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Message-ID: <55F371AB.20805@draigBrady.com>
Date: Sat, 12 Sep 2015 01:28:27 +0100
From: =?UTF-8?B?UMOhZHJhaWcgQnJhZHk=?= <P@draigBrady.com>
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CC: avagin@openvz.org, =?UTF-8?B?THVkb3ZpYyBDb3VydMOocw==?= <ludo@gnu.org>
Subject: query re unlink() ... inotify ... open() race
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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.