Re: [PATCH 1/5][next] net: dev: Introduce struct sockaddr_legacy

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

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@kernel.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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