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
next prev parent 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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