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

From: Al Viro <viro@zeniv.linux.org.uk>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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, greg@kroah.com
Subject: Re: [PATCH 6/9] debugfs: Implement debugfs_create_str()
Date: Sat, 27 Mar 2021 22:24:45 +0000	[thread overview]
Message-ID: <YF+wrUjnGTsH6FGT@zeniv-ca.linux.org.uk> (raw)
In-Reply-To: <20210326103935.183934395@infradead.org>

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.