Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v4 1/2] hugetlb: use f_mode & FMODE_HUGETLBFS to identify
 hugetlbfs files
To:     Miklos Szeredi <miklos@szeredi.hu>,
        Amir Goldstein <amir73il@gmail.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, Linux MM <linux-mm@kvack.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Colin Walters <walters@verbum.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        syzbot <syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
References: <20200612004644.255692-1-mike.kravetz@oracle.com>
 <20200612015842.GC23230@ZenIV.linux.org.uk>
 <b1756da5-4e91-298f-32f1-e5642a680cbf@oracle.com>
 <CAOQ4uxg=o2SVbfUiz0nOg-XHG8irvAsnXzFWjExjubk2v_6c_A@mail.gmail.com>
 <CAJfpegv28Z2aECcb+Yfqum54zfwV=k1G1n_o3o6O-QTWOy3T4Q@mail.gmail.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <33cbc2b6-a37a-8bd1-d896-f4318a61b8ca@oracle.com>
Date:   Mon, 15 Jun 2020 10:48:11 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.8.0
MIME-Version: 1.0
In-Reply-To: <CAJfpegv28Z2aECcb+Yfqum54zfwV=k1G1n_o3o6O-QTWOy3T4Q@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 6/15/20 1:24 AM, Miklos Szeredi wrote:
> On Sat, Jun 13, 2020 at 8:53 AM Amir Goldstein <amir73il@gmail.com> wrote:
> 
>>> I also looked at normal filesystem lower and hugetlbfs upper.  Yes, overlayfs
>>> allows this.  This is somewhat 'interesting' as write() is not supported in
>>> hugetlbfs.  Writing to files in the overlay actually ended up writing to
>>> files in the lower filesystem.  That seems wrong, but overlayfs is new to me.
> 
> Yes, this very definitely should not happen.
> 
>> I am not sure how that happened, but I think that ovl_open_realfile()
>> needs to fixup f_mode flags FMODE_CAN_WRITE | FMODE_CAN_READ
>> after open_with_fake_path().
> 
> Okay, but how did the write actually get to the lower layer?
> 
> I failed to reproduce this.  Mike, how did you trigger this?

My apologies!!!

I reviewed my testing and found that it was incorrectly writing to the
lower filesystem.  Writing to any file in the union will fail.
-- 
Mike Kravetz