Re: Do we really need d_weak_revalidate???

[NeilBrown <neilb@suse.com>]

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On Wed, Aug 23 2017, 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 ....
>

Why do you think that __look_hash() tries too hard?
It does call into the filesystem ->lookup() if the name isn't in the
cache, which probably isn't strictly needed, but that isn't harmful and
the current code does that.

Some the following seems sensible to me (though I haven't tested it).

Thanks,
NeilBrown


diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c
index dd9f1bebb5a3..859c198d0163 100644
--- a/fs/autofs4/dev-ioctl.c
+++ b/fs/autofs4/dev-ioctl.c
@@ -208,12 +208,16 @@ static int find_autofs_mount(const char *pathname,
 {
 	struct path path;
 	int err;
+	struct dentry *de, *parent;
+
+	de = kern_path_locked(pathname, &path);
+	if (IS_ERR(de))
+		return PTR_ERR(de);
+	parent = path.dentry;
+	path.dentry = de;
 
-	err = kern_path_mountpoint(AT_FDCWD, pathname, &path, 0);
-	if (err)
-		return err;
 	err = -ENOENT;
-	while (path.dentry == path.mnt->mnt_root) {
+	do {
 		if (path.dentry->d_sb->s_magic == AUTOFS_SUPER_MAGIC) {
 			if (test(&path, data)) {
 				path_get(&path);
@@ -222,10 +226,11 @@ static int find_autofs_mount(const char *pathname,
 				break;
 			}
 		}
-		if (!follow_up(&path))
-			break;
-	}
+	} while (follow_down_one(&path));
+
 	path_put(&path);
+	inode_unlock(d_inode(parent));
+	dput(parent);
 	return err;
 }
 


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