Re: [PATCH] dcache: Translating dentry into pathname without taking rename_lock

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Sep 04, 2013 at 03:05:23PM -0400, Waiman Long wrote:
>  
>  static int prepend_name(char **buffer, int *buflen, struct qstr *name)
>  {
> -	return prepend(buffer, buflen, name->name, name->len);
> +	/*
> +	 * With RCU path tracing, it may race with rename. Use
> +	 * ACCESS_ONCE() to make sure that it is either the old or
> +	 * the new name pointer. The length does not really matter as
> +	 * the sequence number check will eventually catch any ongoing
> +	 * rename operation.
> +	 */
> +	const char *dname = ACCESS_ONCE(name->name);
> +	int   dlen = name->len;
> +
> +	if (unlikely(!dname || !dlen))
> +		return -EINVAL;
> +	return prepend(buffer, buflen, dname, dlen);

NAK.  A race with d_move() can very well leave you with dname pointing into
an object of length smaller than dlen.  You *can* copy it byte-by-byte
and rely on NUL-termination, but you can't rely on length being accurate -
not without having excluded d_move().