linux-kernel - Re: [PATCH v2 4/7] tracefs: dentry lookup crapectomy

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Date: Thu, 1 Feb 2024 03:02:05 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Christian Brauner <brauner@...nel.org>,
	Ajay Kaher <ajay.kaher@...adcom.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH v2 4/7] tracefs: dentry lookup crapectomy

On Wed, Jan 31, 2024 at 09:26:42PM -0500, Steven Rostedt wrote:

> > Huh?  Just return NULL and be done with that - you'll get an
> > unhashed negative dentry and let the caller turn that into
> > -ENOENT...
> 
> We had a problem here with just returning NULL. It leaves the negative
> dentry around and doesn't get refreshed.

Why would that dentry stick around?  And how would anyone find
it, anyway, when it's not hashed?

> I did this:
> 
>  # cd /sys/kernel/tracing
>  # ls events/kprobes/sched/
> ls: cannot access 'events/kprobes/sched/': No such file or directory
>  # echo 'p:sched schedule' >> kprobe_events
>  # ls events/kprobes/sched/
> ls: cannot access 'events/kprobes/sched/': No such file or directory
> 
> When it should have been:
> 
>  # ls events/kprobes/sched/
> enable  filter  format  hist  hist_debug  id  inject  trigger
> 
> Leaving the negative dentry there will have it fail when the directory
> exists the next time.

Then you have something very deeply fucked up.  NULL or ERR_PTR(-ENOENT)
from ->lookup() in the last component of open() would do exactly the
same thing: dput() whatever had been passed to ->lookup() and fail
open(2) with -ENOENT.