Re: Do we really need d_weak_revalidate???

[Ian Kent]

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 ....

> 
> 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);
>>
>