Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 4 Dec 2018 10:17:36 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Chanho Min <chanho.min@lge.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Pavel Machek <pavel@ucw.cz>, Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux
 4.20-rc4)
Message-ID: <20181204091736.GD73770@gmail.com>
References: <CAHk-=wgdsXyCaLsFEpyUpAeRqVS69u=xo4rzEN+cS=xwz2gajg@mail.gmail.com>
 <20181203074700.GA21240@gmail.com>
 <20181203115601.GA31795@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20181203115601.GA31795@redhat.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


* Oleg Nesterov <oleg@redhat.com> wrote:

> > I reviewed the ->cred_guard_mutex code, and the mutex is held across all
> > of exec() - and we always did this.
> 
> Yes, and this was always wrong. For example, this test-case hangs:
> 
> 	#include <unistd.h>
> 	#include <signal.h>
> 	#include <sys/ptrace.h>
> 	#include <pthread.h>
> 
> 	void *thread(void *arg)
> 	{
> 		ptrace(PTRACE_TRACEME, 0,0,0);
> 		return NULL;
> 	}
> 
> 	int main(void)
> 	{
> 		int pid = fork();
> 
> 		if (!pid) {
> 			pthread_t pt;
> 			pthread_create(&pt, NULL, thread, NULL);
> 			pthread_join(pt, NULL);
> 			execlp("echo", "echo", "passed", NULL);
> 		}
> 
> 		sleep(1);
> 		// or anything else which needs ->cred_guard_mutex,
> 		// say open(/proc/$pid/mem)
> 		ptrace(PTRACE_ATTACH, pid, 0,0);
> 		kill(pid, SIGCONT);
> 
> 		return 0;
> 	}
> 
> we really need to narrow the (huge) scope of ->cred_guard_mutex in exec paths.
> 
> my attempt to fix this was nacked, and nobody suggested a better solution so far.

Any link to your patch and the NAK?

Thanks,

	Ingo