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Date: Mon, 1 Apr 2019 14:30:19 -0700 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Christian Brauner <christian@...uner.io> Cc: Jann Horn <jannh@...gle.com>, Daniel Colascione <dancol@...gle.com>, Aleksa Sarai <cyphar@...har.com>, Andy Lutomirski <luto@...capital.net>, Andrew Lutomirski <luto@...nel.org>, David Howells <dhowells@...hat.com>, "Serge E. Hallyn" <serge@...lyn.com>, Linux API <linux-api@...r.kernel.org>, Linux List Kernel Mailing <linux-kernel@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>, "Eric W. Biederman" <ebiederm@...ssion.com>, Konstantin Khlebnikov <khlebnikov@...dex-team.ru>, Kees Cook <keescook@...omium.org>, Alexey Dobriyan <adobriyan@...il.com>, Thomas Gleixner <tglx@...utronix.de>, Michael Kerrisk-manpages <mtk.manpages@...il.com>, Jonathan Kowalski <bl0pbl33p@...il.com>, "Dmitry V. Levin" <ldv@...linux.org>, Andrew Morton <akpm@...ux-foundation.org>, Oleg Nesterov <oleg@...hat.com>, Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>, Al Viro <viro@...iv.linux.org.uk>, Joel Fernandes <joel@...lfernandes.org> Subject: Re: [PATCH v2 0/5] pid: add pidfd_open() On Mon, Apr 1, 2019 at 12:42 PM Christian Brauner <christian@...uner.io> wrote: > > From what I gather from this thread we are still best of with using fds > to /proc/<pid> as pidfds. Linus, do you agree or have I misunderstood? That does seem to be the most flexible option. > Yes, we can have an internal mount option to restrict access to various > parts of procfs from such pidfds I suspect you'd find that other parties might want such a "restrict proc" mount option too, so I don't think it needs to be anything internal. But it would be pretty much independent of the pidfd issue, of course. > One thing is that we also need something to disable access to the > "/proc/<pid>/net". One option could be to give the files in "net/" an > ->open-handler which checks that our file->f_path.mnt is not one of our > special clone() mounts and if it is refuse the open. I would expect that that would be part of the "restrict proc" mount options, no? > Basically, if you have a system without CONFIG_PROC_FS it makes sense > that clone gives back an anon inode file descriptor as pidfds because > you can still signal threads in a race-free way. But it doesn't make a > lot of sense to have pidfd_open() in this scenario because you can't > really do anything with that pidfd apart from sending signals. Well, people might want that. But realistically, everybody enables /proc support anyway. Even if you don't actually fully *mount* it in some restricted area, the support is pretty much always there in any kernel config. But yes, in general I agree that that also most likely means that a separate system call for "open_pidfd()" isn't worth it. Because if the main objection to /proc is that it exposes too much, then I think a much better option is to see how to sanely restrict the "too much" parts. Because I think there might be a lot of people who want a restricted /proc, in various container models etc. Linus
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