[PATCH 1/5] stddef: Add flexible array union helper

[Kees Cook <keescook@chromium.org>]

Many places in the kernel want to use a flexible array in a union. This
is especially common when wanting several different typed trailing
flexible arrays. Since GCC and Clang don't (on the surface) allow this,
such structs have traditionally used combinations of zero-element arrays
instead. This is usually seen in this form, implying a union of "foo"
and "bar":

struct thing {
	...
	struct type1 foo[0];
	struct type2 bar[];
};

This causes problems with size checks against such zero-element arrays
(for example with -Warray-bounds and -Wzero-length-bounds), so they must
all be converted to "real" flexible arrays, avoiding warnings like this:

fs/hpfs/anode.c: In function 'hpfs_add_sector_to_btree':
fs/hpfs/anode.c:209:27: warning: array subscript 0 is outside the bounds of an interior zero-length array 'struct bplus_internal_node[0]' [-Wzero-length-bounds]
  209 |    anode->btree.u.internal[0].down = cpu_to_le32(a);
      |    ~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from fs/hpfs/hpfs_fn.h:26,
                 from fs/hpfs/anode.c:10:
fs/hpfs/hpfs.h:412:32: note: while referencing 'internal'
  412 |     struct bplus_internal_node internal[0]; /* (internal) 2-word entries giving
      |                                ^~~~~~~~

drivers/net/can/usb/etas_es58x/es58x_fd.c: In function 'es58x_fd_tx_can_msg':
drivers/net/can/usb/etas_es58x/es58x_fd.c:360:35: warning: array subscript 65535 is outside the bounds of an interior zero-length array 'u8[0]' {aka 'unsigned char[]'} [-Wzero-length-bounds]
  360 |  tx_can_msg = (typeof(tx_can_msg))&es58x_fd_urb_cmd->raw_msg[msg_len];
      |                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/net/can/usb/etas_es58x/es58x_core.h:22,
                 from drivers/net/can/usb/etas_es58x/es58x_fd.c:17:
drivers/net/can/usb/etas_es58x/es58x_fd.h:231:6: note: while referencing 'raw_msg'
  231 |   u8 raw_msg[0];
      |      ^~~~~~~

Introduce flex_array() in support of flexible arrays in unions. It is
entirely possible to have a flexible array in a union: it just has to
be in a struct. And since it cannot be alone in a struct, such a struct
must have at least 1 other named member (here provided by __UNIQUE_ID),
but that member can be zero sized.

As with struct_group(), this is needed in UAPI headers as well, so a
minimal implementation (without the __UNIQUE_ID magic) is available for
UAPI.

https://github.com/KSPP/linux/issues/137

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/stddef.h      | 10 ++++++++++
 include/uapi/linux/stddef.h | 13 +++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index f2aefdb22d1d..c7c5d25ac184 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -83,4 +83,14 @@ enum {
 #define struct_group_tagged(TAG, NAME, MEMBERS...) \
 	__struct_group(TAG, NAME, /* no attrs */, MEMBERS)
 
+/**
+ * flex_array(DECL)
+ *
+ * In order to have a flexible array member in a union, it needs to be
+ * wrapped in an anonymous struct with at least 1 named member, but that
+ * member can be empty.
+ */
+#define flex_array(DECL)	\
+	__flex_array(__UNIQUE_ID(__flex_array), DECL)
+
 #endif
diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
index 0fbdf2f711aa..6320bbc90721 100644
--- a/include/uapi/linux/stddef.h
+++ b/include/uapi/linux/stddef.h
@@ -25,3 +25,16 @@
 		struct { MEMBERS } ATTRS; \
 		struct TAG { MEMBERS } ATTRS NAME; \
 	}
+
+/**
+ * __flex_array(UNIQUE_MEMBER_NAME, DECL)
+ *
+ * In order to have a flexible array member in a union, it needs to be
+ * wrapped in an anonymous struct with at least 1 named member, but that
+ * member can be empty.
+ */
+#define __flex_array(UNIQUE_MEMBER_NAME, DECL)	\
+	struct { \
+		struct { } UNIQUE_MEMBER_NAME; \
+		DECL; \
+	}
-- 
2.30.2