[PATCH 5.15 112/230] stddef: Introduce DECLARE_FLEX_ARRAY() helper

Greg Kroah-Hartman
2022-07-11T09:50:47Z

From: Kees Cook <keescook@chromium.org>

[ Upstream commit 3080ea5553cc909b000d1f1d964a9041962f2c5b ]

There are many places where kernel code wants to have several different
typed trailing flexible arrays. This would normally be done with multiple
flexible arrays in a union, but since GCC and Clang don't (on the surface)
allow this, there have been many open-coded workarounds, usually involving
neighboring 0-element arrays at the end of a structure. For example,
instead of something like this:

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

code works around the compiler with:

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

Another case is when a flexible array is wanted as the single member
within a struct (which itself is usually in a union). For example, this
would be worked around as:

union many {
...
struct {
struct type3 baz[0];
};
};

These kinds of work-arounds cause problems with size checks against such
zero-element arrays (for example when building with -Warray-bounds and
-Wzero-length-bounds, and with the coming FORTIFY_SOURCE improvements),
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];
| ^~~~~~~

However, it _is_ entirely possible to have one or more flexible arrays
in a struct or union: it just has to be in another struct. And since it
cannot be alone in a struct, such a struct must have at least 1 other
named member -- but that member can be zero sized. Wrap all this nonsense
into the new DECLARE_FLEX_ARRAY() in support of having flexible arrays
in unions (or alone in a struct).

As with struct_group(), since this is needed in UAPI headers as well,
implement the core there, with a non-UAPI wrapper.

Additionally update kernel-doc to understand its existence.

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/stddef.h | 13 +++++++++++++
include/uapi/linux/stddef.h | 16 ++++++++++++++++
scripts/kernel-doc | 2 ++
3 files changed, 31 insertions(+)

diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index 938216f8ab7e..31fdbb784c24 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -84,4 +84,17 @@ enum {
#define struct_group_tagged(TAG, NAME, MEMBERS...) \
__struct_group(TAG, NAME, /* no attrs */, MEMBERS)

+/**
+ * DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
+ *
+ * @TYPE: The type of each flexible array element
+ * @NAME: The name of the flexible array member
+ *
+ * In order to have a flexible array member in a union or alone in a
+ * struct, it needs to be wrapped in an anonymous struct with at least 1
+ * named member, but that member can be empty.
+ */
+#define DECLARE_FLEX_ARRAY(TYPE, NAME) \
+ __DECLARE_FLEX_ARRAY(TYPE, NAME)
+
#endif
diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
index 610204f7c275..3021ea25a284 100644
--- a/include/uapi/linux/stddef.h
+++ b/include/uapi/linux/stddef.h
@@ -25,3 +25,19 @@
struct { MEMBERS } ATTRS; \
struct TAG { MEMBERS } ATTRS NAME; \
}
+
+/**
+ * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
+ *
+ * @TYPE: The type of each flexible array element
+ * @NAME: The name of the flexible array member
+ *
+ * In order to have a flexible array member in a union or alone in a
+ * struct, it needs to be wrapped in an anonymous struct with at least 1
+ * named member, but that member can be empty.
+ */
+#define __DECLARE_FLEX_ARRAY(TYPE, NAME) \
+ struct { \
+ struct { } __empty_ ## NAME; \
+ TYPE NAME[]; \
+ }
diff --git a/scripts/kernel-doc b/scripts/kernel-doc
index 38aa799a776c..5d54b57ff90c 100755
--- a/scripts/kernel-doc
+++ b/scripts/kernel-doc
@@ -1263,6 +1263,8 @@ sub dump_struct($$) {
$members =~ s/DECLARE_KFIFO\s*\($args,\s*$args,\s*$args\)/$2 \*$1/gos;
# replace DECLARE_KFIFO_PTR
$members =~ s/DECLARE_KFIFO_PTR\s*\($args,\s*$args\)/$2 \*$1/gos;
+ # replace DECLARE_FLEX_ARRAY
+ $members =~ s/(?:__)?DECLARE_FLEX_ARRAY\s*\($args,\s*$args\)/$1 $2\[\]/gos;
my $declaration = $members;

# Split nested struct/union elements as newer ones
--
2.35.1