linux-kernel - [PATCH 3/6] sysfs: introduce __SYSFS_FUNCTION

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Message-Id: <20250116-sysfs-const-attr-prep-v1-3-15e72dba4205@weissschuh.net>
Date: Thu, 16 Jan 2025 18:32:29 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 "Rafael J. Wysocki" <rafael@...nel.org>
Cc: linux-kernel@...r.kernel.org, 
 Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH 3/6] sysfs: introduce __SYSFS_FUNCTION_ALTERNATIVE()

For the constification phase of 'struct attribute' various callback
struct members will need to exist in both const and non-const variants.
Keeping both members in a union avoids memory and CPU overhead but will
be detected and trapped by Control Flow Integrity (CFI).
By deciding between a struct and a union depending whether CFI is
enabled, most configurations can avoid this overhead.
Code using these callbacks will still need to be updated to handle both
members explicitly.
In the union case the compiler will recognize that testing for one union
member is enough and optimize away the code for the other one.

Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
---
 include/linux/sysfs.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index 52d4862bbcef2a6b143710a19801222bf7abc346..46e9261bc35f248a733d27c05c931080b0c58e17 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -58,6 +58,12 @@ do {							\
 #define sysfs_attr_init(attr) do {} while (0)
 #endif

+#ifdef CONFIG_CFI_CLANG
+#define __SYSFS_FUNCTION_ALTERNATIVE(MEMBERS...) struct { MEMBERS }
+#else
+#define __SYSFS_FUNCTION_ALTERNATIVE(MEMBERS...) union { MEMBERS }
+#endif
+
 /**
  * struct attribute_group - data structure used to declare an attribute group.
  * @name:	Optional: Attribute group name

-- 
2.48.1