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    SubjectRe: [RFC PATCH 09/35] ovl: stack file ops
    On Thu, Apr 12, 2018 at 05:08:00PM +0200, Miklos Szeredi wrote:

    [..]
    > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
    > new file mode 100644
    > index 000000000000..a0b606885c41
    > --- /dev/null
    > +++ b/fs/overlayfs/file.c
    > @@ -0,0 +1,76 @@
    > +/*
    > + * Copyright (C) 2017 Red Hat, Inc.
    > + *
    > + * This program is free software; you can redistribute it and/or modify it
    > + * under the terms of the GNU General Public License version 2 as published by
    > + * the Free Software Foundation.
    > + */
    > +
    > +#include <linux/cred.h>
    > +#include <linux/file.h>
    > +#include <linux/xattr.h>
    > +#include "overlayfs.h"
    > +
    > +static struct file *ovl_open_realfile(const struct file *file)
    > +{
    > + struct inode *inode = file_inode(file);
    > + struct inode *upperinode = ovl_inode_upper(inode);
    > + struct inode *realinode = upperinode ?: ovl_inode_lower(inode);
    > + struct file *realfile;
    > + const struct cred *old_cred;
    > +
    > + old_cred = ovl_override_creds(inode->i_sb);
    > + realfile = path_open(&file->f_path, file->f_flags | O_NOATIME,
    > + realinode, current_cred(), false);
    > + revert_creds(old_cred);
    > +
    > + pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",
    > + file, file, upperinode ? 'u' : 'l', file->f_flags,
    > + realfile, IS_ERR(realfile) ? 0 : realfile->f_flags);
    > +
    > + return realfile;
    > +}
    > +
    > +static int ovl_open(struct inode *inode, struct file *file)
    > +{
    > + struct dentry *dentry = file_dentry(file);

    Hi Miklos,

    There is one thing I can't wrap my head around, so I better ask.

    file_dentry() will call ovl_d_real() and try to find dentry based on
    inode installed in f->f_inode. If ovl_d_real() can't find inode dentry
    matching the passed in inode, it warns.

    Assume, I have a stacked overlay configuration. Let me call top level
    overlay layer ovl1 and lower level overlay layer ovl2. Say I open a
    file foo.txt. Now ovl_open() in ovl1 decides that realinode is a lower
    inode and installs that inode f->f_inode of realfile. (This should be
    ovl2 layer inode, let me call it ovl2_inode). Now ovl_open() of ovl2 layer
    will be called and it will call file_dentry() and will look for dentry
    corresponding to ovl2_inode. I am wondering what if a copy up of foo.txt
    was triggered in ovl1 and by the time we called ovl_d_real(dentry,
    ovl2_inode), it will start comparing with inode of ovl1_upper and never
    find ovl2_inode.

    IOW, I am not able to figure out how do we protect agains copy up races
    when ovl_open() calls file_dentry().

    Thanks
    Vivek

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