linux-kernel - Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)

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Date:   Tue, 4 Dec 2018 10:17:36 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Chanho Min <chanho.min@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Pavel Machek <pavel@....cz>, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux
 4.20-rc4)


* Oleg Nesterov <oleg@...hat.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