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Date:   Tue, 3 Mar 2020 17:50:34 +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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        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        linux-api@vger.kernel.org
Subject: Re: [PATCHv5] exec: Fix a deadlock in ptrace
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On Tue, Mar 03, 2020 at 09:18:44AM -0600, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
> 
> > This fixes a deadlock in the tracer when tracing a multi-threaded
> > application that calls execve while more than one thread are running.
> >
> > I observed that when running strace on the gcc test suite, it always
> > blocks after a while, when expect calls execve, because other threads
> > have to be terminated.  They send ptrace events, but the strace is no
> > longer able to respond, since it is blocked in vm_access.
> >
> > The deadlock is always happening when strace needs to access the
> > tracees process mmap, while another thread in the tracee starts to
> > execve a child process, but that cannot continue until the
> > PTRACE_EVENT_EXIT is handled and the WIFEXITED event is received:
> 
> A couple of things.
> 
> Why do we think it is safe to change the behavior exposed to userspace?
> Not the deadlock but all of the times the current code would not
> deadlock?
> 
> Especially given that this is a small window it might be hard for people
> to track down and report so we need a strong argument that this won't
> break existing userspace before we just change things.
> 
> Usually surveying all of the users of a system call that we can find
> and checking to see if they might be affected by the change in behavior
> is difficult enough that we usually opt for not being lazy and
> preserving the behavior.
> 
> This patch is up to two changes in behavior now, that could potentially
> affect a whole array of programs.  Adding linux-api so that this change
> in behavior can be documented if/when this change goes through.
> 
> If you can split the documentation and test fixes out into separate
> patches that would help reviewing this code, or please make it explicit
> that the your are changing documentation about behavior that is changing
> with this patch.

Agreed. I think it'd be good to do it in three patches:
1. unrelated documentation update
2. fix + documentation changes specific to the fix
3. test(s)

Christian