Re: -Wsometimes-uninitialized Clang warning in net/tipc/node.c

From: Arnd Bergmann
Date: Thu Mar 21 2019 - 07:45:22 EST

On Wed, Mar 20, 2019 at 9:51 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> On Wed, Mar 20, 2019 at 12:07 PM Nathan Chancellor
> <natechancellor@xxxxxxxxx> wrote:
> The use in tipc_bearer_xmit() isn't even guaranteed to set the in/out
> parameter, so even if the if is taken doesn't guarantee that maddr is
> always initialized before calling tipc_bearer_xmit().

Right, it is only initialized in certain states. It was always
initialized until commit 598411d70f85 ("tipc: make resetting of
links non-atomic"), afterwards only if the link was not reset,
and as of commit 73f646cec354 ("tipc: delay ESTABLISH
state event when link is established") only if it's not
'establishing' or 'reset'.

> At the minimum, we should initialize maddr to NULL. I think we'd
> prefer to risk the possibility of a null pointer dereference to the
> possibility of working with uninitialized memory. To be clear, both
> are bad, but one is easier to spot/debug later than the other.

I disagree with setting it to NULL, given that it is still an obviously
incorrect value. We could add a if(maddr) check before calling
tipc_bearer_xmit(), but I think it would be clearer to check
skb_queue_empty(xmitq)) if that avoids the warning:

diff --git a/net/tipc/node.c b/net/tipc/node.c
index 2dc4919ab23c..147786795e48 100644
--- a/net/tipc/node.c
+++ b/net/tipc/node.c
@@ -844,7 +844,8 @@ static void tipc_node_link_down(struct tipc_node
*n, int bearer_id, bool delete)
if (delete)
tipc_mon_remove_peer(n->net, n->addr, old_bearer_id);
- tipc_bearer_xmit(n->net, bearer_id, &xmitq, maddr);
+ if (skb_queue_empty(xmitq))
+ tipc_bearer_xmit(n->net, bearer_id, &xmitq, maddr);
tipc_sk_rcv(n->net, &le->inputq);

This duplicates the check inside of skb_queue_empty(),
but I don't know if the compiler can see through the
logic behind the inlined function calls.