On Tue, Sep 16, 2014 at 07:57:27PM +0100, Martin Townsend wrote:
> Hi Alex,
>
> On 16/09/14 18:38, Alexander Aring wrote:
> >Hi Martin,
> >
> >On Tue, Sep 16, 2014 at 03:44:43PM +0100, Martin Townsend wrote:
> >>I would like to keep freeing skb's out of process_data as process_data will
> >>become something like iphc_decompress_hdr and it would be good if that's
> >>all it did. Otherwise I feel we are going to put a constraint on all
> >>future header decompression routines in that they must free the skb on
> >>error. I think it would be better to defer this so on error you might want
> >>to try something else with the skb, maybe not but at least the option is
> >>there.
> >>So how about
> >>
> >> struct sk_buff * ret_skb;
> >> switch (skb->data[0] & 0xe0) {
> >> case LOWPAN_DISPATCH_IPHC: /* ipv6 datagram */
> >> ret_skb = process_data(skb, &hdr);
> >> if (IS_ERR(ret_skb))
> >> goto drop_skb;
> >> else
> >> skb = ret_skb;
> >> break;
> >>
> >>I know we currently have 3 calls to process_data so it will look fairly
> >>ugly in this patch but in my next patch to fix lowpan_rcv to handle
> >>uncompressed IPv6 packets that are fragmented there will only be one call
> >>to process_data so it won't look so bad. You could even wrap it in a macro
> >>but I'm not a fan of this as they can obfuscate the code a bit.
> >>
> >>Thoughts?
> >>
> >sorry, I can't follow how this solve the issue if the "parameter skb" is
> >already consumed or not. If process_data returns a error before
> >parameter consume, then we should run kfree_skb(parameter_skb), if it's
> >afterwards we should do nothing. Point is we don't know that there. I
> >suppose if we do consume_skb and refcount reach 0 the parameter_skb
> >becomes a dangling pointer.
> >
> >- Alex
>
> process_data never consumes the skb, it may copy_expand and then consume the
> old one so it will either return an error or an skb that contains the
> uncompressed ipv6 header. By calling process_data using a different sk_buff
> pointer (ret_skb) that the parameter we can check this for an error and if
> so goto drop_skb which will kfree_skb(skb) which is fine as skb is still
are you sure it's still valid? I don't get it. :-(
> valid. if ret_skb is good and we assign to skb and carry on to the
> function that passes the skb up the stack, lowpan_give_skb_to_devices,
> which deals with either consuming or kfreeing.
>
> Or am I missing something?
>
I make another c example, hopeful more correct than the last one:
char *foo(char *skb)
{
char *new;
if (some_error_before_consume)
return ERR_PTR(-EINVAL); /* here we need to do a free(skb) */
/* UDP expand */
new = expand(skb, 16);
if (!new)
return ERR_PTR(-ENOMEM);
consume(skb); /* parameter skb becomes dangling pointer */
skb = new; /* doesn't rescue it, it is different than skb from caller
function
at this point, the skb_inout had rescue it, because it
was a pointer
of pointer */
/* IPv6 expand */
new = expand(skb, 40);
if (!new) /* some error after a consume(skb), will crash at drop_skb
label */
return ERR_PTR(-ENOMEM);
consume(skb);
skb = new;
return skb;
}
int main(int argc, const char *argv[])
{
char *local_buf = malloc(42);
char *skb;
local_skb = foo(skb);
if (IS_ERR(local_skb))
goto drop_skb;
else
skb = local_skb; /* ??? */
return NET_RX_SUCCESS;
drop_skb:
free(skb); /* dangling pointer will be freed if foo called consume(skb)
it's correct when foo returned on
some_error_before_consume
condition. */
drop:
return NET_RX_DROP;
}
I don't know what "skb = local_skb" did now there.
- Alex
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