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Re: [PATCH v2] unshare: Unsharing a thread does not require unsharing a vm

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Oleg Nesterov <oleg@redhat.com> writes:

> On 08/12, Eric W. Biederman wrote:
>>
>> +	if (unshare_flags & (CLONE_SIGHAND | CLONE_VM)) {
>> +		if (atomic_read(&current->sighand->count) > 1)
>> +			return -EINVAL;
>> +	}
>
> I am still not sure we want this... please the the previous email.

Reading your other email I did not see why you thought this check was
unnecessary.

> But perhaps I missed something.

In short:
clone(VM) --> mm_users > 1 && sighand_struct->count == 1
followed by:
unshare(SIGHAND)
the unshare should succeed.

Meanwhile:
clone(VM|SIGHAND) --> mm_users > 1 && sighand_struct->count > 1
followed by:
unshare(SIGHAND)
the unshare should fail.

I actually tested both of these cases and my patch works properly.

Not that I expect that there is anyone actually calling unshare(SIGHAND)
but unless we figure out how to remove the code, the code should
function correctly.  If for no other reason than to not confuse people
reading and maintaining the code.

Further I have audited the callers and we don't have anyone playing
games with sighand->count.  There is an implementation of unsharing the
sighand_struct in dethread in fs/exec.c that relies on this.

Other possible tests such as current_is_single_threaded() and
thread_group_empty() fail at the wrong times to be used.

So I think it is clear that testing for a private sighand_struct is
necessaring and testing sighand->count is a perfectly fine test.

Eric

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Re: [PATCH] user_ns: use correct check for single-threadedness Oleg Nesterov <oleg@redhat.com> - 2015-08-05 19:30 +0200
  Re: [PATCH] user_ns: use correct check for single-threadedness ebiederm@xmission.com (Eric W. Biederman) - 2015-08-05 20:10 +0200
    Re: [PATCH] user_ns: use correct check for single-threadedness ebiederm@xmission.com (Eric W. Biederman) - 2015-08-05 21:00 +0200
      Re: [PATCH] user_ns: use correct check for single-threadedness Oleg Nesterov <oleg@redhat.com> - 2015-08-06 16:10 +0200
        [PATCH 0/2] userns: Creation logic fixes ebiederm@xmission.com (Eric W. Biederman) - 2015-08-12 03:30 +0200
          [PATCH 2/2] userns,pidns: Force thread group sharing, not signal handler sharing ebiederm@xmission.com (Eric W. Biederman) - 2015-08-12 03:40 +0200
            Re: [PATCH 2/2] userns,pidns: Force thread group sharing, not  signal handler sharing Oleg Nesterov <oleg@redhat.com> - 2015-08-12 19:30 +0200
          [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm ebiederm@xmission.com (Eric W. Biederman) - 2015-08-12 03:40 +0200
            Re: [PATCH 1/2] unshare: Unsharing a thread does not require  unsharing a vm Oleg Nesterov <oleg@redhat.com> - 2015-08-12 20:00 +0200
              Re: [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm ebiederm@xmission.com (Eric W. Biederman) - 2015-08-12 20:50 +0200
                Re: [PATCH 1/2] unshare: Unsharing a thread does not require  unsharing a vm Oleg Nesterov <oleg@redhat.com> - 2015-08-13 15:00 +0200
                Re: [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm ebiederm@xmission.com (Eric W. Biederman) - 2015-08-13 17:50 +0200
                Re: [PATCH 1/2] unshare: Unsharing a thread does not require  unsharing a vm Oleg Nesterov <oleg@redhat.com> - 2015-08-13 18:20 +0200
                Re: [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm ebiederm@xmission.com (Eric W. Biederman) - 2015-08-13 18:40 +0200
                Re: [PATCH 1/2] unshare: Unsharing a thread does not require  unsharing a vm Oleg Nesterov <oleg@redhat.com> - 2015-08-13 19:00 +0200
                Re: [PATCH 1/2] unshare: Unsharing a thread does not require  unsharing a vm Oleg Nesterov <oleg@redhat.com> - 2015-08-14 20:10 +0200
              [PATCH v2] unshare: Unsharing a thread does not require unsharing a vm ebiederm@xmission.com (Eric W. Biederman) - 2015-08-12 22:10 +0200
                Re: [PATCH v2] unshare: Unsharing a thread does not require  unsharing a vm Oleg Nesterov <oleg@redhat.com> - 2015-08-13 15:00 +0200
                Re: [PATCH v2] unshare: Unsharing a thread does not require unsharing a vm ebiederm@xmission.com (Eric W. Biederman) - 2015-08-13 18:10 +0200
                Re: [PATCH v2] unshare: Unsharing a thread does not require  unsharing a vm Oleg Nesterov <oleg@redhat.com> - 2015-08-13 18:40 +0200
                Re: [PATCH v2] unshare: Unsharing a thread does not require unsharing a vm ebiederm@xmission.com (Eric W. Biederman) - 2015-08-13 18:50 +0200
          Re: [PATCH 0/2] userns: Creation logic fixes Kees Cook <keescook@chromium.org> - 2015-08-12 08:30 +0200
        Re: [PATCH] user_ns: use correct check for single-threadedness ebiederm@xmission.com (Eric W. Biederman) - 2015-08-12 03:30 +0200
          Re: [PATCH] user_ns: use correct check for single-threadedness Oleg Nesterov <oleg@redhat.com> - 2015-08-12 16:50 +0200
            Re: [PATCH] user_ns: use correct check for single-threadedness ebiederm@xmission.com (Eric W. Biederman) - 2015-08-12 17:20 +0200
      Re: [PATCH] user_ns: use correct check for single-threadedness Oleg Nesterov <oleg@redhat.com> - 2015-08-06 17:20 +0200
        Re: [PATCH] user_ns: use correct check for single-threadedness ebiederm@xmission.com (Eric W. Biederman) - 2015-08-06 23:30 +0200
      Re: [PATCH] user_ns: use correct check for single-threadedness Oleg Nesterov <oleg@redhat.com> - 2015-08-07 04:50 +0200

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