Hi Jeff, > On Oct 30, 2024, at 5:23 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote: [...] >> If the subtree is all in the same file system, we can attach fanotify to >> the whole file system, and then use some dget_parent() and follow_up() >> to walk up the directory tree in the fastpath handler. However, if there >> are other mount points in the subtree, we will need more logic to handle >> these mount points. >> > > My 2 cents... > > I'd just confine it to a single vfsmount. If you want to monitor in > several submounts, then you need to add new fanotify watches. > > Alternately, maybe there is some way to designate that an entire > vfsmount is a child of a watched (or ignored) directory? > >> @Christian, I would like to know your thoughts on this (walking up the >> directory tree in fanotify fastpath handler). It can be expensive for >> very very deep subtree. >> > > I'm not Christian, but I'll make the case for it. It's basically a > bunch of pointer chasing. That's probably not "cheap", but if you can > do it under RCU it might not be too awful. It might still suck with > really deep paths, but this is a sample module. It's not expected that > everyone will want to use it anyway. Thanks for the suggestion! I will try to do it under RCU. > >> How should we pass in the subtree? I guess we can just use full path in >> a string as the argument. >> > > I'd stay away from string parsing. How about this instead? > > Allow a process to open a directory fd, and then hand that fd to an > fanotify ioctl that says that you want to ignore everything that has > that directory as an ancestor. Or, maybe make it so that you only watch > dentries that have that directory as an ancestor? I'm not sure what > makes the most sense. Yes, directory fd is another option. Currently, the "attach to group" function only takes a string as input. I guess it makes sense to allow taking a fd, or maybe we should allow any random format (pass in a pointer to a structure. Let me give it a try. Thanks again! Song > > Then, when you get a dentry-based event, you just walk the d_parent > pointers back up to the root of the vfsmount. If one of them matches > the dentry in your fd then you're done. If you hit the root of the > vfsmount, then you're also done (and know that it's not a child of that > dentry). > -- > Jeff Layton <jlayton@xxxxxxxxxx>
