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

From: Nathan Fontenot <[email-masked]>
Date: Tue, 27 Oct 2015 08:51:35 -0500

On 10/27/2015 03:17 AM, Andy Shevchenko wrote:
> 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;

Agreed. I'll send out a v2 of the patch with this update and add a comment
to indicate we do not want to include the trailing '\' character.

-Nathan

> ...
> if (tail > path) {
>>  		if (!parent_path)
>>  			return ERR_PTR(-ENOMEM);
>>  	}