linux-kernel - Re: [PATCH v4 1/2] hugetlb: use f_mode & FMODE

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Message-ID: <33cbc2b6-a37a-8bd1-d896-f4318a61b8ca@oracle.com>
Date:   Mon, 15 Jun 2020 10:48:11 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miklos Szeredi <miklos@...redi.hu>,
        Amir Goldstein <amir73il@...il.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Colin Walters <walters@...bum.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+d6ec23007e951dadf3de@...kaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH v4 1/2] hugetlb: use f_mode & FMODE_HUGETLBFS to identify
 hugetlbfs files

On 6/15/20 1:24 AM, Miklos Szeredi wrote:
> On Sat, Jun 13, 2020 at 8:53 AM Amir Goldstein <amir73il@...il.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