Message-ID: <1c12601bea3e9c18da6adc106bfcf5b7569e5dfb.1729037131.git.gustavoars@kernel.org>
Date: Tue, 15 Oct 2024 18:27:16 -0600
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: Andrew Lunn <andrew+netdev@...n.ch>,
	Johannes Berg <johannes@...solutions.net>,
	David Ahern <dsahern@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-hardening@...r.kernel.org, linux-wireless@...r.kernel.org,
	Kees Cook <kees@...nel.org>
Subject: [PATCH 1/5][next] net: dev: Introduce struct sockaddr_legacy

We are currently working on enabling the -Wflex-array-member-not-at-end
compiler option. This option has helped us detect several objects of
the type `struct sockaddr` that appear in the middle of composite
structures like `struct rtentry`, `struct compat_rtentry`, and others:

include/uapi/linux/wireless.h:751:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/wireless.h:776:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/wireless.h:833:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/wireless.h:857:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/wireless.h:864:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/route.h:33:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/route.h:34:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/route.h:35:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/if_arp.h:118:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/if_arp.h:119:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/if_arp.h:121:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/if_arp.h:126:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/uapi/linux/if_arp.h:127:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/net/compat.h:34:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
include/net/compat.h:35:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/nfsd/nfsd.h:74:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/nfsd/nfsd.h:75:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

In order to fix the warnings above, we introduce `struct sockaddr_legacy`.
The intention is to use it to replace the type of several struct members
in the middle of composite structures, currently of type `struct sockaddr`.

These middle struct members are currently causing thousands of warnings
because `struct sockaddr` contains a flexible-array member, introduced
by commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible array in
struct sockaddr").

The new `struct sockaddr_legacy` doesn't include a flexible-array
member, making it suitable for use as the type of middle members
in composite structs that don't really require the flexible-array
member in `struct sockaddr`, thus avoiding -Wflex-array-member-not-at-end
warnings.

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 include/linux/socket.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index d18cc47e89bd..f370ae0e6c82 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -40,6 +40,25 @@ struct sockaddr {
 	};
 };
 
+/*
+ * This is the legacy form of `struct sockaddr`. The original `struct sockaddr`
+ * was modified in commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible
+ * array in struct sockaddR") due to the fact that "One of the worst offenders
+ * of "fake flexible arrays" is struct sockaddr". This means that the original
+ * `char sa_data[14]` behaved as a flexible array at runtime, so a proper
+ * flexible-array member was introduced.
+ *
+ * This caused several flexible-array-in-the-middle issues:
+ * https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wflex-array-member-not-at-end
+ *
+ * `struct sockaddr_legacy` replaces `struct sockaddr` in all instances where
+ * objects of this type do not appear at the end of composite structures.
+ */
+struct sockaddr_legacy {
+	sa_family_t	sa_family;	/* address family, AF_xxx	*/
+	char 		sa_data[14];	/* 14 bytes of protocol address	*/
+};
+
 struct linger {
 	int		l_onoff;	/* Linger active		*/
 	int		l_linger;	/* How long to linger for	*/
-- 
2.34.1

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