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Date:   Tue, 10 Mar 2020 20:15:02 +0100
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        David Howells <dhowells@redhat.com>,
        James Morris <jamorris@linux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Christian Kellner <christian@kellner.me>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
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        Arnd Bergmann <arnd@arndb.de>,
        Sargun Dhillon <sargun@sargun.me>
Subject: Re: [PATCH] pidfd: Stop taking cred_guard_mutex
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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@sargun.me>
> Cc: Christian Brauner <christian.brauner@ubuntu.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.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@ubuntu.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.