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

From: Vasiliy Kulikov
Date: Sun Mar 25 2012 - 11:27:56 EST

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@xxxxxxxxxxxx>
> 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 - bringing security into open computing environments
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