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@xxxxxxxxxx>
---
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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