linux-kernel - Re: [PATCH] proc: fix mount -t proc -o AAA

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Date:	Sun, 25 Mar 2012 19:27:36 +0400
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Arkadiusz Miśkiewicz <a.miskiewicz@...il.com>,
	linux-kernel@...r.kernel.org, Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH] proc: fix mount -t proc -o AAA

On Fri, Mar 23, 2012 at 16:15 -0700, Andrew Morton wrote:
> I'm surprised.  "mount -o remount,<options>" doesn't work on a mounted
> procfs, and nobody noticed until now?
> 
> The patch looks OK - has it been tested with both valid and invalid
> mount options?
> 
> 
> I redid the changelog:
> 
> 
> From: Vasiliy Kulikov <segoon@...nwall.com>
> Subject: proc: fix mount -t proc -o AAA
> 
> The proc_parse_options() call from proc_mount() runs only once at boot
> time.  So on any later mount attempt, any mount options are ignored
> because ->s_root is already initialized.
> 
> As a consequence, "mount -o remount,<options>" will ignore the options.

No, remount works as it should.  _mount_ doesn't work.  Why it was not spotted:
Live case is:

1) upstart and systemd don't use /etc/fstab for /proc when mounting it at
the boot time.
2) dbus, etc. use /proc/ from the boot, so /proc cannot be umounted without
dbus stop.

So, to apply hidepid=X without system reboot procfs should be remounted
instead of umount+mount.

> To fix this, parse the mount options unconditionally.

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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