Subject: Re: [PATCH] powerpc/pseries: Correct string length in pseries_of_derive_parent()

From: Andy Shevchenko <[email-masked]>
Date: Tue, 27 Oct 2015 10:17:35 +0200

On Mon, 2015-10-26 at 14:33 -0500, Nathan Fontenot wrote:
> Commit a030e1e4bbd085bbcfd0a23f8d355fcd41f39bed made a change to use
> kstrndup() instead of kmalloc() + strlcpy() in
> pseries_of_derive_parent()
> which introduces a subtle change in the parent path name generated.
> The kstrndup() routine will copy n characters followed by a
> terminating null,
> whereas strlcpy() will copy n-1 characters and add a terminating
> null.

Nice catch!
One comment below.

> 
> This slight difference results in having a parent path that includes
> the
> trailing '/' character, i.e. "/cpus/" vs. "/cpus". This then causes
> the
> subsequent call to of_find_node_by_path() to fail, and in the case of
> DLPAR add operations, the DLPAR request fails.
> 
> This patch reduces the total length of the string to copy in kstrndup
> by 1
> so we no longer copy the trailing '/'.
> 
> Signed-off-by: Nathan Fontenot <[email-masked]>
> ---
>  arch/powerpc/platforms/pseries/of_helpers.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/of_helpers.c
> b/arch/powerpc/platforms/pseries/of_helpers.c
> index 4417afe..6d90378 100644
> --- a/arch/powerpc/platforms/pseries/of_helpers.c
> +++ b/arch/powerpc/platforms/pseries/of_helpers.c
> @@ -24,7 +24,7 @@ struct device_node *pseries_of_derive_parent(const
> char *path)
>  		return ERR_PTR(-EINVAL);
>  
>  	if (tail > path + 1) {
> -		parent_path = kstrndup(path, tail - path,
> GFP_KERNEL);
> +		parent_path = kstrndup(path, (tail - 1) - path,
> GFP_KERNEL);

Since  previous line has (tail > path + 1) which is equivalent to (tail
- 1 > path), can we amend both?

For example (might be better, but first comes to my mind)

const char *tail = kbasename(path) - 1;

...

if (tail > path) {

>  		if (!parent_path)
>  			return ERR_PTR(-ENOMEM);
>  	}

-- 
Andy Shevchenko <[email-masked]>
Intel Finland Oy