linux-kernel - Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard

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Date:   Mon, 09 Mar 2020 13:24:30 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Bernd Edlinger <bernd.edlinger@...mail.de>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        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\@vger.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm\@kvack.org" <linux-mm@...ck.org>,
        "stable\@vger.kernel.org" <stable@...r.kernel.org>,
        "linux-api\@vger.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

ebiederm@...ssion.com (Eric W. Biederman) writes:

> Bernd Edlinger <bernd.edlinger@...mail.de> writes:
>
>> On 3/9/20 6:40 PM, Eric W. Biederman wrote:
>>> Bernd Edlinger <bernd.edlinger@...mail.de> writes:
>>> 
>>>> On 3/8/20 10:38 PM, Eric W. Biederman wrote:
>>>>>
>>>>> The cred_guard_mutex is problematic.  The cred_guard_mutex is held
>>>>> over the userspace accesses as the arguments from userspace are read.
>>>>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>>>                                 ^ over
>>>>
>>>> ... is held while waiting for the trace parent to handle PTRACE_EVENT_EXIT
>>>> or something?
>>> 
>>> Yes.  Let me see if I can phrase that better.
>>> 
>>>> I wonder if we also should mention that
>>>> it is held while waiting for the trace parent to
>>>> receive the exit code with "wait"?
>>> 
>>> I don't think we have to spell out the details of how it all works,
>>> unless that makes things clearer.  Kernel developers can be expected
>>> to figure out how the kernel works.  The critical thing is that it is
>>> an indefinite wait for userspace to take action.
>>> 
>>> But I will look.
>>> 
>>>>> threads are killed.  The cred_guard_mutex is held over
>>>>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>>>>>
>>>>> Any of those can result in deadlock, as the cred_guard_mutex is held
>>>>> over a possible indefinite userspace waits for userspace.
>>>>>
>>>>> Add exec_update_mutex that is only held over exec updating process
>>>>
>>>> Add ?
>>> 
>>> Yes.  That is what the change does: add exec_update_mutex.
>>> 
>>
>> I just kind of missed the "subject" in this sentence,
>> like "This patch adds an exec_update_mutex that is ..."
>> but english is a foreign language for me, so may be okay as is.
>
> English has a lot of options.  I think this is a stylistic difference.
>
> Instead of being an observer and describing what the change does:
> "This patch adds exec_update_mutex ..."  
>
> I was being there in the moment and saying/commading what is happening:
> "Add exec_update_mutex ..."
>
> Using the more immdediate form ends up with more concise and clearer
> sentences.
>
> Every one of my writing teachers in school emphasized that point
> and I see the who it works when I write things.  But writing is hard and
> I still tend toward long rambling sentences with many qualifiers that
> confuse and detract from the point rather than make it clear what is
> happening.

And reading through it all now I can see your confusion.  That
description of my changes was not well done.  Reworking it now.

Eric