linux-kernel - query re unlink() ... inotify ... open() race

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Date:	Sat, 12 Sep 2015 01:28:27 +0100
From:	Pádraig Brady <P@...igBrady.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
CC:	avagin@...nvz.org,
	Ludovic Courtès <ludo@....org>
Subject: query re unlink() ... inotify ... open() race

Hi,

We're noticing a rare race here with open() in tail(1), where this happens:

  tail --follow=name "file"
    /* "file" is unlinked() by another process */
    read(IN_ATTRIB from inotify); /* for st_nlink-- */
    open("file") /* Done to check if deleted, but this succeeds! */

The open() succeeding is surprising. Is that allowed?
The summary of the sequence in the kernel is:

  vfs_unlink() {
    mutex_lock(&(dentry->d_inode->i_mutex));
    security_inode_unlink(dir, dentry);
    try_break_deleg(target, delegated_inode);
    dir->i_op->unlink(dir, dentry);
    dont_mount(dentry);
    detach_mounts(dentry);
    mutex_unlock(&(dentry->d_inode->i_mutex));

    fsnotify_link_count(target)
    d_delete(dentry);
  }

thanks,
Pádraig.
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