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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Mon, 15 Jun 2020 10:24:42 +0200
Message-ID: <CAJfpegv28Z2aECcb+Yfqum54zfwV=k1G1n_o3o6O-QTWOy3T4Q@mail.gmail.com>
Subject: Re: [PATCH v4 1/2] hugetlb: use f_mode & FMODE_HUGETLBFS to identify
 hugetlbfs files
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        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>
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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?

Thanks,
Miklos