Re: [RFC][PATCH] inotify 0.10.0

[Ray Lee <ray-lk@madrabbit.org>]

On Tue, 2004-09-28 at 16:34 -0400, John McCutchan wrote:
> On Tue, 2004-09-28 at 13:32, Ray Lee wrote:
> > On Tue, 2004-09-28 at 12:53 -0400, Robert Love wrote:
> > > On Tue, 2004-09-28 at 10:41 -0600, Chris Friesen wrote:
> > > > Andrew Morton wrote:
> > > > 
> > > > > Why don't you pass a file descriptor into the syscall instead of a pathname?
> > > > > You can then take a ref on the inode and userspace can close the file.
> > > > > That gets you permission checking for free.
> > > > 
> > > > For passing in the data, that would work.  Wouldn't you still need a name or 
> > > > path when getting data back though?
> > > 
> > > Does Andrew mean an fd on the thing being watched?
> > > 
> > > That is what we are trying to fix with dnotify: the open fd's are pin
> > > the device and prevent unmount, making notification on removable devices
> > > impossible.
> > 
> > That's why he said to close the fd right after the syscall. But yeah,
> > for a case of someone wanting to watch their 1700 directories underneath
> > ~/, thems a lot of open calls.
> > 
> > > Such a 1:1 relationship also opens way too many fd's.
> > 
> > ...I'm not sure I follow. If you're talking about the IN_CREATE and
> > IN_DELETE events available when watching a parent directory, then I
> > don't think anything would change. IOW, why not do an open(2) on the
> > directory in question, and pass that fd in?
> > 
> > Regardless, Andrew's point still stands. What do we want the permission
> > semantics to be? One would think that a normal user account should not
> > be able to watch the contents of some other user's 0600 directories, for
> > example. open(2) already does all the correct checks. We should inherit
> > that work if at all possible.
> 
> Yes we should, but I think the inotify interface would be cleaner if we
> just factored out this permission code and called it from open() and
> from the inotify code.

I don't know enough to have an opinion on this. I'll leave that up to
actual arbiters of taste.

> > 
> > Another benefit of passing in an fd, by the way, would be to make it
> > easier to make a write(2) interface to inotify, and get rid of the ioctl
> > one.
> > 
> 
> I don't see how passing directories/files to inotify by fd not filename,
> makes providing a write(2) interface to inotify any easier.

There seemed to be some unwillingness to deal with variable length items
being passed back and forth, to and fro userspace. Placing an fd in the
watch structure would make it fixed size, rather than variable length.
Frankly, I find either as easy to deal with.

> To me they are mutually exclusive.

To me as well. Robert seems to have issues with making the userspace
interface as simple as possible, hence my suggestion. I'm happy to
withdraw it.

> When you open up /dev/inotify, you get an fd,
> you read events from it. We could provide write on that fd instead of
> the ioctl() interface. 

I agree completely.

> > As Chris points out, we still need a way to pass the name or path back
> > to userspace when an event occurs, which is the interface I was harping
> > on a few messages back.
> > 
> > It seems we're trying to recreate a variant struct dirent for
> > communicating changes to userspace. Perhaps we can learn something from
> > already trodden ground? Just sayin'.
> 
> Yes the current method of passing the name back to user space is
> definitely sub par. But I don't think passing a full path to user space
> is reasonable, as that would require walking the dirent tree for every
> event. Really the best we can provide user space is the filename/dirname
> (relative to the directory you are currently watching).

Apologies, I was unclear. I should not have said that we would pass the
full path back. In fact, we can't, as the same directory may be mounted
in multiple places. I knew this, but I blame lack of sleep for my slip.

Ray