linux-hardening - [RFC] string: Allow 2-argument strscpy()

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Date: Mon, 29 Jan 2024 12:29:04 -0800
From: Kees Cook <keescook@...omium.org>
To: Andy Shevchenko <andy@...nel.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Kees Cook <keescook@...omium.org>,
	linux-hardening@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [RFC] string: Allow 2-argument strscpy()

Using sizeof(dst) is the overwhelmingly common case for strscpy().
Instead of requiring this everywhere, allow a 2-argument version to be
used that will use the sizeof() internally.

Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Andy Shevchenko <andy@...nel.org>
Cc: linux-hardening@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
What do people think of this idea? It's usually very redundant to
include the 3rd argument, so this might improve readability (and
perhaps make things more robust by avoiding mistakes when the
destination name changes).
---
 include/linux/fortify-string.h | 4 ++--
 include/linux/string.h         | 9 ++++++++-
 lib/string.c                   | 4 ++--
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index 89a6888f2f9e..56be4d4a5dea 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -215,7 +215,7 @@ __kernel_size_t __fortify_strlen(const char * const POS p)
 }
 
 /* Defined after fortified strnlen() to reuse it. */
-extern ssize_t __real_strscpy(char *, const char *, size_t) __RENAME(strscpy);
+extern ssize_t __real_strscpy(char *, const char *, size_t) __RENAME(sized_strscpy);
 /**
  * strscpy - Copy a C-string into a sized buffer
  *
@@ -234,7 +234,7 @@ extern ssize_t __real_strscpy(char *, const char *, size_t) __RENAME(strscpy);
  * Returns the number of characters copied in @p (not including the
  * trailing %NUL) or -E2BIG if @size is 0 or the copy of @q was truncated.
  */
-__FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, size_t size)
+__FORTIFY_INLINE ssize_t sized_strscpy(char * const POS p, const char * const POS q, size_t size)
 {
 	/* Use string size rather than possible enclosing struct size. */
 	const size_t p_size = __member_size(p);
diff --git a/include/linux/string.h b/include/linux/string.h
index ab148d8dbfc1..0bb1c8d05f18 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -67,9 +67,16 @@ extern char * strcpy(char *,const char *);
 extern char * strncpy(char *,const char *, __kernel_size_t);
 #endif
 #ifndef __HAVE_ARCH_STRSCPY
-ssize_t strscpy(char *, const char *, size_t);
+ssize_t sized_strscpy(char *, const char *, size_t);
 #endif
 
+#define __strscpy0(dst, src, ...)	sized_strscpy(dst, src, sizeof(dst))
+
+#define __strscpy1(dst, src, size)	sized_strscpy(dst, src, size)
+
+#define strscpy(dst, src, ...)	\
+	CONCATENATE(__strscpy, COUNT_ARGS(__VA_ARGS__))(dst, src, __VA_ARGS__)
+
 /* Wraps calls to strscpy()/memset(), no arch specific code required */
 ssize_t strscpy_pad(char *dest, const char *src, size_t count);
 
diff --git a/lib/string.c b/lib/string.c
index 6891d15ce991..2869895a1180 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -104,7 +104,7 @@ EXPORT_SYMBOL(strncpy);
 #endif
 
 #ifndef __HAVE_ARCH_STRSCPY
-ssize_t strscpy(char *dest, const char *src, size_t count)
+ssize_t sized_strscpy(char *dest, const char *src, size_t count)
 {
 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
 	size_t max = count;
@@ -170,7 +170,7 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
 
 	return -E2BIG;
 }
-EXPORT_SYMBOL(strscpy);
+EXPORT_SYMBOL(sized_strscpy);
 #endif
 
 /**
-- 
2.34.1