linux-kernel - Re: [PATCH v2] dns_resolver: assure that dns

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Date:	Sat, 7 Jun 2014 14:42:59 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Manuel Schölling <manuel.schoelling@....de>
cc:	davem@...emloft.net, jeffrey.t.kirsher@...el.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] dns_resolver: assure that dns_query() result is
 null-terminated

On Sat, 7 Jun 2014, Manuel Schölling wrote:

> dns_query() credulously assumes that keys are null-terminated and
> returns a copy of a memory block that is off by one.

No sign-off?  Please read Documentation/SubmittingPatches.

> ---
>  net/dns_resolver/dns_query.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/dns_resolver/dns_query.c b/net/dns_resolver/dns_query.c
> index e7b6d53..84871a2 100644
> --- a/net/dns_resolver/dns_query.c
> +++ b/net/dns_resolver/dns_query.c
> @@ -145,11 +145,11 @@ int dns_query(const char *type, const char *name, size_t namelen,
>  	len = upayload->datalen;
>  
>  	ret = -ENOMEM;
> -	*_result = kmalloc(len + 1, GFP_KERNEL);
> +	*_result = kzalloc(len + 1, GFP_KERNEL);
>  	if (!*_result)
>  		goto put;
>  
> -	memcpy(*_result, upayload->data, len + 1);
> +	memcpy(*_result, upayload->data, len);
>  	if (_expiry)
>  		*_expiry = rkey->expiry;
>  

kzalloc() would be unnecessary overhead (zeroing definitely comes with a 
cost) if you're going to copy to the memory immediately afterwards.  Just 
leave the kmalloc(), do the memcpy() and explicitly zero terminate it 
_result.