On Wed, 28 Oct 2020, Tom Rix wrote:

On 10/28/20 4:35 AM, Lukas Bulwahn wrote:

quoted

make clang-analyzer on x86_64 defconfig caught my attention with:

net/sched/cls_api.c:2964:3: warning: Value stored to 'parent' is never read
  [clang-analyzer-deadcode.DeadStores]
                parent = 0;
                ^

net/sched/cls_api.c:2977:4: warning: Value stored to 'parent' is never read
  [clang-analyzer-deadcode.DeadStores]
                        parent = q->handle;
                        ^

Commit 32a4f5ecd738 ("net: sched: introduce chain object to uapi")
introduced tc_dump_chain() and this initial implementation already
contained these unneeded dead stores.

Simplify the code to make clang-analyzer happy.

As compilers will detect these unneeded assignments and optimize this
anyway, the resulting binary is identical before and after this change.

No functional change. No change in object code.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
---
applies cleanly on current master and next-20201028

Jamal, Cong, Jiri, please ack.
David, Jakub, please pick this minor non-urgent clean-up patch.

 net/sched/cls_api.c | 16 +++-------------
 1 file changed, 3 insertions(+), 13 deletions(-)

diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index faeabff283a2..8ce830ca5f92 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c

@@ -2940,7 +2940,6 @@ static int tc_dump_chain(struct sk_buff *skb, struct netlink_callback *cb)
 	struct tcf_chain *chain;
 	long index_start;
 	long index;
-	u32 parent;
 	int err;
 
 	if (nlmsg_len(cb->nlh) < sizeof(*tcm))

@@ -2955,13 +2954,6 @@ static int tc_dump_chain(struct sk_buff *skb, struct netlink_callback *cb)
 		block = tcf_block_refcnt_get(net, tcm->tcm_block_index);
 		if (!block)
 			goto out;
-		/* If we work with block index, q is NULL and parent value
-		 * will never be used in the following code. The check
-		 * in tcf_fill_node prevents it. However, compiler does not
-		 * see that far, so set parent to zero to silence the warning
-		 * about parent being uninitialized.
-		 */
-		parent = 0;
 	} else {
 		const struct Qdisc_class_ops *cops;
 		struct net_device *dev;

@@ -2971,13 +2963,11 @@ static int tc_dump_chain(struct sk_buff *skb, struct netlink_callback *cb)
 		if (!dev)
 			return skb->len;
 
-		parent = tcm->tcm_parent;
-		if (!parent) {
+		if (!tcm->tcm_parent)
 			q = dev->qdisc;
-			parent = q->handle;

This looks like a an unused error handler.

and the later call to

if (TC_H_MIN(tcm->tcm_parent)

maybe should be

if (TC_H_MIN(parent))

so I am skeptical that this change is ok because the code around it looks buggy.

Maybe that is the case.

Certainly the comment above about being uninitialized is outdated as 
parent is not used in tc_chain_fill_node().

I had another look and I noticed a copy of this same pattern (with the 
same comment) in tc_dump_tfilter(), but it seems that the two copies have 
somehow diverged over time. Certainly, something is fishy here.

I guess it needs some more digging in the code...

Lukas

Tom

quoted

-		} else {
+		else
 			q = qdisc_lookup(dev, TC_H_MAJ(tcm->tcm_parent));
-		}
+
 		if (!q)
 			goto out;
 		cops = q->ops->cl_ops;