net: sched: Annotate struct tc_pedit with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct tc_pedit.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

Link: https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci [1]
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tc_act/tc_pedit.h

@@ -62,7 +62,7 @@ struct tc_pedit_sel {
 	tc_gen;
 	unsigned char           nkeys;
 	unsigned char           flags;
-	struct tc_pedit_key     keys[0];
+	struct tc_pedit_key     keys[] __counted_by(nkeys);
 };
 
 #define tc_pedit tc_pedit_sel

net/sched/act_pedit.c

@@ -515,11 +515,11 @@ static int tcf_pedit_dump(struct sk_buff *skb, struct tc_action *a,
 		spin_unlock_bh(&p->tcf_lock);
 		return -ENOBUFS;
 	}
+	opt->nkeys = parms->tcfp_nkeys;
 
 	memcpy(opt->keys, parms->tcfp_keys,
 	       flex_array_size(opt, keys, parms->tcfp_nkeys));
 	opt->index = p->tcf_index;
-	opt->nkeys = parms->tcfp_nkeys;
 	opt->flags = parms->tcfp_flags;
 	opt->action = p->tcf_action;
 	opt->refcnt = refcount_read(&p->tcf_refcnt) - ref;