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Date:   Sun, 17 Sep 2017 17:38:28 +0100
From:   Al Viro <>
To:     Linus Torvalds <>
Cc:     Mimi Zohar <>,
        Christoph Hellwig <>,
        LSM List <>,
        Christoph Hellwig <>,,
        James Morris <>,
        Linux Kernel Mailing List <>,
        Matthew Garrett <>,
        Jan Kara <>, Theodore Ts'o <>,
        Andreas Dilger <>,
        Jaegeuk Kim <>, Chao Yu <>,
        Steven Whitehouse <>,
        Bob Peterson <>,
        David Woodhouse <>,
        Dave Kleikamp <>,
        Ryusuke Konishi <>,
        Mark Fasheh <>,
        Joel Becker <>,
        Richard Weinberger <>,
        "Darrick J. Wong" <>,
        Hugh Dickins <>, Chris Mason <>
Subject: Re: [PATCH 3/3] ima: use fs method to read integrity data (updated
 patch description)

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...

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