From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: procfs/d_invalidate break userland
Date: Thu, 23 Apr 2015 00:51:35 +0100
Message-ID: <20150422235135.GY889@ZenIV.linux.org.uk>
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Cc: Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= <mic@digikod.net>
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On Thu, Apr 23, 2015 at 01:41:02AM +0200, Micka=EBl Sala=FCn wrote:
> Hi,
>=20
> For your information, the commit bbd5192412fdedbae00888316bfe350bf89d=
0458 (proc: Update proc_flush_task_mnt to use d_invalidate), in the v3.=
17, break some userland use of procfs.
>=20
> In a user namespace, I'm using /proc/<pid>/task/<tid>/fdinfo to mount=
 a tmpfs, chdir into it, terminate the <tid> thread, and use the tmpfs =
as a private working directory for PID 1. This trick is useful to disco=
nnect/d_drop a directory from the current file system and make it inacc=
essible to the other processes (kind of an O_TMPFILE for directory).
>=20
> With the v3.17 this is no more possible because it's no more possible=
 to mount anything in this tmpfs nor use the previously mounted file sy=
stems in this tmpfs.
>=20
> The new d_invalidate is really good to lazily invalidate/umount an en=
tire hierarchy but I'm not sure the intention was to remove this useful=
 "feature" ;)

And what will happen if you mount stuff there and exit?  Dirty filesyst=
em
after reboot?
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