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xen-devel
[Xen-devel] RE: [PATCH ixgbe] Don't depend of skb->data for VMDq
>> Occasionally, vmq_alloc_skb() will give us an skb with a nonzero but
>> bogus data pointer. Dereferencing this pointer will then cause a
>> panic.
>That's disturbing. The SKB needs to have a data area (because
>otherwise there's nowhere to put the skb_shinfo, and hence no frags
>list), so the data pointer really ought to be valid (i.e. NULL is also
>bad here).
>
OK, I need to eat some crow here. The patch is correct, but my description is
completely wrong, and is based on false assumptions. I completely forgot that
skb_shinfo hangs off the data area, and assumed that "no packet data in
skb->data" was equivalent to "skb->data is NULL". Completely, totally, wrong.
As you noted, skb->data has got to be valid for the skb to be useful at all.
The use of "if (skb->data)" is completely incorrect in any context. I should
know this -- I've stared at skbuff.h long enough over the years.
In my defense, the patch went through two internal review cycles and nobody
else noticed it either. So I feel a little better. It's a use case that just
doesn't come up that often.
Please keep the patch, but ignore (or change) the description. Sorry about the
confusion.
-Mitch
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