Re: [PATCH] tracefs/eventfs: Use root and instance inodes as default ownership

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 8 Jan 2024 12:04:54 +0100
Christian Brauner <brauner@kernel.org> wrote:

> > > IOW, the inode_permission() in lookup_one_len() that eventfs does is
> > > redundant and just wrong.  
> > 
> > I don't think so.  
> 
> I'm very well aware that the dentries and inode aren't created during
> mkdir but the completely directory layout is determined. You're just
> splicing in dentries and inodes during lookup and readdir.
> 
> If mkdir /sys/kernel/tracing/instances/foo has succeeded and you later
> do a lookup/readdir on
> 
> ls -al /sys/kernel/tracing/instances/foo/events
> 
> Why should the creation of the dentries and inodes ever fail due to a
> permission failure?

They shouldn't.

> The vfs did already verify that you had the required
> permissions to list entries in that directory. Why should filling up
> /sys/kernel/tracing/instances/foo/events ever fail then? It shouldn't
> That tracefs instance would be half-functional. And again, right now
> that inode_permission() check cannot even fail.

And it shouldn't. But without dentries and inodes, how does VFS know what
is allowed to open the files?

-- Steve