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

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Date:   Thu, 20 Feb 2020 23:03:09 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
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

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