium.org> To: Christian Brauner <christian.brauner@...ntu.com> Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>, Tycho Andersen <tycho@...ho.ws>, Matt Denton <mpdenton@...gle.com>, Sargun Dhillon <sargun@...gun.me>, Jann Horn <jannh@...gle.com>, Chris Palmer <palmer@...gle.com>, Aleksa Sarai <cyphar@...har.com>, Robert Sesek <rsesek@...gle.com>, Jeffrey Vander Stoep <jeffv@...gle.com>, Linux Containers <containers@...ts.linux-foundation.org> Subject: Re: [PATCH 1/2] seccomp: notify user trap about unused filter On Thu, May 28, 2020 at 12:45:01AM +0200, Christian Brauner wrote: > On Wed, May 27, 2020 at 03:37:58PM -0700, Kees Cook wrote: > > But there's a mapping between pidfd and task struct that is separate > > from task struct itself, yes? I.e. keeping a pidfd open doesn't pin > > struct task in memory forever, right? > > No, but that's an implementation detail and we discussed that. It pins > struct pid instead of task_struct. Once the process is fully gone you > just get ESRCH. Oh right! struct pid, yes. Okay, that's quite a bit smaller. > For example, fds to /proc/<pid>/<tid>/ fds aren't just closed once the > task has gone away, userspace will just get ESRCH when it tries to open > files under there but the fd remains valid until close() is called. > > In addition, of all the anon inode fds, none of them have the "close the > file behind userspace back" behavior: io_uring, signalfd, timerfd, btf, > perf_event, bpf-prog, bpf-link, bpf-map, pidfd, userfaultfd, fanotify, > inotify, eventpoll, fscontext, eventfd. These are just core kernel ones. > I'm pretty sure that it'd be very odd behavior if we did that. I'd > rather just notify userspace and leave the close to them. But maybe I'm > missing something. Well, they have a "you are now disconnected" state, which I was thinking could be done entirely entirely on the VFS side of things, but it looks like it's not. So, yes, okay, thank you for walking me through all that. I still want to take a closer look at all the notify calls in here. It seems strange that seccomp has to do all the wakeups (but I guess there are no "generic" poll helpers?) -- Kees Cook
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