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Subject: Re: Do we really need d_weak_revalidate???
From: Ian Kent <ikent@redhat.com>
To: NeilBrown <neilb@suse.com>, Jeff Layton <jlayton@redhat.com>,
        Trond Myklebust <trondmy@primarydata.com>,
        "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "mkoutny@suse.com" <mkoutny@suse.com>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>
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Date: Wed, 23 Aug 2017 10:54:27 +0800
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On 23/08/17 10:40, Ian Kent wrote:
> On 23/08/17 10:32, Ian Kent wrote:
>> On 23/08/17 09:06, NeilBrown wrote:
>>> On Mon, Aug 21 2017, Ian Kent wrote:
>>>
>>>>>
>>>>> A mount isn't triggered by kern_path(pathname, 0, &path).
>>>>> That '0' would need to include one of
>>>>>   LOOKUP_PARENT | LOOKUP_DIRECTORY |
>>>>>   LOOKUP_OPEN | LOOKUP_CREATE | LOOKUP_AUTOMOUNT
>>>>>
>>>>> to trigger an automount (otherwise you just get -EISDIR).
>>>>
>>>> It's perfectly sensible to think that but there is a case where a
>>>> a mount is triggered when using kern_path().
>>>>
>>>> The EISDIR return occurs for positive dentrys, negative dentrys
>>>> will still trigger an automount (which is autofs specific,
>>>> indirect mount map using nobrowse option, the install default).
>>>
>>> Ok, I understand this better now.  This difference between direct and
>>> indirect mounts is slightly awkward. It is visible from user-space, but
>>> not elegant to document.
>>> When you use O_PATH to open a direct automount that has not already been
>>> triggered, the open returns the underlying directory (and fstatfs
>>> confirms that it is AUTOFS_SUPER_MAGIC).  When you use O_PATH on
>>> an indirect automount, it *will* trigger the automount when "nobrowse" is
>>> in effect, but it won't when "browse" is in effect.
>>
>> That inconsistency has bothered me for quite a while now.
>>
>> It was carried over from the autofs module behavior when automounting
>> support was added to the VFS. What's worse is it prevents the use of
>> the AT_NO_AUTOMOUNT flag from working properly with fstatat(2) and with
>> statx().
>>
>> There is some risk in changing that so it does work but it really does
>> need to work to enable userspace to not trigger an automount by using
>> this flag.
>>
>> So that's (hopefully) going to change soonish, see:
>> http://ozlabs.org/~akpm/mmotm/broken-out/autofs-fix-at_no_automount-not-being-honored.patch
>>
>> The result should be that stat family calls don't trigger automounts except
>> for fstatat(2) and statx() which will require the AT_NO_AUTOMOUNT flag.
>>
>>>
>>> So we cannot just say "O_PATH doesn't trigger automounts", which is
>>> essentially what I said in
>>>
>>> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=97a45d02e6671482e8b2cdcce3951930bf6bdb94
>>>
>>> It might be possible to modify automount so that it was more consistent
>>> - i.e. if the point is triggered by a mkdir has been done, just to the
>>> mkdir.  If it is triggered after a mkdir has been done, do the mount.  I
>>> guess that might be racy, and in any case is hard to justify.
>>>
>>> Maybe I should change it to be about "direct automounts", and add a note
>>> that indirect automounts aren't so predictable.
>>
>> Right and the semantics should be much more consistent in the near future.
>> I hope (and expect) this semantic change won't cause problems.
>>
>>>
>>> But back to my original issue of wanting to discard
>>> kern_path_mountpoint, what would you think of the following approach -
>>> slight revised from before.
>>>
>>> Thanks,
>>> NeilBrown
>>>
>>> diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
>>> index beef981aa54f..7663ea82e68d 100644
>>> --- a/fs/autofs4/autofs_i.h
>>> +++ b/fs/autofs4/autofs_i.h
>>> @@ -135,10 +135,13 @@ static inline struct autofs_info *autofs4_dentry_ino(struct dentry *dentry)
>>>  /* autofs4_oz_mode(): do we see the man behind the curtain?  (The
>>>   * processes which do manipulations for us in user space sees the raw
>>>   * filesystem without "magic".)
>>> + * A process performing certain ioctls can get temporary oz status.
>>>   */
>>> +extern struct task_struct *autofs_tmp_oz;
>>>  static inline int autofs4_oz_mode(struct autofs_sb_info *sbi)
>>>  {
>>> -	return sbi->catatonic || task_pgrp(current) == sbi->oz_pgrp;
>>> +	return sbi->catatonic || task_pgrp(current) == sbi->oz_pgrp ||
>>> +		autofs_tmp_oz == current;
>>>  }
>>>  
>>>  struct inode *autofs4_get_inode(struct super_block *, umode_t);
>>> diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c
>>> index dd9f1bebb5a3..d76401669a20 100644
>>> --- a/fs/autofs4/dev-ioctl.c
>>> +++ b/fs/autofs4/dev-ioctl.c
>>> @@ -200,6 +200,20 @@ static int autofs_dev_ioctl_protosubver(struct file *fp,
>>>  	return 0;
>>>  }
>>>  
>>> +struct task_struct *autofs_tmp_oz;
>>> +int kern_path_oz(const char *pathname, int flags, struct path *path)
>>> +{
>>> +	static DEFINE_MUTEX(autofs_oz);
>>> +	int err;
>>> +
>>> +	mutex_lock(&autofs_oz);
>>> +	autofs_tmp_oz = current;
>>> +	err = kern_path(pathname, flags, path);
>>> +	autofs_tmp_oz = NULL;
>>> +	mutex_unlock(&autofs_oz);
>>> +	return err;
>>> +}
>>> +
>>
>> It's simple enough but does look like it will attract criticism as being
>> a hack!
>>
>> The kern_path_locked() function is very similar to what was originally
>> done, along with code to look down the mount stack (rather than up the
>> way it does now) to get the mount point. In this case, to be valid the
>> dentry can't be a symlink so that fits kern_path_locked() too.
> 
> Oh wait, that __lookup_hash() tries too hard to resolve the dentry,
> that won't quite work, and maybe d_lookup() can't be used safely in
> this context either ....

Umm .. d_lookup() does look ok so maybe path_parentat() + d_lookup()
would be ok.

> 
>>
>> So maybe it is worth going back to the way it was in the beginning and
>> be done with it .... OTOH Al must have had a reason for changing the
>> way it was done that I didn't get.
>>
>>>  /* Find the topmost mount satisfying test() */
>>>  static int find_autofs_mount(const char *pathname,
>>>  			     struct path *res,
>>> @@ -209,7 +223,8 @@ static int find_autofs_mount(const char *pathname,
>>>  	struct path path;
>>>  	int err;
>>>  
>>> -	err = kern_path_mountpoint(AT_FDCWD, pathname, &path, 0);
>>> +	err = kern_path_oz(pathname, 0, &path);
>>> +
>>>  	if (err)
>>>  		return err;
>>>  	err = -ENOENT;
>>> @@ -552,8 +567,7 @@ static int autofs_dev_ioctl_ismountpoint(struct file *fp,
>>>  
>>>  	if (!fp || param->ioctlfd == -1) {
>>>  		if (autofs_type_any(type))
>>> -			err = kern_path_mountpoint(AT_FDCWD,
>>> -						   name, &path, LOOKUP_FOLLOW);
>>> +			err = kern_path_oz(name, LOOKUP_FOLLOW, &path);
>>>  		else
>>>  			err = find_autofs_mount(name, &path,
>>>  						test_by_type, &type);
>>>
>>
>