linux-kernel - Re: [PATCH 0/7] proc: Dentry flushing without proc

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Message-ID: <87mu9c7ruf.fsf@x220.int.ebiederm.org>
Date:   Thu, 20 Feb 2020 17:37:44 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 0/7] proc: Dentry flushing without proc_mnt

Al Viro <viro@...iv.linux.org.uk> writes:

> On Thu, Feb 20, 2020 at 03:02:22PM -0800, Linus Torvalds wrote:
>> On Thu, Feb 20, 2020 at 12:48 PM Eric W. Biederman
>> <ebiederm@...ssion.com> wrote:
>> >
>> > Linus, does this approach look like something you can stand?
>> 
>> A couple of worries, although one of them seem to have already been
>> resolved by Al.
>> 
>> I think the real gatekeeper should be Al in general.  But other than
>> the small comments I had, I think this might work just fine.
>> 
>> Al?
>
> I'll need to finish RTFS there; I have initially misread that patch,
> actually - Eric _is_ using that thing both for those directories
> and for sysctl inodes.  And the prototype for that machinery (the
> one he'd pulled from proc_sysctl.c) is playing with pinning superblocks
> way too much; for per-pid directories that's not an issue, but
> for sysctl table removal you are very likely to hit a bunch of
> evictees on the same superblock...

I saw that was possible.  If the broad strokes look correct I don't have
a problem at all with optimizing for the case where many of the entries
are for inodes on the same superblock.  I just had enough other details
on my mind I was afraid if I got a little more clever I would have
introduced a typo somewhere.


I wish I could limit the sysctl parts to just directories, but
unfortunately the sysctl tables don't always give a guarantee that a
directory is what will be removed.  But sysctls do have one name per
inode invarant like fat.  There is no way to express a sysctl
table that doesn't have that invariant.

As for d_find_alias/d_invalidate.

Just for completeness I wanted to write a loop:

	while (dentry = d_find_alias(inode)) {
        	d_invalidate(dentry);
                dput(dentry);
        }

Unfortunately that breaks on directories, because for directories
d_find_alias turns into d_find_any_alias, and continues to return aliases
even when they are unhashed.

It might be nice to write a cousin of d_prune_aliases call
it d_invalidate_aliases that just does that loop the correct way
in dcache.c

Eric