linux-hardening - [PATCH][next] stddef: Introduce struct_group_tagged

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Date: Mon, 18 Mar 2024 17:00:33 -0600
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-hardening@...r.kernel.org
Subject: [PATCH][next] stddef: Introduce struct_group_tagged_attr() helper
 macro

We need a new `struct_group()` helper that allows for both having the
struct be tagged, and specifying struct attributes like `__packed`
or `__align(x)`.

This new helper will initially be used to address
-Wflex-array-member-not-at-end warnings, where a tagged struct is used
to separate the flexible-array member from the rest of the members in
the flexible structure [1]. There are some scenarios in which those
members need to be packed, as well.

So, `struct_group_tagged_attr()` is introduced for this.

Link: https://lore.kernel.org/linux-hardening/ZeIgeZ5Sb0IZTOyt@neat/ [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 include/linux/stddef.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index 929d67710cc5..919df9453257 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -80,6 +80,26 @@ enum {
 #define struct_group_tagged(TAG, NAME, MEMBERS...) \
 	__struct_group(TAG, NAME, /* no attrs */, MEMBERS)
 
+/**
+ * struct_group_tagged_attr() - Create a struct_group with a reusable
+ * tag and trailing attributes.
+ *
+ * @TAG: The tag name for the named sub-struct
+ * @NAME: The identifier name of the mirrored sub-struct
+ * @ATTRS: Any struct attributes to apply
+ * @MEMBERS: The member declarations for the mirrored structs
+ *
+ * Used to create an anonymous union of two structs with identical
+ * layout and size: one anonymous and one named. The former can be
+ * used normally without sub-struct naming, and the latter can be
+ * used to reason about the start, end, and size of the group of
+ * struct members. Includes struct tag argument for the named copy,
+ * so the specified layout can be reused later. Also includes
+ * structure attributes argument.
+ */
+#define struct_group_tagged_attr(TAG, NAME, ATTRS, MEMBERS...) \
+	__struct_group(TAG, NAME, ATTRS, MEMBERS)
+
 /**
  * DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
  *
-- 
2.34.1