On Wed, 28 Oct 2020, Tom Rix wrote:
>
> On 10/28/20 4:35 AM, Lukas Bulwahn wrote:
> > 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@xxxxxxxxx>
> > ---
> > 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
>
> > - } else {
> > + else
> > q = qdisc_lookup(dev, TC_H_MAJ(tcm->tcm_parent));
> > - }
> > +
> > if (!q)
> > goto out;
> > cops = q->ops->cl_ops;
>
>
