linux-hardening - [PATCH 2/3] fortify: cosmetic cleanups to __compiletime

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Message-ID: <20220830205309.312864-3-ndesaulniers@google.com>
Date:   Tue, 30 Aug 2022 13:53:08 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nathan Chancellor <nathan@...nel.org>, Tom Rix <trix@...hat.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH 2/3] fortify: cosmetic cleanups to __compiletime_strlen

Two things I noticed in __compiletime_strlen:
1. A temporary, __p, is created+used to avoid repeated side effects from
   multiple evaluation of the macro parameter, but the macro parameter
   was being used accidentally in __builtin_object_size.
2. The temporary has a curious signedness and const-less qualification.
   Just use __auto_type.
3. (size_t)-1 is perhaps more readable as -1UL.
4. __p_size == -1UL when __builtin_object_size can't evaluate the
   object size at compile time. We could just reuse __ret and use one
   less variable here.

Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 include/linux/fortify-string.h | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index c5adad596a3f..aaf73575050f 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -22,11 +22,10 @@ void __write_overflow_field(size_t avail, size_t wanted) __compiletime_warning("
 
 #define __compiletime_strlen(p)					\
 ({								\
-	unsigned char *__p = (unsigned char *)(p);		\
-	size_t __ret = (size_t)-1;				\
-	size_t __p_size = __object_size(p, 1);			\
-	if (__p_size != (size_t)-1) {				\
-		size_t __p_len = __p_size - 1;			\
+	__auto_type __p = (p);					\
+	size_t __ret = __object_size(__p, 1);			\
+	if (__ret != -1UL) {					\
+		size_t __p_len = __ret - 1;			\
 		if (__builtin_constant_p(__p[__p_len]) &&	\
 		    __p[__p_len] == '\0')			\
 			__ret = __builtin_strlen(__p);		\
-- 
2.37.2.672.g94769d06f0-goog