Date: Sun, 17 Sep 2017 17:38:28 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>,
        Christoph Hellwig <hch@infradead.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>, linux-ima-devel@lists.sourceforge.net,
        James Morris <jmorris@namei.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Matthew Garrett <mjg59@srcf.ucam.org>, Jan Kara <jack@suse.com>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <yuchao0@huawei.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Bob Peterson <rpeterso@redhat.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Dave Kleikamp <shaggy@kernel.org>,
        Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>,
        Mark Fasheh <mfasheh@versity.com>, Joel Becker <jlbec@evilplan.org>,
        Richard Weinberger <richard@nod.at>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Hugh Dickins <hughd@google.com>, Chris Mason <clm@fb.com>
Subject: Re: [PATCH 3/3] ima: use fs method to read integrity data (updated
 patch description)
Message-ID: <20170917163828.GE5426@ZenIV.linux.org.uk>
References: <1505451494-30228-1-git-send-email-zohar@linux.vnet.ibm.com>
 <1505451494-30228-4-git-send-email-zohar@linux.vnet.ibm.com>
 <1505507142.4200.103.camel@linux.vnet.ibm.com>
 <CA+55aFxmeff53zeioTiQY6m=F18ekgX1-HWZyQUor=NJYrxM9A@mail.gmail.com>
 <20170917151757.GA14262@infradead.org>
 <CA+55aFyrZ=YsfNc1vp=vArNgotLXGPr4F6uZiz22Uj2XHGUvaw@mail.gmail.com>
 <1505664935.4200.191.camel@linux.vnet.ibm.com>
 <CA+55aFyBhb_hQ84heyWnAP+1vn48Wjpnas0YSUY-R6Ej6bv_Dw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CA+55aFyBhb_hQ84heyWnAP+1vn48Wjpnas0YSUY-R6Ej6bv_Dw@mail.gmail.com>
User-Agent: Mutt/1.8.3 (2017-05-23)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1374
Lines: 27

On Sun, Sep 17, 2017 at 09:34:01AM -0700, Linus Torvalds wrote:
> Now, I suspect most (all?) do, but that's a historical artifact rather
> than "design". In particular, the VFS layer used to do the locking for
> the filesystems, to guarantee the POSIX requirements (POSIX requires
> that writes be seen atomically).
> 
> But that lock was pushed down into the filesystems, since some
> filesystems really wanted to have parallel writes (particularly for
> direct IO, where that POSIX serialization requirement doesn't exist).
> 
> That's all many years ago, though. New filesystems are likely to have
> copied the pattern from old ones, but even then..
> 
> Also, it's worth noting that "inode->i_rwlock" isn't even well-defined
> as a lock. You can have the question of *which* inode gets talked
> about when you have things like eoverlayfs etc. Normally it would be
> obvious, but sometimes you'd use "file->f_mapping->host" (which is the
> same thing in the simple cases), and sometimes it really wouldn't be
> obvious at all..
> 
> So... I'm really not at all convinced that i_rwsem is sensible. It's
> one of those things that are "mostly right for the simple cases",
> but...

The thing pretty much common to all of them is that write() might need
to modify permissions (suid removal), which brings ->i_rwsem in one
way or another - notify_change() needs that held...