linux-ext4 - Re: [PATCH v27 03/21] vfs: Add MAY_DELETE_SELF and MAY_DELETE

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Message-ID: <CAHc6FU4h-LCH_4VPiybakGWUXothvr+KzjZGjRaH_kEjZhd=TQ@mail.gmail.com>
Date:   Mon, 13 Feb 2017 16:40:41 +0100
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Jeremy Allison <jra@...ba.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jeff Layton <jlayton@...chiereds.net>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Dave Chinner <david@...morbit.com>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux NFS list <linux-nfs@...r.kernel.org>,
        linux-cifs@...r.kernel.org, Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v27 03/21] vfs: Add MAY_DELETE_SELF and MAY_DELETE_CHILD
 permission flags

On Tue, Dec 6, 2016 at 10:25 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Tue, Dec 6, 2016 at 10:13 PM, Jeremy Allison <jra@...ba.org> wrote:
>> On Tue, Dec 06, 2016 at 03:15:29PM -0500, J. Bruce Fields wrote:
>>> On Fri, Dec 02, 2016 at 10:57:42AM +0100, Miklos Szeredi wrote:
>>> > On Tue, Oct 11, 2016 at 2:50 PM, Andreas Gruenbacher
>>> > <agruenba@...hat.com> wrote:
>>> > > Normally, deleting a file requires MAY_WRITE access to the parent
>>> > > directory.  With richacls, a file may be deleted with MAY_DELETE_CHILD access
>>> > > to the parent directory or with MAY_DELETE_SELF access to the file.
>>> > >
>>> > > To support that, pass the MAY_DELETE_CHILD mask flag to inode_permission()
>>> > > when checking for delete access inside a directory, and MAY_DELETE_SELF
>>> > > when checking for delete access to a file itself.
>>> > >
>>> > > The MAY_DELETE_SELF permission overrides the sticky directory check.
>>> >
>>> > And MAY_DELETE_SELF seems totally inappropriate to any kind of rename,
>>> > since from the point of view of the inode we are not doing anything at
>>> > all.  The modifications are all in the parent(s), and that's where the
>>> > permission checks need to be.
>>>
>>> I'm having a hard time finding an authoritative reference here (Samba
>>> people might be able to help), but my understanding is that Windows
>>> gives this a meaning something like "may I delete a link to this file".
>>>
>>> (And not even "may I delete the *last* link to this file", which might
>>> also sound more logical.)
>>
>> I just did a recent patch here. In Samba we now check for
>> SEC_DIR_ADD_FILE/SEC_DIR_ADD_SUBDIR on the target directory
>> (depending on if the object being moved is a file or dir).
>
> And MAY_DELETE_SELF as well, for rename?  That's really counterintuitive for me.

Yes, MAY_DELETE_SELF applies to delete as well as rename; otherwise
rename() would behave different from link() + unlink(); when a user
has the appropriate permissions, the result should be the same though.

Thanks,
Andreas