linux-kernel - Re: [PATCH v2 4/5] exec: Move exec_mmap right after de_thread in flush_old

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Message-ID: <87tv2xz510.fsf@x220.int.ebiederm.org>
Date:   Mon, 09 Mar 2020 14:45:15 -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 4/5] exec: Move exec_mmap right after de_thread in flush_old_exec

Bernd Edlinger <bernd.edlinger@...mail.de> writes:

> On 3/8/20 10:38 PM, Eric W. Biederman wrote:
>> 
>> This consolidation allows the creation of a mutex to replace
>> cred_guard_mutex that is not held of possible indefinite userspace
>
> can you also reword this "held of" thing here as well?

Done:

    exec: Move exec_mmap right after de_thread in flush_old_exec
    
    I have read through the code in exec_mmap and I do not see anything
    that depends on sighand or the sighand lock, or on signals in anyway
    so this should be safe.
    
    This rearrangement of code has two siginficant benefits.  It makes
    the determination of passing the point of no return by testing bprm->mm
    accurate.  All failures prior to that point in flush_old_exec are
    either truly recoverable or they are fatal.
    
    Futher this consolidates all of the possible indefinite waits for
    userspace together at the top of flush_old_exec.  The possible wait
    for a ptracer on PTRACE_EVENT_EXIT, the possible wait for a page fault
    to be resolved in clear_child_tid, and the possible wait for a page
    fault in exit_robust_list.
    
    This consolidation allows the creation of a mutex to replace
    cred_guard_mutex that is not held over possible indefinite userspace
    waits.  Which will allow removing deadlock scenarios from the kernel.
    
    Reviewed-by: Bernd Edlinger <bernd.edlinger@...mail.de>
    Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Eric