[PATCH] fanotify: dont destroy mark when ignore mask is cleared

[PATCH] fanotify: dont destroy mark when ignore mask is cleared

[Lino Sanfilippo]

In mark_remove_from_mask() the mark is destroyed regardless of whether the
event mask or ignore mask is cleared. We should only destroy the mark if the
event mask is cleared.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
---
 fs/notify/fanotify/fanotify_user.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

Please apply this after patch "Dont allow a mask of 0 if setting or removing a
mark" which i sent today.

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 207cdeb..c9143a0 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -518,7 +518,7 @@ static __u32 fanotify_mark_remove_from_mask(struct fsnotify_mark *fsn_mark,
 	}
 	spin_unlock(&fsn_mark->lock);
 
-	if (!(oldmask & ~mask))
+	if (!(flags & FAN_MARK_IGNORED_MASK) && !(oldmask & ~mask))
 		fsnotify_destroy_mark(fsn_mark);
 
 	return mask & oldmask;
-- 
1.5.6.5

Re: [PATCH] fanotify: dont destroy mark when ignore mask is cleared

[Eric Paris]

On Mon, 2010-11-22 at 18:52 +0100, Lino Sanfilippo wrote:
> In mark_remove_from_mask() the mark is destroyed regardless of whether the
> event mask or ignore mask is cleared. We should only destroy the mark if the
> event mask is cleared.
> 
> Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>

Hmmmm, really I'm not sure if that is right either (but it's certainly
closer)  What about something like:

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 81df3ad..29fbf17 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -527,7 +527,7 @@ static __u32 fanotify_mark_remove_from_mask(struct fsnotify_mark *fsn_mark,
 	}
 	spin_unlock(&fsn_mark->lock);
 
-	if (!(oldmask & ~mask))
+	if (!fsn_mark->mask && !fsn_mark->ignored_mask)
 		fsnotify_destroy_mark(fsn_mark);
 
 	return mask & oldmask;

Re: [PATCH] fanotify: dont destroy mark when ignore mask is cleared

[Lino Sanfilippo]

On Tue, Nov 23, 2010 at 02:51:19PM -0500, Eric Paris wrote:

> 
> Hmmmm, really I'm not sure if that is right either (but it's certainly
> closer)  What about something like:
> 
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 81df3ad..29fbf17 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -527,7 +527,7 @@ static __u32 fanotify_mark_remove_from_mask(struct fsnotify_mark *fsn_mark,
>  	}
>  	spin_unlock(&fsn_mark->lock);
>  
> -	if (!(oldmask & ~mask))
> +	if (!fsn_mark->mask && !fsn_mark->ignored_mask)
>  		fsnotify_destroy_mark(fsn_mark);
>  
>  	return mask & oldmask;
> 
> 

Yep, youre right, we should also check the ignore mask before we destroy the mark.

BUT:
1. There is this flag FAN_MARK_ONDIR which is set implicitly in the ignore mask
whenever it has not explicitly been set by the user (see fanotify_mark_add_to_mask()).
If that flag has been set the ignore mask will never get cleared unless the
user does something like

fanotify_mark(fd, FAN_MARK_REMOVE | FAN_MARK_IGNORED_MASK, FAN_MARK_ONDIR,...)

which presumes that he knows that this flag has been set. 



Btw.: 
every time a user _forgets_ to explicitly set FAN_MARK_ONDIR, it will be set
in the ignored mask and thus events on dirs are skipped. Thus calls like

fanotify_mark(fd, FAN_MARK_ADD, FAN_MARK_ONDIR, ...)  /* get dir events */
fanotify_mark(fd, FAN_MARK_ADD, ...) /* add mark for some kind of event */

will result in dir events being ignored by the second call to fanotify_mark(),
although the user has requested those events in his first call.
This is very likely not what the user expected. Is there a reason why ONDIR is 
set implicitly in the ignore mask?
Otherwise i would suggest to not set it implicitly in mark->ignored_mask,
but to set it in mark->mask if requested by the user. Then we could ignore dir 
events as long as the flag has not been set there. 


2. I just realized that we cant simply call destroy_mark() if the masks are 0. 
There may be one or more concurrent processes calling fsnotify_find_inode_mark() 
(see fanotify_add_inode_mark()) and get the mark we are about to destroy at the 
same time.

I will take a closer look at it, but it seems to be difficult to me to safely 
call destroy_mark() as long as we are not in the context of fanotify_release() (in 
which we dont have to deal with concurrency like that any more). 





 

Re: [PATCH] fanotify: dont destroy mark when ignore mask is cleared

[Eric Paris]

On Wed, 2010-11-24 at 13:31 +0100, Lino Sanfilippo wrote:

> 2. I just realized that we cant simply call destroy_mark() if the masks are 0. 
> There may be one or more concurrent processes calling fsnotify_find_inode_mark() 
> (see fanotify_add_inode_mark()) and get the mark we are about to destroy at the 
> same time.
> 
> I will take a closer look at it, but it seems to be difficult to me to safely 
> call destroy_mark() as long as we are not in the context of fanotify_release() (in 
> which we dont have to deal with concurrency like that any more). 

I guess it is a question of safe vs racy.  Yes it is safe, nothing will
explode or panic.  But we might have a race between one task removing an
event type causing the mask to go to 0 and we should destroy the mark
and another task adding an event type.  If it raced just right we might
destroy the mark after the second task added to it.  I guess we really
need to serialize fsnotify_mark() per group to solve the race...

Do you want to take a stab at fixing these things or should I?

-Eric

Re: [PATCH] fanotify: dont destroy mark when ignore mask is cleared

[Lino Sanfilippo]

On Tue, Nov 30, 2010 at 01:16:35PM +0100, Lino Sanfilippo wrote:

> I guess it is a question of safe vs racy.  Yes it is safe, nothing will
> explode or panic.  But we might have a race between one task removing an
> event type causing the mask to go to 0 and we should destroy the mark
> and another task adding an event type.  If it raced just right we might
> destroy the mark after the second task added to it.  I guess we really
> need to serialize fsnotify_mark() per group to solve the race...
> 
> Do you want to take a stab at fixing these things or should I?
> 
> -Eric

IMHO the right thing to serialize this would be to do 

LOCK(groups->mark_lock)
- get the inode mark
- set the marks mask
- possibly destroy the mask
UNLOCK(groups->mark_lock)

But we cant do this since setting the marks mask requires the lock of the mark 
- which would mean an incorrect lock order according to fsnotify_add_mark():

mark->lock
group->mark_lock
inode->i_lock

What we could do very easily is use another mutex instead (use an existing one like the 
groups notification_mutex, or a completely new one) which is responsible for synchronising
add_mark()/remove_mark().  

If that solution is ok I'll prepare the patches for it. Otherwise i am not sure how to solve this... 


Lino

Re: [PATCH] fanotify: dont destroy mark when ignore mask is cleared

[Eric Paris]

On Tue, 2010-11-30 at 16:59 +0100, Lino Sanfilippo wrote:
> On Tue, Nov 30, 2010 at 01:16:35PM +0100, Lino Sanfilippo wrote:
> 
> > I guess it is a question of safe vs racy.  Yes it is safe, nothing will
> > explode or panic.  But we might have a race between one task removing an
> > event type causing the mask to go to 0 and we should destroy the mark
> > and another task adding an event type.  If it raced just right we might
> > destroy the mark after the second task added to it.  I guess we really
> > need to serialize fsnotify_mark() per group to solve the race...
> > 
> > Do you want to take a stab at fixing these things or should I?
> > 
> > -Eric
> 
> IMHO the right thing to serialize this would be to do 
> 
> LOCK(groups->mark_lock)
> - get the inode mark
> - set the marks mask
> - possibly destroy the mask
> UNLOCK(groups->mark_lock)
> 
> But we cant do this since setting the marks mask requires the lock of the mark 
> - which would mean an incorrect lock order according to fsnotify_add_mark():
> 
> mark->lock
> group->mark_lock
> inode->i_lock
> 
> What we could do very easily is use another mutex instead (use an existing one like the 
> groups notification_mutex, or a completely new one) which is responsible for synchronising
> add_mark()/remove_mark().

I'd think a new per group mutex would be the right way to go.  I'm not
sure how I feel about notification_mutex.  I guess you can go ahead and
overload it and we can split it off later if someone finds it to be a
performance blocker.

-Eric