linux-hardening - [PATCH 1/1] Fortify: Use C arithmetic not FIELD_xxx() in FORTIFY

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Message-Id: <20251214125857.3308-1-david.laight.linux@gmail.com>
Date: Sun, 14 Dec 2025 12:58:57 +0000
From: david.laight.linux@...il.com
To: Kees Cook <kees@...nel.org>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	linux-hardening@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: David Laight <david.laight.linux@...il.com>
Subject: [PATCH 1/1] Fortify: Use C arithmetic not FIELD_xxx() in FORTIFY_REASON defines

From: David Laight <david.laight.linux@...il.com>

FIELD_GET() and FIELD_PREP() are mainly useful for hardware register
accesses, but here they are being used for some very simple oprations.

This wouldn't matter much, but they contain a lot of compile-time
checks (that really aren't needed here) that bloat the expansion
of FIELD_GET(GENMASK(7, 1), func) to over 18KB.
Even with the 'bloat reduced' FIELD_GET/PREP they are still hundreds of
characters.

Replace FIELD_GET(BIT(0), r) with ((r) & 1), FIELD_GET(GENMASK(7, 1), r) with
(r) >> 1), and (FIELD_PREP(BIT(0), write) | FIELD_PREP(GENMASK(7, 1), func))
with ((func) << 1 | (write)).

The generated code is the same, but it makes the .c file less obfuctaced,
the .i file much easier to read, and should marginally decrease compilation
time.

Signed-off-by: David Laight <david.laight.linux@...il.com>
---

Note that changing 'const u8 reason' to 'const unsigned int reason' generates
better code - in this case removing 2 instructions (one in each of the called
functions).

 include/linux/fortify-string.h | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index b3b53f8c1b28..171982e53c9a 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -2,7 +2,6 @@
 #ifndef _LINUX_FORTIFY_STRING_H_
 #define _LINUX_FORTIFY_STRING_H_
 
-#include <linux/bitfield.h>
 #include <linux/bug.h>
 #include <linux/const.h>
 #include <linux/limits.h>
@@ -10,10 +9,9 @@
 #define __FORTIFY_INLINE extern __always_inline __gnu_inline __overloadable
 #define __RENAME(x) __asm__(#x)
 
-#define FORTIFY_REASON_DIR(r)		FIELD_GET(BIT(0), r)
-#define FORTIFY_REASON_FUNC(r)		FIELD_GET(GENMASK(7, 1), r)
-#define FORTIFY_REASON(func, write)	(FIELD_PREP(BIT(0), write) | \
-					 FIELD_PREP(GENMASK(7, 1), func))
+#define FORTIFY_REASON_DIR(r)		((r) & 1)
+#define FORTIFY_REASON_FUNC(r)		((r) >> 1)
+#define FORTIFY_REASON(func, write)	((func) << 1 | (write))
 
 /* Overridden by KUnit tests. */
 #ifndef fortify_panic
-- 
2.39.5