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

[Andrew Vagin]

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.

On Mon, May 16, 2016 at 10:30:47PM +0000, Tycho Andersen wrote:
> See the comment for details, but basically tracefs is automounted by the
> kernel, so we can just mount debugfs with MS_REC and get the right result.
> 
> v2: rebase on criu-dev
> v3: don't use a new fstype->flags, just always set MS_REC in debugfs'
>     ->parse
> 
> Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
> ---
> This is kind of similarly ugly to to a cgroup patch I sent earlier, where
> we needed some filesystem specific code and checks in various parts of
> mount.c. I somehow can't figure out a nicer way to do this, but I am
> definitely open to suggestions.
> ---
>  criu/mount.c      | 15 +++++++++++++++
>  criu/proc_parse.c |  7 +++++++
>  images/mnt.proto  |  1 +
>  3 files changed, 23 insertions(+)
> 
> diff --git a/criu/mount.c b/criu/mount.c
> index 6f47655..e85e5d9 100644
> --- a/criu/mount.c
> +++ b/criu/mount.c
> @@ -1679,6 +1679,17 @@ out:
>  	return ret;
>  }
>  
> +static int debugfs_parse(struct mount_info *pm)
> +{
> +	/* tracefs is automounted underneath debugfs sometimes, and the
> +	 * kernel's overmounting protection prevents us from mounting debugfs
> +	 * first without tracefs, so let's always mount debugfs MS_REC.
> +	 */
> +	pm->flags |= MS_REC;
> +
> +	return 0;
> +}
> +
>  static int cgroup_parse(struct mount_info *pm)
>  {
>  	if (!(root_ns_mask & CLONE_NEWCGROUP))
> @@ -1780,6 +1791,10 @@ static struct fstype fstypes[] = {
>  	}, {
>  		.name = "debugfs",
>  		.code = FSTYPE__DEBUGFS,
> +		.parse = debugfs_parse,
> +	}, {
> +		.name = "tracefs",
> +		.code = FSTYPE__TRACEFS,
>  	}, {
>  		.name = "cgroup",
>  		.code = FSTYPE__CGROUP,
> diff --git a/criu/proc_parse.c b/criu/proc_parse.c
> index cebf21c..db1d0af 100644
> --- a/criu/proc_parse.c
> +++ b/criu/proc_parse.c
> @@ -1381,6 +1381,13 @@ struct mount_info *parse_mountinfo(pid_t pid, struct ns_id *nsid, bool for_dump)
>  			goto end;
>  		}
>  
> +		if (new->fstype->code == FSTYPE__TRACEFS) {
> +			pr_info("\tskipping tracefs mounted at %s\n", new->mountpoint + 1);
> +			mnt_entry_free(new);
> +			new = NULL;
> +			goto end;
> +		}
> +
>  		pr_info("\ttype %s source %s mnt_id %d s_dev %#x %s @ %s flags %#x options %s\n",
>  				fsname, new->source,
>  				new->mnt_id, new->s_dev, new->root, new->mountpoint,
> diff --git a/images/mnt.proto b/images/mnt.proto
> index fe1e4bb..9338ecb 100644
> --- a/images/mnt.proto
> +++ b/images/mnt.proto
> @@ -20,6 +20,7 @@ enum fstype {
>  	AUTO			= 16;
>  	OVERLAYFS		= 17;
>  	AUTOFS			= 18;
> +	TRACEFS			= 19;
>  };
>  
>  message mnt_entry {
> -- 
> 2.7.4
> 
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