linux-kernel - Re: [PATCH v7 07/11] proc: flush task dcache entries from all procfs instances

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Date:   Sat, 25 Jan 2020 10:45:25 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alexey Gladkov <gladkov.alexey@...il.com>
Cc:     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>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        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>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        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>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH v7 07/11] proc: flush task dcache entries from all procfs instances

On Sat, Jan 25, 2020 at 5:06 AM Alexey Gladkov <gladkov.alexey@...il.com> wrote:
>
> This allows to flush dcache entries of a task on multiple procfs mounts
> per pid namespace.

>From a quick read-through, this is the only one I really react negatively to.

The locking looks odd. It only seems to protect the new proc_mounts
list, but then it's a whole big rwsem, and it's taken over all of
proc_flush_task_mnt(), and the locking is exported to all over as a
result of that - including the dummy functions for "there is no proc"
case.

And proc_flush_task_mnt() itself should need no locking over any of
it, so it's all just for the silly looping over the list.

So

 (a) this looks fishy and feels wrong - I get a very strong feeling
that the locking is wrong to begin with, and could/should have been
done differently

 (b) all the locking should have been internal to /proc, and those
wrappers shouldn't exist in a common header file (and certainly not
for the non-proc case).

Yes, (a) is just a feeling, and I don't have any great suggestions.
Maybe make it an RCU list and use a spinlock for updating it?

But (b) is pretty much a non-starter in this form. Those wrappers
shouldn't be in a globally exported core header file. No way.

               Linus