netdev - RE: [PATCH net-next] netdevsim: correctly check return value of debugfs_create

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Date:   Wed, 20 Dec 2017 09:38:52 +0900
From:   "Prashant Bhole" <bhole_prashant_q7@....ntt.co.jp>
To:     "'Jakub Kicinski'" <jakub.kicinski@...ronome.com>
Cc:     "'David Miller'" <davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next] netdevsim: correctly check return value of debugfs_create_dir

> From: Jakub Kicinski [mailto:jakub.kicinski@...ronome.com]
> 
> On Wed, 20 Dec 2017 09:16:01 +0900, Prashant Bhole wrote:
> > > From: David Miller [mailto:davem@...emloft.net]
> > >
> > > From: "Prashant Bhole" <bhole_prashant_q7@....ntt.co.jp>
> > > Date: Tue, 19 Dec 2017 13:45:47 +0900
> > >
> > > > I tried to evaluate whether fixing return value of
> > > > debugfs_create_dir() (and
> > > > friends) will be useful or not because it has not been changed
> > > > since very long time. Now I am not much convinced about changing
this api.
> > > >
> > > > Important and possible error codes could be -EEXIST and -ENOMEM.
> > > > Suppose -EEXIST is returned, IMO the directory shouldn't exists in
> > > > the first place because it is specific to particular module. Also,
> > > > there is no point in creating file in such directory, because
> > > > directory owner (creator) might remove it too. This means there
> > > > are less chances that api change will be useful. Please let me know
your
> opinion on it.
> > > >
> > > > If you are ok with above explanation, shall I submit v2 for this
patch?
> > >
> > > Well, something is seriously wrong if the directory exists already.
> > >
> > > It could be that two netdevsim modules, independantly compiled, are
> > > trying
> > to
> > > be loaded.
> > >
> > > Wouldn't it clearly be desirable to fail and not load the module in
> > > that
> > case?
> > >
> > > This is why I think ignoring debugfs errors is foolish.
> >
> > Right. I am planning to do following (quoting previous mail), In
> > debugfs error will not be ignored in modules load.
> > -----------
> > Dave,
> > Thanks for comments. I will try to fix error handling in netdevsim
first.
> >
> > Jakub,
> > Let's decide with an example. The typical directory structure for
> > netdevsim interface is as below:
> > /sys/kernel/debug/netdevsim/sim0/bpf_bound_progs/
> > Please let me know if you are ok with following:
> >
> > 1) If debugfs_create_dir() fails in module_init, let's keep it fatal
> > error with corrected condition:
> > +	if (IS_ERR_OR_NULL(nsim_ddir))
> > +		return -ENOMEM;
> 
> Ack.
> 
> > 2) In case sim0 or bpf_bound_progs are  fail to create, we need to add
> > checks before creating any file in them.
> 
> What do you mean by "check before"?  Checking if creation of each file
fails or
> not, or something different?

For example:
I will check if state->ddir is not NULL before creating files in it.

if (state->ddir) {
	debugfs_create_u32("id", 0400, state->ddir, &prog->aux->id);
	debugfs_create_file("state", 0400, state->ddir,
			    &state->state, &nsim_bpf_string_fops);
	debugfs_create_bool("loaded", 0400, state->ddir, &state->is_loaded);
}

-Prashant