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Date:   Mon, 18 May 2020 19:41:18 -0400
From:   "Colin Walters" <walters@verbum.org>
To:     "Miklos Szeredi" <miklos@szeredi.hu>,
        "Mike Kravetz" <mike.kravetz@oracle.com>
Cc:     syzbot <syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, "Miklos Szeredi" <mszeredi@redhat.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        "Al Viro" <viro@zeniv.linux.org.uk>
Subject: Re: kernel BUG at mm/hugetlb.c:LINE!
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On Tue, May 12, 2020, at 11:04 AM, Miklos Szeredi wrote:

> > However, in this syzbot test case the 'file' is in an overlayfs filesystem
> > created as follows:
> >
> > mkdir("./file0", 000)                   = 0
> > mount(NULL, "./file0", "hugetlbfs", MS_MANDLOCK|MS_POSIXACL, NULL) = 0
> > chdir("./file0")                        = 0
> > mkdir("./file1", 000)                   = 0
> > mkdir("./bus", 000)                     = 0
> > mkdir("./file0", 000)                   = 0
> > mount("\177ELF\2\1\1", "./bus", "overlay", 0, "lowerdir=./bus,workdir=./file1,u"...) = 0

Is there any actual valid use case for mounting an overlayfs on top of hugetlbfs?  I can't think of one.  Why isn't the response to this to instead only allow mounting overlayfs on top of basically a set of whitelisted filesystems?