linux-kernel - Re: [PATCH 4/7] proc: Use d_invalidate in proc_prune_siblings

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Message-ID: <87blps7rrx.fsf@x220.int.ebiederm.org>
Date:   Thu, 20 Feb 2020 17:39:14 -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 4/7] proc: Use d_invalidate in proc_prune_siblings_dcache

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

> On Thu, Feb 20, 2020 at 10:54:20PM +0000, Al Viro wrote:
>> On Thu, Feb 20, 2020 at 02:49:53PM -0600, Eric W. Biederman wrote:
>> > 
>> > The function d_prune_aliases has the problem that it will only prune
>> > aliases thare are completely unused.  It will not remove aliases for
>> > the dcache or even think of removing mounts from the dcache.  For that
>> > behavior d_invalidate is needed.
>> > 
>> > To use d_invalidate replace d_prune_aliases with d_find_alias
>> > followed by d_invalidate and dput.  This is safe and complete
>> > because no inode in proc has any hardlinks or aliases.
>> 
>> s/no inode.*/it's a fucking directory inode./
>
> Wait... You are using it for sysctls as well?  Ho-hum...  The thing is,
> for sysctls you are likely to run into consequent entries with the
> same superblock, making for a big pile of useless playing with
> ->s_active...  And yes, that applied to mainline as well

Which is why I worked to merge the two cases since they were so close.
Fewer things to fix and more eyeballs on the code.

Eric