[CRIU] [PATCH v3 3/4] mount: Forced mount unmounted binfmt_misc to do not lost its content

[Andrew Vagin]

On Wed, Jun 29, 2016 at 02:49:25PM +0300, Kirill Tkhai wrote:
> Umount does not remove binfmt_misc content. If it's mounted once again,
> the same entries remain registered.
> 
> Criu does not dump content of umounted binfmt_misc. So, after C/R we
> lose it at all.
> 
> This patch forces mounting of unmounted binfmt_misc before we collect
> mountpoints. If it's unmounted, we mount it back and add this mount
> to the list of forced mounted mountpoints. Next patch need this
> list to mark the mount in special way in dump image.
> 
> v2: Print error in case of umount() fail.
>     Move add_forced_mount() to another patch.
> v3: Close binfmt_misc dir before its umount().
> 
> Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
> ---
>  criu/cr-dump.c       |   13 ++++++++++++
>  criu/include/mount.h |    1 +
>  criu/mount.c         |   56 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 70 insertions(+)
> 
> diff --git a/criu/cr-dump.c b/criu/cr-dump.c
> index 7609544..90da57b 100644
> --- a/criu/cr-dump.c
> +++ b/criu/cr-dump.c
> @@ -747,6 +747,16 @@ static int dump_task_core_all(struct parasite_ctl *ctl,
>  	return ret;
>  }
>  
> +static int prepare_dump__tasks_freezed(void)
> +{
> +	int ret;
> +
> +	/* Tasks freezed, so we do not race with systemd's autofs unmounter */
> +	ret = try_mount_binfmt_misc(root_item->pid.real);
> +
> +	return ret;
> +}
> +
>  static int collect_pstree_ids_predump(void)
>  {
>  	struct pstree_item *item;
> @@ -1682,6 +1692,9 @@ int cr_dump_tasks(pid_t pid)
>  	if (collect_pstree())
>  		goto err;
>  
> +	if (prepare_dump__tasks_freezed())
> +		goto err;
> +
>  	if (collect_pstree_ids())
>  		goto err;
>  
> diff --git a/criu/include/mount.h b/criu/include/mount.h
> index 423b6d0..bd15468 100644
> --- a/criu/include/mount.h
> +++ b/criu/include/mount.h
> @@ -129,4 +129,5 @@ extern int read_mnt_ns_img(void);
>  extern void cleanup_mnt_ns(void);
>  extern void cleanup_forced_mounts(void);
>  
> +extern int try_mount_binfmt_misc(pid_t pid);
>  #endif /* __CR_MOUNT_H__ */
> diff --git a/criu/mount.c b/criu/mount.c
> index 4a288e4..df8a87d 100644
> --- a/criu/mount.c
> +++ b/criu/mount.c
> @@ -3761,4 +3761,60 @@ void cleanup_forced_mounts(void)
>  	}
>  }
>  
> +#define BINFMT_MISC_HOME "/proc/sys/fs/binfmt_misc"
> +
> +int try_mount_binfmt_misc(pid_t pid)
> +{
> +	int num, mnt_fd, ret, exit_code = -1;
> +	struct dirent *de;
> +	DIR *dir;
> +
> +	ret = switch_ns(pid, &mnt_ns_desc, &mnt_fd);
> +	if (ret < 0) {
> +		pr_err("Can't switch mnt_ns\n");
> +		goto out;
> +	}
> +
> +	ret = mount("binfmt_misc", BINFMT_MISC_HOME, "binfmt_misc", 0, NULL);
> +	if (ret < 0) {
> +		if (errno == EPERM) {
> +			pr_info("Can't mount binfmt_misc: EPERM. Running in user_ns?\n");
> +			exit_code = 0;
> +			goto restore_ns;
> +		}
> +		if (errno != EBUSY && errno != ENODEV && errno != ENOENT) {
> +			pr_perror("Can't mount binfmt_misc");
> +			goto restore_ns;
> +		}
> +		pr_info("Prepare binfmt_misc: skipping(%d)\n", errno);
> +	} else {
> +		dir = opendir(BINFMT_MISC_HOME);
> +		if (!dir) {
> +			pr_perror("Can't read binfmt_misc dir");
> +			goto restore_ns;
> +		}
> +
> +		num = 0;
> +		/* ".", "..", "register", "status" */
> +		while (num <= 4 && (de = readdir(dir)) != NULL)
> +			num++;
> +		closedir(dir);
> +		if (num <= 4) {
> +			/* No entries */
> +			if (umount(BINFMT_MISC_HOME) < 0)
> +				pr_perror("Can't umount "BINFMT_MISC_HOME"\n");
> +		} else {
> +			ret = add_forced_mount(pid, BINFMT_MISC_HOME);

Why do we need to add a fake mount? Maybe it's better to dump it right
here?

> +			if (ret)
> +				goto restore_ns;
> +		}
> +	}
> +
> +	exit_code = 0;
> +restore_ns:
> +	ret = restore_ns(mnt_fd, &mnt_ns_desc);
> +out:
> +	return ret ? -1 : exit_code;
> +}
> +
>  struct ns_desc mnt_ns_desc = NS_DESC_ENTRY(CLONE_NEWNS, "mnt");
> 
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