[PATCH net] ipmr, ip6mr: annotate data-races in vif_seq_show()

[Linkui Xiao]

From: Linkui Xiao <xiaolinkui@xxxxxxxxxx>

ipmr_vif_seq_show() and ip6mr_vif_seq_show() read vif->bytes_in,
vif->pkt_in, vif->bytes_out and vif->pkt_out with only rcu_read_lock()
held: since commit b96ef16d2f83 ("ipmr: convert /proc handlers to
rcu_read_lock()") both seq_start helpers are annotated __acquires(RCU)
and no longer take mrt_lock.

Those counters are updated from softirq context and the writers already
use WRITE_ONCE(): ipmr_prepare_xmit() and ip_mr_forward() on the IPv4
side, ip6mr_prepare_xmit() and ip6_mr_forward() on the IPv6 side. The
other lockless readers use READ_ONCE() as well - ipmr_ioctl(),
ipmr_compat_ioctl(), ipmr_fill_vif(), ip6mr_ioctl() and
ip6mr_compat_ioctl().

The two vif_seq_show() helpers are the only remaining bare readers, so
KCSAN flags them and the compiler is free to tear or reload the values
while the /proc/net/ip_mr_vif and /proc/net/ip6_mr_vif lines are being
formatted. Annotate them like the other readers; these are plain
statistics, no locking is needed.

Fixes: b96ef16d2f83 ("ipmr: convert /proc handlers to rcu_read_lock()")
Assisted-by: Qwen:Qwen3.8-max
Signed-off-by: Linkui Xiao <xiaolinkui@xxxxxxxxxx>
---
 net/ipv4/ipmr.c  | 5 +++--
 net/ipv6/ip6mr.c | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index e5f2b1c6150d..36d1b0410a08 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -3178,8 +3178,9 @@ static int ipmr_vif_seq_show(struct seq_file *seq, void *v)
 		seq_printf(seq,
 			   "%2td %-10s %8ld %7ld  %8ld %7ld %05X %08X %08X\n",
 			   vif - mrt->vif_table,
-			   name, vif->bytes_in, vif->pkt_in,
-			   vif->bytes_out, vif->pkt_out,
+			   name,
+			   READ_ONCE(vif->bytes_in), READ_ONCE(vif->pkt_in),
+			   READ_ONCE(vif->bytes_out), READ_ONCE(vif->pkt_out),
 			   vif->flags, vif->local, vif->remote);
 	}
 	return 0;
diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 3f2ed9b77deb..7a3c795ec68b 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -485,8 +485,9 @@ static int ip6mr_vif_seq_show(struct seq_file *seq, void *v)
 		seq_printf(seq,
 			   "%2td %-10s %8ld %7ld  %8ld %7ld %05X\n",
 			   vif - mrt->vif_table,
-			   name, vif->bytes_in, vif->pkt_in,
-			   vif->bytes_out, vif->pkt_out,
+			   name,
+			   READ_ONCE(vif->bytes_in), READ_ONCE(vif->pkt_in),
+			   READ_ONCE(vif->bytes_out), READ_ONCE(vif->pkt_out),
 			   vif->flags);
 	}
 	return 0;
-- 
2.25.1