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Re: [libvirt] [RFC] Create ptmx as a device



On Wed, Aug 31, 2011 at 01:18:33PM -0500, Serge Hallyn wrote:
> Hi,
> 
> I'm seeing an issue with udev and libvirt-lxc.  Libvirt-lxc creates
> /dev/ptmx as a symlink to /dev/pts/ptmx.  When udev starts up, it
> checks the device type, sees ptmx is 'not right', and replaces it
> with a 'proper' ptmx.
> 
> In lxc, /dev/ptmx is bind-mounted from /dev/pts/ptmx instead of being
> symlinked, so udev sees the right device type and leaves it alone.
> 
> A patch like the following seems to work for me.  Would there be
> any objections to this?
> 
> >From 4c5035de52de7e06a0de9c5d0bab8c87a806cba7 Mon Sep 17 00:00:00 2001
> From: Ubuntu <ubuntu domU-12-31-39-14-F0-B3 compute-1 internal>
> Date: Wed, 31 Aug 2011 18:15:54 +0000
> Subject: [PATCH 1/1] make ptmx a bind mount rather than symlink
> 
> udev on some systems checks the device type of /dev/ptmx, and replaces it if
> not as expected.  The symlink created by libvirt-lxc therefore gets replaced.
> By creating it as a bind mount, the device type is correct and udev leaves it
> alone.
> 
> Signed-off-by: Serge Hallyn <serge hallyn canonical com>
> ---
>  src/lxc/lxc_container.c |   20 ++++++++++----------
>  1 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/src/lxc/lxc_container.c b/src/lxc/lxc_container.c
> index e425328..6991aec 100644
> --- a/src/lxc/lxc_container.c
> +++ b/src/lxc/lxc_container.c
> @@ -543,18 +543,18 @@ static int lxcContainerPopulateDevices(void)
>          }
>      }
>  
> +    dev_t dev = makedev(LXC_DEV_MAJ_TTY, LXC_DEV_MIN_PTMX);
> +    if (mknod("/dev/ptmx", S_IFCHR, dev) < 0 ||
> +        chmod("/dev/ptmx", 0666)) {
> +        virReportSystemError(errno, "%s",
> +                             _("Failed to make device /dev/ptmx"));
> +        return -1;
> +    }
> +
>      if (access("/dev/pts/ptmx", W_OK) == 0) {
> -        if (symlink("/dev/pts/ptmx", "/dev/ptmx") < 0) {
> -            virReportSystemError(errno, "%s",
> -                                 _("Failed to create symlink /dev/ptmx to /dev/pts/ptmx"));
> -            return -1;
> -        }
> -    } else {
> -        dev_t dev = makedev(LXC_DEV_MAJ_TTY, LXC_DEV_MIN_PTMX);
> -        if (mknod("/dev/ptmx", S_IFCHR, dev) < 0 ||
> -            chmod("/dev/ptmx", 0666)) {
> +        if (mount("/dev/pts/ptmx", "/dev/ptmx", "ptmx", MS_BIND, NULL) < 0) {
>              virReportSystemError(errno, "%s",
> -                                 _("Failed to make device /dev/ptmx"));
> +                                 _("Failed to bind-mount /dev/ptmx to /dev/pts/ptmx"));
>              return -1;
>          }
>      }

  Hum, if we do a mount, I would expect to do an unmount somewhere.
Also the lifetime of the mount and the symlink is really different,
a recursive remove when destroying the container would lead to no
resource leak but I think that for a bind mount we absolutely have
to clean it up.

Daniel

-- 
Daniel Veillard      | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
daniel veillard com  | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library  http://libvirt.org/


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