On Sat, Jun 06, 2015 at 08:16:50PM -0700, Alexander Duyck wrote:
>On 06/06/2015 05:05 AM, 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 <fir...@gmail.com>
>>>---
>>> 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 <eduma...@google.com>
>>
>>Thanks.
>
>kv->kv isn't a dereference. kv is an array contained within the tnode.
>Below is the layout of struct tnode:
>
>|
>struct tnode{
> struct rcu_head rcu;
> t_key empty_children; /* KEYLENGTH bits needed */
> t_key full_children; /* KEYLENGTH bits needed */
> struct key_vector __rcu*parent;
> struct key_vector kv[1];
>#define tn_bits kv[0].bits
>};|
>
>
>As such kv->kv doesn't actually dereference anything, it is simply a means
>for getting the offset to the array from the pointer kv. It should be
>equivalent to &kv->kv[0] which last I knew was how the offsetof macro
>basically worked and it uses a NULL pointer. The first actual derference
>occurs at "l->key = key" which should be safe since writing to a memory
>location has side effects so the compiler cannot reorder the writes before
>the !kv check.
>
>The patch itself is fine. However what it is fixing is the smatch false
>positive, not a null pointer deference. As such you may need to update the
>comments to reflect that.
Thanks Alex for firguring out my false cognition!
I will update the patch with new comments.
Firo
>
>- Alex
>
>
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