[CRIU] [PATCH 1/2] mount: handle tracefs more gracefully

[Tycho Andersen]

Hi guys,

On Tue, May 17, 2016 at 10:37:22AM -0700, Andrew Vagin wrote:
> Hi Tycho,
> 
> Honestly, I don't like this hack. I think it will not work in all cases.
> 
> For example, we create a clean mount, if an required path is
> overmounted:
> 
>         if (&c->siblings != &mi->bind->children) {
>                 /* Get a copy of mi->bind without child mounts */
>                 if (mount(mnt_path, mnt_clean_path, NULL, MS_BIND, NULL)) {
>                         pr_perror("Unable to bind-mount %s to %s",
>                                         mi->bind->mountpoint, mnt_clean_path);
>                 }
>                 mnt_path = mnt_clean_path;
>                 umount_mnt_path = true;
>         }
> 
> so if we will create a clean mount for debugfs, we will lost tracefs.
> 
> Do you bind-mount debugfs from host in a container? If it's yes, you probably
> use --mnt-ext-map options for it. Maybe we can add recursive external mounts
> or something like this.

>From our IRC discussion, avagin suggested,

avagin | --mnt-ext-map xxx:yyy:opts

which is fine with me. For now, we only need a few options:

'r' == MS_REC
'i' == ignore the mount on dump (i.e. don't even put it in the images)

with a combination of these two options, I think that we can make the
tracefs case work without any other changes to CRIU.

One other thing I noticed is that --ext-mount-map splits on `:`, which
will be wrong for paths with a `:` in them. Maybe we don't care about
that right now, though :)

What do you guys think? It will work, but it will require everyone who
mounts debugfs in a container to handle it this same way, as opposed
to being in CRIU proper.

Another way would be to drop the FSTYPE__TRACEFS special case from
proc_parse.c in the first patch, and add some kind of hook to the
struct fstype which says whether or not to record the fs in the dump
image (maybe just a different return value from ->parse?). See the
attached patch for what that would look like.

Tycho
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