[PATCH] net/sch_generic.h: use sizeof_member() and get rid of unused variable

[PATCH] net/sch_generic.h: use sizeof_member() and get rid of unused variable

[Antonio Quartulli]

Compiling with -Wunused triggers the following warning:

./include/net/sch_generic.h: In function ‘qdisc_cb_private_validate’:
./include/net/sch_generic.h:464:23: warning: unused variable ‘qcb’ [-Wunused-variable]
  464 |  struct qdisc_skb_cb *qcb;
      |                       ^~~

as the qcb variable is only used to compute the sizeof one of its members.

Get rid of the warning by using the provided sizeof_member() macro
and avoid having a variable at all.

At the same time use sizeof_member() also for computing the sizeof
skb->cb, thus avoiding ‘qdisc_cb_private_validate’ to have an skb
argument at all.

Cc: Toke Høiland-Jørgensen <toke@toke.dk>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>

---

Affected code has been compile-tested on x86_64

 include/net/codel_qdisc.h | 2 +-
 include/net/pie.h         | 2 +-
 include/net/sch_generic.h | 9 ++++-----
 net/sched/sch_cake.c      | 2 +-
 net/sched/sch_choke.c     | 2 +-
 net/sched/sch_fq.c        | 2 +-
 net/sched/sch_netem.c     | 2 +-
 net/sched/sch_sfb.c       | 2 +-
 8 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/include/net/codel_qdisc.h b/include/net/codel_qdisc.h
index 098630f83a55..c59d17de4ef3 100644
--- a/include/net/codel_qdisc.h
+++ b/include/net/codel_qdisc.h
@@ -57,7 +57,7 @@ struct codel_skb_cb {
 
 static struct codel_skb_cb *get_codel_cb(const struct sk_buff *skb)
 {
-	qdisc_cb_private_validate(skb, sizeof(struct codel_skb_cb));
+	qdisc_cb_private_validate(sizeof(struct codel_skb_cb));
 	return (struct codel_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 
diff --git a/include/net/pie.h b/include/net/pie.h
index 3fe2361e03b4..c15fe3032ad0 100644
--- a/include/net/pie.h
+++ b/include/net/pie.h
@@ -109,7 +109,7 @@ static inline void pie_vars_init(struct pie_vars *vars)
 
 static inline struct pie_skb_cb *get_pie_cb(const struct sk_buff *skb)
 {
-	qdisc_cb_private_validate(skb, sizeof(struct pie_skb_cb));
+	qdisc_cb_private_validate(sizeof(struct pie_skb_cb));
 	return (struct pie_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index c510b03b9751..8e1f7a0d7572 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -459,12 +459,11 @@ static inline bool lockdep_tcf_proto_is_locked(struct tcf_proto *tp)
 #define tcf_proto_dereference(p, tp)					\
 	rcu_dereference_protected(p, lockdep_tcf_proto_is_locked(tp))
 
-static inline void qdisc_cb_private_validate(const struct sk_buff *skb, int sz)
+static inline void qdisc_cb_private_validate(int sz)
 {
-	struct qdisc_skb_cb *qcb;
-
-	BUILD_BUG_ON(sizeof(skb->cb) < offsetof(struct qdisc_skb_cb, data) + sz);
-	BUILD_BUG_ON(sizeof(qcb->data) < sz);
+	BUILD_BUG_ON(sizeof_field(struct sk_buff, cb) <
+		     offsetof(struct qdisc_skb_cb, data) + sz);
+	BUILD_BUG_ON(sizeof_field(struct qdisc_skb_cb, data) < sz);
 }
 
 static inline int qdisc_qlen_cpu(const struct Qdisc *q)
diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 1496e87cd07b..2800551b7465 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -281,7 +281,7 @@ static u64 us_to_ns(u64 us)
 
 static struct cobalt_skb_cb *get_cobalt_cb(const struct sk_buff *skb)
 {
-	qdisc_cb_private_validate(skb, sizeof(struct cobalt_skb_cb));
+	qdisc_cb_private_validate(sizeof(struct cobalt_skb_cb));
 	return (struct cobalt_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 
diff --git a/net/sched/sch_choke.c b/net/sched/sch_choke.c
index bd618b00d319..6badd324df43 100644
--- a/net/sched/sch_choke.c
+++ b/net/sched/sch_choke.c
@@ -137,7 +137,7 @@ struct choke_skb_cb {
 
 static inline struct choke_skb_cb *choke_skb_cb(const struct sk_buff *skb)
 {
-	qdisc_cb_private_validate(skb, sizeof(struct choke_skb_cb));
+	qdisc_cb_private_validate(sizeof(struct choke_skb_cb));
 	return (struct choke_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 
diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 8f06a808c59a..0c2497509a3d 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -56,7 +56,7 @@ struct fq_skb_cb {
 
 static inline struct fq_skb_cb *fq_skb_cb(struct sk_buff *skb)
 {
-	qdisc_cb_private_validate(skb, sizeof(struct fq_skb_cb));
+	qdisc_cb_private_validate(sizeof(struct fq_skb_cb));
 	return (struct fq_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 84f82771cdf5..2b930d928c5c 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -161,7 +161,7 @@ struct netem_skb_cb {
 static inline struct netem_skb_cb *netem_skb_cb(struct sk_buff *skb)
 {
 	/* we assume we can use skb next/prev/tstamp as storage for rb_node */
-	qdisc_cb_private_validate(skb, sizeof(struct netem_skb_cb));
+	qdisc_cb_private_validate(sizeof(struct netem_skb_cb));
 	return (struct netem_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 
diff --git a/net/sched/sch_sfb.c b/net/sched/sch_sfb.c
index 4074c50ac3d7..ffaf145a7b05 100644
--- a/net/sched/sch_sfb.c
+++ b/net/sched/sch_sfb.c
@@ -91,7 +91,7 @@ struct sfb_skb_cb {
 
 static inline struct sfb_skb_cb *sfb_skb_cb(const struct sk_buff *skb)
 {
-	qdisc_cb_private_validate(skb, sizeof(struct sfb_skb_cb));
+	qdisc_cb_private_validate(sizeof(struct sfb_skb_cb));
 	return (struct sfb_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 
-- 
2.26.2

Re: [PATCH] net/sch_generic.h: use sizeof_member() and get rid of unused variable

[David Miller]

From: Antonio Quartulli <a@unstable.cc>
Date: Tue, 19 May 2020 11:13:33 +0200

> Compiling with -Wunused triggers the following warning:
> 
> ./include/net/sch_generic.h: In function ‘qdisc_cb_private_validate’:
> ./include/net/sch_generic.h:464:23: warning: unused variable ‘qcb’ [-Wunused-variable]
>   464 |  struct qdisc_skb_cb *qcb;
>       |                       ^~~
> 
> as the qcb variable is only used to compute the sizeof one of its members.

It's referenced in the code, therefore it is not "unused".

If in some configuration BUILD_BUG_ON() does not reference it's arguments,
that's the bug that needs to be fixed.

Re: [PATCH] net/sch_generic.h: use sizeof_member() and get rid of unused variable

[Antonio Quartulli]

Hi David,

On 20/05/2020 00:40, David Miller wrote:
> From: Antonio Quartulli <a@unstable.cc>
> Date: Tue, 19 May 2020 11:13:33 +0200
> 
>> Compiling with -Wunused triggers the following warning:
>>
>> ./include/net/sch_generic.h: In function ‘qdisc_cb_private_validate’:
>> ./include/net/sch_generic.h:464:23: warning: unused variable ‘qcb’ [-Wunused-variable]
>>   464 |  struct qdisc_skb_cb *qcb;
>>       |                       ^~~
>>
>> as the qcb variable is only used to compute the sizeof one of its members.
> 
> It's referenced in the code, therefore it is not "unused".

True.

> 
> If in some configuration BUILD_BUG_ON() does not reference it's arguments,
> that's the bug that needs to be fixed.
> 

I don't think it's BUILD_BUG_ON()'s fault, because qcb->data is passed
to sizeof() first.

My best guess is that gcc is somewhat optimizing the sizeof(gcb->data)
and thus leaving the gcb variable unused.


This said, I think it's better for the code style (and for the compiler)
if we used sizeof_member().

Should I resend the patch with a commit message that does not mention
the "unused" warning?


Thanks a lot.
Best Regards,


-- 
Antonio Quartulli

Re: [PATCH] net/sch_generic.h: use sizeof_member() and get rid of unused variable

[David Miller]

From: Antonio Quartulli <a@unstable.cc>
Date: Wed, 20 May 2020 10:39:33 +0200

> I don't think it's BUILD_BUG_ON()'s fault, because qcb->data is passed
> to sizeof() first.
> 
> My best guess is that gcc is somewhat optimizing the sizeof(gcb->data)
> and thus leaving the gcb variable unused.

If you remove the argument from the function but leave the BUILD_BUG_ON()
calls the same, the compilation will fail.

Any such optimization is therefore unreasonable.

The variable is used otherwise compilation would not fail when you
remove it right?

Re: [PATCH] net/sch_generic.h: use sizeof_member() and get rid of unused variable

[Antonio Quartulli]

On 20/05/2020 20:17, David Miller wrote:
> From: Antonio Quartulli <a@unstable.cc>
> Date: Wed, 20 May 2020 10:39:33 +0200
> 
>> I don't think it's BUILD_BUG_ON()'s fault, because qcb->data is passed
>> to sizeof() first.
>>
>> My best guess is that gcc is somewhat optimizing the sizeof(gcb->data)
>> and thus leaving the gcb variable unused.
> 
> If you remove the argument from the function but leave the BUILD_BUG_ON()
> calls the same, the compilation will fail.
> 
> Any such optimization is therefore unreasonable.
> 
> The variable is used otherwise compilation would not fail when you
> remove it right?

You're correct.
I guess this should be reported to gcc then.


Regards,

-- 
Antonio Quartulli