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 13:10:48 -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

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.

Eric