Re: [RFC 2/2] fanotify: emit FAN_MODIFY_DIR on filesystem changes

[Amir Goldstein <amir73il@gmail.com>]

On Sun, Mar 19, 2017 at 2:04 PM, Jan Kara <jack@suse.cz> wrote:
> On Sun 19-03-17 11:37:39, Filip Štědronský wrote:
>> On Sun, Mar 19, 2017 at 11:19:43AM +0100, Jan Kara wrote:
>> > However if you can really call fsnotify hooks with 'path' available in all
>> > the places, it should be equally hard to just pass 'path' to
>> > vfs_(create|mkdir|...) and that way we don't have to sprinkle fsnotify
>> > calls into several call sites but keep them local to vfs_(create|mkdir|...)
>> > helpers. Hmm?
>>
>> the problem is: not absolutely all. One illuminating example is the use
>> of vfs_mknod in devtmpfs. There a struct path is not only unavailable
>> but makes not semantic sense: the changes do not go thru any mountpoint.
>
> How come? In current kernel the call looks like:
>
> vfs_mknod(d_inode(path.dentry), dentry, mode, dev->devt);
>
> so the path is available there... I've actually quickly checked all
> vfs_mknod() callers and they all seem to have path available.
>
>> And in general I think there will be situations where you would need
>> to call VFS functions without paths.
>>
>> Thus I suggested either
>> (a) wrapping the VFS functions with path variants, or
>> (b) giving them an optional vfsmount argument that can be set to NULL
>>     when it does not make sense
>
> So my first take is that fsnotify calls happen still relatively high in the
> call stack where we should mostly have mount point available from the path
> lookup. That being said there may be places where we've lost that
> information and it will not be easy to propagate it there and that would
> have to be dealt with on case-by-case basis. But mountpoint is needed for
> other stuff like security checks these days as well so we should have it
> available in principle.
>

I agree that propagating the path to fsnotify seem like the right thing to do.
fsnotify_inoderemove() is an example (the only one I know of) where path
is not available (when called down from from dput()), but frankly, I can't
think of any use cases that really needs to make use of the
FS_DELETE_SELF event in that case.

d_delete() which also calls fsnotify_inoderemove() already calls
fsnotify_nameremove() hook with the exact same dentry, so
the FS_DELETE_SELF event can be generated by that hook as well
as the FS_DELETE event.