Subject: Re: [PATCH] fib_trie: Fix potential null pointer dereference

From: Firo Yang <[email-masked]>
Date: Sun, 7 Jun 2015 21:19:24 +0800

On Sun, Jun 07, 2015 at 08:36:45AM +0200, Julia Lawall wrote:
>
>
>On Sun, 7 Jun 2015, Firo Yang wrote:
>
>> On Sat, Jun 06, 2015 at 05:05:04AM -0700, Eric Dumazet wrote:
>> >On Sat, 2015-06-06 at 19:35 +0800, Firo Yang wrote:
>> >> A smatch warning.
>> >> When kmem_cache_alloc() failed to alloc memory, a null pointer
>> >> will be returned. Redeference null pointer will generate
>> >
>> >
>> >Dereferencing a null pointer will crash.
>> >
>> >> an unnecessary oops. So, use it after check.
>> >> 
>> >> Signed-off-by: Firo Yang <[email-masked]>
>> >> ---
>> >>  net/ipv4/fib_trie.c | 3 ++-
>> >>  1 file changed, 2 insertions(+), 1 deletion(-)
>> >> 
>> >> diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
>> >> index 01bce15..34094c7 100644
>> >> --- a/net/ipv4/fib_trie.c
>> >> +++ b/net/ipv4/fib_trie.c
>> >> @@ -326,12 +326,13 @@ static inline void empty_child_dec(struct key_vector *n)
>> >>  static struct key_vector *leaf_new(t_key key, struct fib_alias *fa)
>> >>  {
>> >>  	struct tnode *kv = kmem_cache_alloc(trie_leaf_kmem, GFP_KERNEL);
>> >> -	struct key_vector *l = kv->kv;
>> >> +	struct key_vector *l;
>> >>  
>> >>  	if (!kv)
>> >>  		return NULL;
>> >>  
>> >>  	/* initialize key vector */
>> >> +	l = kv->kv;
>> >>  	l->key = key;
>> >>  	l->pos = 0;
>> >>  	l->bits = 0;
>> >
>> >Fixes: dc35dbeda3e0 ("fib_trie: Add tnode struct as a container for fields not needed in key_vector")
>> >Acked-by: Eric Dumazet <[email-masked]>
>> >
>> >Thanks.
>> 
>> Hi Eric,
>> Please discard this useless patch figured out by Alexander Duyck. 
>> I will send a patch to Smatch for eliminating the negative warning.
>
>I think that many people would make the same mistake when looking at the 
>code.  The change doesn't seem to hurt anything?
Actually, yes. But it does imporve nothing but coding style.
I think Alexander's code style make function more compact is also a
good code style. So, just keep it original.

Firo
>julia

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