linux-kernel - Re: [PATCH 6/9] debugfs: Implement debugfs_create

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Date:   Sat, 27 Mar 2021 20:33:15 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        mgorman@...e.de, juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, bsegall@...gle.com, bristot@...hat.com,
        joshdon@...gle.com, valentin.schneider@....com,
        linux-kernel@...r.kernel.org, greg@...ah.com
Subject: Re: [PATCH 6/9] debugfs: Implement debugfs_create_str()

On Sat, 27 Mar 2021 22:24:45 +0000
Al Viro <viro@...iv.linux.org.uk> wrote:

> On Fri, Mar 26, 2021 at 11:33:58AM +0100, Peter Zijlstra wrote:
> 
> > +again:
> > +	rcu_read_lock();
> > +	str = rcu_dereference(*(char **)file->private_data);
> > +	len = strlen(str) + 1;
> > +
> > +	if (!copy || copy_len < len) {
> > +		rcu_read_unlock();
> > +		kfree(copy);
> > +		copy = kmalloc(len + 1, GFP_KERNEL);
> > +		if (!copy) {
> > +			debugfs_file_put(dentry);
> > +			return -ENOMEM;
> > +		}
> > +		copy_len = len;
> > +		goto again;
> > +	}
> > +
> > +	strncpy(copy, str, len);
> > +	copy[len] = '\n';
> > +	copy[len+1] = '\0';
> > +	rcu_read_unlock();  
> 
> *Ow*
> 
> 	If the string can't change under you, what is RCU use about?
> And if it can, any use of string functions is asking for serious
> trouble; they are *not* guaranteed to be safe when any of the strings
> involved might be modified under them.

Just from looking at the above, RCU isn't protecting that the string
can change under you, but the pointer to file->private_data can.

	str = rcu_dereference(*(char **)file->private_data);

That's just getting a pointer to the string. While under rcu, the value
of that string wont change nor will it be free. But file->private_data
might change, and it might free its old value, but will do so after a
RCU grace period (which is why the above has rcu_read_lock).

What the above looks like to me is a way to copy that string safely,
without worrying that it will be freed underneath you. But there's no
worry that it will change.

-- Steve