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Date: Wed, 12 Feb 2020 19:47:28 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
LKML <linux-kernel@...r.kernel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Linux API <linux-api@...r.kernel.org>,
Linux FS Devel <linux-fsdevel@...r.kernel.org>,
Linux Security Module <linux-security-module@...r.kernel.org>,
Akinobu Mita <akinobu.mita@...il.com>,
Alexey Dobriyan <adobriyan@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>,
Daniel Micay <danielmicay@...il.com>,
Djalal Harouni <tixxdz@...il.com>,
"Dmitry V . Levin" <ldv@...linux.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Ingo Molnar <mingo@...nel.org>,
"J . Bruce Fields" <bfields@...ldses.org>,
Jeff Layton <jlayton@...chiereds.net>,
Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>,
Oleg Nesterov <oleg@...hat.com>,
Solar Designer <solar@...nwall.com>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs
instances
On Wed, Feb 12, 2020 at 10:45:06AM -0800, Linus Torvalds wrote:
> On Wed, Feb 12, 2020 at 7:01 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
> >
> > Fundamentally proc_flush_task is an optimization. Just getting rid of
> > dentries earlier. At least at one point it was an important
> > optimization because the old process dentries would just sit around
> > doing nothing for anyone.
>
> I'm pretty sure it's still important. It's very easy to generate a
> _ton_ of dentries with /proc.
>
> > I wonder if instead of invalidating specific dentries we could instead
> > fire wake up a shrinker and point it at one or more instances of proc.
>
> It shouldn't be the dentries themselves that are a freeing problem.
> They're being RCU-free'd anyway because of lookup. It's the
> proc_mounts list that is the problem, isn't it?
>
> So it's just fs_info that needs to be rcu-delayed because it contains
> that list. Or is there something else?
Large part of the headache is the possibility that some joker has
done something like mounting tmpfs on /proc/<pid>/map_files, or
binding /dev/null on top of /proc/<pid>/syscall, etc.
IOW, that d_invalidate() can very well have to grab namespace_sem.
And possibly do a full-blown fs shutdown of something NFS-mounted,
etc...
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