Re: [PATCH v2 6/9] debugfs: Implement debugfs_create_str()

From: Peter Zijlstra <peterz@infradead.org>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Greg KH <greg@kroah.com>,
	mingo@kernel.org, mgorman@suse.de, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com, bristot@redhat.com,
	joshdon@google.com, valentin.schneider@arm.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 6/9] debugfs: Implement debugfs_create_str()
Date: Fri, 26 Mar 2021 15:22:30 +0100	[thread overview]
Message-ID: <20210326142230.GJ4746@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <70594935-18e6-0791-52f9-0448adf37155@rasmusvillemoes.dk>

On Fri, Mar 26, 2021 at 01:53:59PM +0100, Rasmus Villemoes wrote:
> On 26/03/2021 12.38, 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();
> 
> As noted (accidentally off-list), this is broken. I think you want this
> on top
> 
> - len = strlen(str) + 1;
> + len = strlen(str);

  kmalloc(len + 2, ...);

> - strncpy(copy, str, len);
> + memcpy(copy, str, len);
>   copy[len] = '\n';
> - copy[len+1] = '\0';

I'll go with strscpy() I tihnk, something like:

	len = strscpy(copy, str, len);
	if (len < 0)
		return len;
	copy[len] = '\n';
	copy[len + 1] = '\0';

> > +EXPORT_SYMBOL_GPL(debugfs_read_file_str);
> 
> Why?

Copy-pasta from debugfs_*_bool(). This thing seems to export everything
and I figured I'd go along with that.