linux-kernel - Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

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Message-ID: <CAHk-=wjePLiQqUfQGCrNb0wp+EtgRddQbcK-pHH=6rxbdYNNOA@mail.gmail.com>
Date:   Thu, 13 Feb 2020 14:47:48 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
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 Thu, Feb 13, 2020 at 2:23 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> I'd been thinking of ->d_fsdata pointing to a structure with list_head
> and a (non-counting) task_struct pointer for those guys.  Allocated
> on lookup, of course (as well as readdir ;-/) and put on the list
> at the same time.

Hmm. That smells like potentially a lot of small allocations, and
making readdir() even nastier.

Do we really want to create the dentries at readdir time? We do now
(with proc_fill_cache()) but do we actually _need_ to?

I guess a lot of readdir users end up doing a stat on it immediately
afterwards. I think right now we do it to get the inode number, and
maybe that is a basic requirement (even if I don't think it's really
stable - an inode could be evicted and then the ino changes, no?)

Ho humm. This all doesn't make me happy. But I guess the proof is in
the pudding - and if you come up with a good patch, I won't complain.

              Linus