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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
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        marcos.souza.org@gmail.com, hoeun.ryu@gmail.com,
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        rppt@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Tejun Heo <tj@kernel.org>
References: <20180503133338.GA23401@redhat.com> <87y3h0x0qg.fsf@xmission.com>
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        <20180509145151.GB25742@redhat.com>
Date:   Wed, 09 May 2018 22:00:00 -0500
In-Reply-To: <20180509145151.GB25742@redhat.com> (Oleg Nesterov's message of
        "Wed, 9 May 2018 16:51:52 +0200")
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Subject: Re: [PATCH 2/2] memcg: Close the race between migration and installing bprm->mm as mm
Sender: linux-kernel-owner@vger.kernel.org
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Oleg Nesterov <oleg@redhat.com> writes:

> On 05/04, Eric W. Biederman wrote:
>>
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1044,6 +1044,8 @@ static int exec_mmap(struct mm_struct *mm)
>>  		return 0;
>>  	}
>>  	mmdrop(active_mm);
>> +	/* The tsk may have migrated before the new mm was attached */
>> +	mm_sync_memcg_from_task(tsk);
>>  	return 0;
>>  }
>
> call_usermodehelper(). perhaps it makes sense to change flush_old_exec()
> to clear PF_KTHREAD/etc before exec_mmap().

Yes.  That does look like something to be fixed.  In practice while it
is rare for anything to be migrated during exec I expect it to be even
rarer for kernel threads.  But that is a legitimate issue.

I expect all of these lines after exec_mmap actually belong
in exec_mmap.  Which would then make rearraning the lines easy to ensure
mm_sync_memcg_from_task sees PF_KTHREAD cleared.
	/*
	 * After clearing bprm->mm (to mark that current is using the
	 * prepared mm now), we have nothing left of the original
	 * process. If anything from here on returns an error, the check
	 * in search_binary_handler() will SEGV current.
	 */
	bprm->mm = NULL;

	set_fs(USER_DS);
	current->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
					PF_NOFREEZE | PF_NO_SETAFFINITY);
	flush_thread();
	current->personality &= ~bprm->per_clear;

Eric