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

[Kirill Tkhai]

Hi, Andrew!

On 16.07.2016 01:07, Andrew Vagin wrote:
> 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?

It was here for using generic dumping code. But you may ignore this, according to Pavel
review, I'll remake patchset in other way.
 
>> +			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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