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Message-ID: <ce6cd8e7-cd59-4fcd-89d4-b62765add7a9@embeddedor.com>
Date: Tue, 22 Oct 2024 11:30:32 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: David Laight <David.Laight@...LAB.COM>,
"'Gustavo A. R. Silva'" <gustavoars@...nel.org>,
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" <netdev@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
Kees Cook <kees@...nel.org>
Subject: Re: [PATCH 1/5][next] net: dev: Introduce struct sockaddr_legacy
On 22/10/24 06:13, David Laight wrote:
> From: Gustavo A. R. Silva
>> Sent: 16 October 2024 01:27
>>
>> 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:
>>
> ...
>>
>> 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
>
> I'd bet that the code even indexed the array?
> So it is all worse that just a compiler warning/
I haven't found evidence of that, but this is precisely what we want to prevent
from happening. :)
>
>> + *
>> + * `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 */
>> +};
>> +
>
> I'm not sure that is a very good name.
> Reading it you don't know when it is 'legacy' from.
Yep, naming is hard sometimes. This is why I added that long comment
above the struct. :)
However, this is changing and now it looks like this:
diff --git a/include/uapi/linux/socket.h b/include/uapi/linux/socket.h
index d3fcd3b5ec53d2..2e179706bec4d8 100644
--- a/include/uapi/linux/socket.h
+++ b/include/uapi/linux/socket.h
@@ -35,4 +35,32 @@ struct __kernel_sockaddr_storage {
#define SOCK_TXREHASH_DISABLED 0
#define SOCK_TXREHASH_ENABLED 1
+typedef __kernel_sa_family_t sa_family_t;
+
+/*
+ * 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 */
+};
+
+#ifdef __KERNEL__
+# define __kernel_sockaddr_legacy sockaddr_legacy
+#else
+# define __kernel_sockaddr_legacy sockaddr
+#endif
+
+
#endif /* _UAPI_LINUX_SOCKET_H */
https://lore.kernel.org/linux-hardening/202410160942.000495E@keescook/
--
Gustavo
> It's size is clearly that of the original IPv4 sockaddr.
> (I'm not sure there was ever an earlier one.)
>
> Perhaps 'strict sockaddr_16' would be better?
> Or, looking at the actual failures, sockaddr_ipv4?
>
> Alternatively revert b5f0de6df6dce and add a new type that has the char[]
> field??
>
> David
>
> -
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