linux-kernel - Re: [PATCH] net: cls_api: remove unneeded local variable in tc_dump

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Date:   Wed, 28 Oct 2020 16:50:53 +0100 (CET)
From:   Lukas Bulwahn <lukas.bulwahn@...il.com>
To:     Tom Rix <trix@...hat.com>
cc:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [PATCH] net: cls_api: remove unneeded local variable in
 tc_dump_chain()



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