Gao feng <gaofeng@xxxxxxxxxxxxxx> writes:
> On 11/02/2013 02:06 PM, Gao feng wrote:
>> Hi Eric,
>>
>> On 08/28/2013 05:44 AM, Eric W. Biederman wrote:
>>>
>>> Rely on the fact that another flavor of the filesystem is already
>>> mounted and do not rely on state in the user namespace.
>>>
>>> Verify that the mounted filesystem is not covered in any significant
>>> way. I would love to verify that the previously mounted filesystem
>>> has no mounts on top but there are at least the directories
>>> /proc/sys/fs/binfmt_misc and /sys/fs/cgroup/ that exist explicitly
>>> for other filesystems to mount on top of.
>>>
>>> Refactor the test into a function named fs_fully_visible and call that
>>> function from the mount routines of proc and sysfs. This makes this
>>> test local to the filesystems involved and the results current of when
>>> the mounts take place, removing a weird threading of the user
>>> namespace, the mount namespace and the filesystems themselves.
>>>
>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>>> ---
>>> fs/namespace.c | 37 +++++++++++++++++++++++++------------
>>> fs/proc/root.c | 7 +++++--
>>> fs/sysfs/mount.c | 3 ++-
>>> include/linux/fs.h | 1 +
>>> include/linux/user_namespace.h | 4 ----
>>> kernel/user.c | 2 --
>>> kernel/user_namespace.c | 2 --
>>> 7 files changed, 33 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/fs/namespace.c b/fs/namespace.c
>>> index 64627f8..877e427 100644
>>> --- a/fs/namespace.c
>>> +++ b/fs/namespace.c
>>> @@ -2867,25 +2867,38 @@ bool current_chrooted(void)
>>> return chrooted;
>>> }
>>>
>>> -void update_mnt_policy(struct user_namespace *userns)
>>> +bool fs_fully_visible(struct file_system_type *type)
>>> {
>>> struct mnt_namespace *ns = current->nsproxy->mnt_ns;
>>> struct mount *mnt;
>>> + bool visible = false;
>>>
>>> - down_read(&namespace_sem);
>>> + if (unlikely(!ns))
>>> + return false;
>>> +
>>> + namespace_lock();
>>> list_for_each_entry(mnt, &ns->list, mnt_list) {
>>> - switch (mnt->mnt.mnt_sb->s_magic) {
>>> - case SYSFS_MAGIC:
>>> - userns->may_mount_sysfs = true;
>>> - break;
>>> - case PROC_SUPER_MAGIC:
>>> - userns->may_mount_proc = true;
>>> - break;
>>> + struct mount *child;
>>> + if (mnt->mnt.mnt_sb->s_type != type)
>>> + continue;
>>> +
>>> + /* This mount is not fully visible if there are any child mounts
>>> + * that cover anything except for empty directories.
>>> + */
>>> + list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
>>> + struct inode *inode = child->mnt_mountpoint->d_inode;
>>> + if (!S_ISDIR(inode->i_mode))
>>> + goto next;
>>> + if (inode->i_nlink != 2)
>>> + goto next;
>>
>>
>> I met a problem that proc filesystem failed to mount in user namespace,
>> The problem is the i_nlink of sysctl entries under proc filesystem is not
>> 2. it always is 1 even it's a directory, see proc_sys_make_inode. and for
>> btrfs, the i_nlink for an empty dir is 2 too. it seems like depends on the
>> filesystem itself,not depends on vfs. In my system binfmt_misc is mounted
>> on /proc/sys/fs/binfmt_misc, and the i_nlink of this directory's inode is
>> 1.
Yes. 1 is what filesystems that are too lazy to count the number of
links to a directory return, and /proc/sys is currently such a
filesystem.
Ordinarily nlink == 2 means a directory does not have any subdirectories.
>> btw, I'm not quite understand what's the inode->i_nlink != 2 here means?
>> is this directory empty? as I know, when we create a file(not dir) under
>> a dir, the i_nlink of this dir will not increase.
>>
>> And another question, it looks like if we don't have proc/sys fs mounted,
>> then proc/sys will be failed to be mounted?
>>
>
> Any Idea?? or should we need to revert this patch??
The patch is mostly doing what it is supposed to be doing.
Now the code is slightly buggy. inode->i_nlink will test to see if a
directory has subdirectories but it won't test to see if a directory is
empty. Where did my brain go when I was writing that test?
Right now I would rather not have the empty directory exception than
remove this code.
The test is a little trickier to write than it might otherwise be
because /proc and /sys tend to be slightly imperfect filesystems.
I think the only way to really test that is to call readdir on the
directory itself :( I don't like that thought.
I don't know what I was thinking when I wrote that test but I definitely
goofed up. Grr!
I can certainly filter out any directory with nlink > 2. That would be
an easy partial step forward.
The real question though is how do I detect directories it is safe to
mount on where there will not be files in them. I can't call iterate
with the namespace_lock held so things are a bit tricky.
Eric
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