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From: Amir Goldstein <amir73il@gmail.com>
Date: Mon, 27 Aug 2018 09:34:47 +0300
Message-ID: <CAOQ4uxgvNoJESGkDmRxGyjvRY7xZzHBwx0txvVOb1dd6Xyv3GA@mail.gmail.com>
Subject: Re: [PATCH v2 3/6] ovl: fix GPF in swapfile_activate of file from
 overlayfs over xfs
To: Dave Chinner <david@fromorbit.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
        Al Viro <viro@zeniv.linux.org.uk>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Mon, Aug 27, 2018 at 6:43 AM Dave Chinner <david@fromorbit.com> wrote:
>
> On Sun, Aug 26, 2018 at 07:25:14PM +0300, Amir Goldstein wrote:
> > Since overlayfs implements stacked file operations, the underlying
> > filesystems are not supposed to be exposed to the overlayfs file,
> > whose f_inode is an overlayfs inode.
> >
> > Assigning an overlayfs file to swap_file results in an attempt of xfs
> > code to dereference an xfs_inode struct from an ovl_inode pointer:
> >
> >  CPU: 0 PID: 2462 Comm: swapon Not tainted
> >  4.18.0-xfstests-12721-g33e17876ea4e #3402
> >  RIP: 0010:xfs_find_bdev_for_inode+0x23/0x2f
> >  Call Trace:
> >   xfs_iomap_swapfile_activate+0x1f/0x43
> >   __se_sys_swapon+0xb1a/0xee9
> >
> > Fix this by not assigning the real inode mapping to f_mapping, which
> > will cause swapon() to return an error (-EINVAL). Although it makes
> > sense not to allow setting swpafile on an overlayfs file, some users
> > may depend on it, so we may need to fix this up in the future.
> >
> > Keeping f_mapping pointing to overlay inode mapping will cause O_DIRECT
> > open to fail. Fix this by installing ovl_aops with noop_direct_IO in
> > overlay inode mapping.
>
> Ummm - shouldn't ovl be checking the real inode's .direct_IO method
> and returning status based on that? i.e. if the underlying fs
> doesn't support O_DIRECT, neither should ovl...
>

ovl_open_realfile() will take care of that later when overlay actually
tried to open the underlying file.

> > +const struct address_space_operations ovl_aops = {
> > +     /* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO */
> > +     .direct_IO              = noop_direct_IO,
> > +};
> > +
> >  /*
> >   * It is possible to stack overlayfs instance on top of another
> >   * overlayfs instance as lower layer. We need to annonate the
> > @@ -575,6 +580,7 @@ static void ovl_fill_inode(struct inode *inode, umode_t mode, dev_t rdev,
> >       case S_IFREG:
> >               inode->i_op = &ovl_file_inode_operations;
> >               inode->i_fop = &ovl_file_operations;
> > +             inode->i_mapping->a_ops = &ovl_aops;
> >               break;
>
> So you put an ovl interposer in the way here - it needs to pass
> through *everything* to the the real inode's aops, right?
>

No. it's just a decoy a_ops, so if we miss another spot like swapon()
user will get an error (i.e. for no a_ops->readpages) rather then calling
into underlying filesystem aops with an overlay file and oopsing.

So the trick of this patch is to change the game from whack-an-oops
to whack-an-einval, which is better for everyone.

Cheers,
Amir.