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From: Kees Cook <keescook@chromium.org>
To: linux-kernel@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Keith Packard <keithp@keithp.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	clang-built-linux@googlegroups.com,
	linux-hardening@vger.kernel.org
Subject: [PATCH v3 1/5] stddef: Introduce DECLARE_FLEX_ARRAY() helper
Date: Fri, 27 Aug 2021 09:30:11 -0700	[thread overview]
Message-ID: <20210827163015.3141722-2-keescook@chromium.org> (raw)
In-Reply-To: <20210827163015.3141722-1-keescook@chromium.org>

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>
---
 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 8b103a53b000..ca507bd5f808 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 d9715efbe0b7..65088b512d14 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
-- 
2.30.2


  reply	other threads:[~2021-08-27 16:30 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-27 16:30 [PATCH v3 0/5] Enable -Warray-bounds and -Wzero-length-bounds Kees Cook
2021-08-27 16:30 ` Kees Cook [this message]
2021-08-27 16:30 ` [PATCH v3 2/5] treewide: Replace open-coded flex arrays in unions Kees Cook
2021-08-28  7:51   ` Vincent MAILHOL
2021-08-27 16:30 ` [PATCH v3 3/5] treewide: Replace 0-element memcpy() destinations with flexible arrays Kees Cook
2021-08-27 16:30 ` [PATCH v3 4/5] Makefile: Enable -Warray-bounds Kees Cook
2021-08-31  4:31   ` Guenter Roeck
2021-08-31 17:46     ` Kees Cook
2021-08-31 19:40       ` Guenter Roeck
2021-08-31 20:18         ` Kees Cook
2021-08-31 20:49           ` Guenter Roeck
2021-08-27 16:30 ` [PATCH v3 5/5] Makefile: Enable -Wzero-length-bounds Kees Cook
2021-08-30 18:44 ` [PATCH v3 0/5] Enable -Warray-bounds and -Wzero-length-bounds Nathan Chancellor
2021-08-30 20:12   ` Kees Cook
2021-08-30 20:16   ` Kees Cook
2021-08-30 22:34     ` Nathan Chancellor

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