linux-kernel - [PATCH 06/15] netfilter: conntrack: avoid gcc-10 zero-length-bounds warning

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Date:   Thu, 30 Apr 2020 23:30:48 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     linux-kernel@...r.kernel.org,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Jeremy Sowden <jeremy@...zel.net>,
        Li RongQing <lirongqing@...du.com>,
        Joe Perches <joe@...ches.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jules Irenge <jbi.octave@...il.com>,
        Dirk Morris <dmorris@...aloft.com>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org
Subject: [PATCH 06/15] netfilter: conntrack: avoid gcc-10 zero-length-bounds warning

gcc-10 warns around a suspicious access to an empty struct member:

net/netfilter/nf_conntrack_core.c: In function '__nf_conntrack_alloc':
net/netfilter/nf_conntrack_core.c:1522:9: warning: array subscript 0 is outside the bounds of an interior zero-length array 'u8[0]' {aka 'unsigned char[0]'} [-Wzero-length-bounds]
 1522 |  memset(&ct->__nfct_init_offset[0], 0,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from net/netfilter/nf_conntrack_core.c:37:
include/net/netfilter/nf_conntrack.h:90:5: note: while referencing '__nfct_init_offset'
   90 |  u8 __nfct_init_offset[0];
      |     ^~~~~~~~~~~~~~~~~~

The code is correct but a bit unusual. Rework it slightly in a way that
does not trigger the warning, using an empty struct instead of an empty
array. There are probably more elegant ways to do this, but this is the
smallest change.

Fixes: c41884ce0562 ("netfilter: conntrack: avoid zeroing timer")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 include/net/netfilter/nf_conntrack.h | 2 +-
 net/netfilter/nf_conntrack_core.c    | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index 9f551f3b69c6..90690e37a56f 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -87,7 +87,7 @@ struct nf_conn {
 	struct hlist_node	nat_bysource;
 #endif
 	/* all members below initialized via memset */
-	u8 __nfct_init_offset[0];
+	struct { } __nfct_init_offset;
 
 	/* If we were expected by an expectation, this will be it */
 	struct nf_conn *master;
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index c4582eb71766..0173398f4ced 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -1519,9 +1519,9 @@ __nf_conntrack_alloc(struct net *net,
 	ct->status = 0;
 	ct->timeout = 0;
 	write_pnet(&ct->ct_net, net);
-	memset(&ct->__nfct_init_offset[0], 0,
+	memset(&ct->__nfct_init_offset, 0,
 	       offsetof(struct nf_conn, proto) -
-	       offsetof(struct nf_conn, __nfct_init_offset[0]));
+	       offsetof(struct nf_conn, __nfct_init_offset));
 
 	nf_ct_zone_add(ct, zone);
 
-- 
2.26.0