netfilter: nft_counter: fix reset of counters on 32bit archs

nft_counter_reset() calls u64_stats_add() with a negative value to reset
the counter. This will work on 64bit archs, hence the negative value
added will wrap as a 64bit value which then can wrap the stat counter as
well.

On 32bit archs, the added negative value will wrap as a 32bit value and
_not_ wrapping the stat counter properly. In most cases, this would just
lead to a very large 32bit value being added to the stat counter.

Fix by introducing u64_stats_sub().

Fixes: 4a1d3ac ("netfilter: nft_counter: Use u64_stats_t for statistic.")
Signed-off-by: Anders Grahn <anders.grahn@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

Author: angrq

include/linux/u64_stats_sync.h

@@ -97,6 +97,11 @@ static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
 	local64_add(val, &p->v);
 }
 
+static inline void u64_stats_sub(u64_stats_t *p, s64 val)
+{
+	local64_sub(val, &p->v);
+}
+
 static inline void u64_stats_inc(u64_stats_t *p)
 {
 	local64_inc(&p->v);
@@ -145,6 +150,11 @@ static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
 	p->v += val;
 }
 
+static inline void u64_stats_sub(u64_stats_t *p, s64 val)
+{
+	p->v -= val;
+}
+
 static inline void u64_stats_inc(u64_stats_t *p)
 {
 	p->v++;

net/netfilter/nft_counter.c

@@ -117,8 +117,8 @@ static void nft_counter_reset(struct nft_counter_percpu_priv *priv,
 	nft_sync = this_cpu_ptr(&nft_counter_sync);
 
 	u64_stats_update_begin(nft_sync);
-	u64_stats_add(&this_cpu->packets, -total->packets);
-	u64_stats_add(&this_cpu->bytes, -total->bytes);
+	u64_stats_sub(&this_cpu->packets, total->packets);
+	u64_stats_sub(&this_cpu->bytes, total->bytes);
 	u64_stats_update_end(nft_sync);
 
 	local_bh_enable();