linux-kernel - Re: [PATCH] pidfd: Stop taking cred_guard

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Message-ID: <20200310191502.qcqxfkhh5k2gxyzp@wittgenstein>
Date:   Tue, 10 Mar 2020 20:15:02 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Bernd Edlinger <bernd.edlinger@...mail.de>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Sargun Dhillon <sargun@...gun.me>
Subject: Re: [PATCH] pidfd: Stop taking cred_guard_mutex

On Tue, Mar 10, 2020 at 01:52:05PM -0500, Eric W. Biederman wrote:
> 
> During exec some file descriptors are closed and the files struct is
> unshared.  But all of that can happen at other times and it has the
> same protections during exec as at ordinary times.  So stop taking the
> cred_guard_mutex as it is useless.
> 
> Furthermore he cred_guard_mutex is a bad idea because it is deadlock
> prone, as it is held in serveral while waiting possibly indefinitely
> for userspace to do something.
> 
> Cc: Sargun Dhillon <sargun@...gun.me>
> Cc: Christian Brauner <christian.brauner@...ntu.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  kernel/pid.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> Christian if you don't have any objections I will take this one through
> my tree.

Sure.
Acked-by: Christian Brauner <christian.brauner@...ntu.com>

> 
> I tried to figure out why this code path takes the cred_guard_mutex and
> the archive on lore.kernel.org was not helpful in finding that part of
> the conversation.

Let me think a little harder and hopefully get back to you with a
sensible explanation.