fortify: Allow strlen() and strnlen() to pass compile-time known lengths

Under CONFIG_FORTIFY_SOURCE, it is possible for the compiler to perform
strlen() and strnlen() at compile-time when the string size is known.
This is required to support compile-time overflow checking in strlcpy().
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/fortify-string.h

@@ -10,6 +10,20 @@ void __read_overflow(void) __compiletime_error("detected read beyond size of obj
 void __read_overflow2(void) __compiletime_error("detected read beyond size of object (2nd parameter)");
 void __write_overflow(void) __compiletime_error("detected write beyond size of object (1st parameter)");
 
+#define __compiletime_strlen(p)				\
+({							\
+	unsigned char *__p = (unsigned char *)(p);      \
+	size_t ret = (size_t)-1;			\
+	size_t p_size = __builtin_object_size(p, 1);	\
+	if (p_size != (size_t)-1) {			\
+		size_t p_len = p_size - 1;		\
+		if (__builtin_constant_p(__p[p_len]) &&	\
+		    __p[p_len] == '\0')			\
+			ret = __builtin_strlen(__p);	\
+	}						\
+	ret;						\
+})
+
 #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
 extern void *__underlying_memchr(const void *p, int c, __kernel_size_t size) __RENAME(memchr);
 extern int __underlying_memcmp(const void *p, const void *q, __kernel_size_t size) __RENAME(memcmp);
@@ -60,21 +74,31 @@ extern __kernel_size_t __real_strnlen(const char *, __kernel_size_t) __RENAME(st
 __FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
 {
 	size_t p_size = __builtin_object_size(p, 1);
-	__kernel_size_t ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
+	size_t p_len = __compiletime_strlen(p);
+	size_t ret;
+
+	/* We can take compile-time actions when maxlen is const. */
+	if (__builtin_constant_p(maxlen) && p_len != (size_t)-1) {
+		/* If p is const, we can use its compile-time-known len. */
+		if (maxlen >= p_size)
+			return p_len;
+	}
 
+	/* Do not check characters beyond the end of p. */
+	ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
 	if (p_size <= ret && maxlen != ret)
 		fortify_panic(__func__);
 	return ret;
 }
 
+/* defined after fortified strnlen to reuse it. */
 __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
 {
 	__kernel_size_t ret;
 	size_t p_size = __builtin_object_size(p, 1);
 
-	/* Work around gcc excess stack consumption issue */
-	if (p_size == (size_t)-1 ||
-		(__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0'))
+	/* Give up if we don't know how large p is. */
+	if (p_size == (size_t)-1)
 		return __underlying_strlen(p);
 	ret = strnlen(p, p_size);
 	if (p_size <= ret)
@@ -86,24 +110,27 @@ __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
 extern size_t __real_strlcpy(char *, const char *, size_t) __RENAME(strlcpy);
 __FORTIFY_INLINE size_t strlcpy(char *p, const char *q, size_t size)
 {
-	size_t ret;
 	size_t p_size = __builtin_object_size(p, 1);
 	size_t q_size = __builtin_object_size(q, 1);
+	size_t q_len;	/* Full count of source string length. */
+	size_t len;	/* Count of characters going into destination. */
 
 	if (p_size == (size_t)-1 && q_size == (size_t)-1)
 		return __real_strlcpy(p, q, size);
-	ret = strlen(q);
-	if (size) {
-		size_t len = (ret >= size) ? size - 1 : ret;
-
-		if (__builtin_constant_p(len) && len >= p_size)
+	q_len = strlen(q);
+	len = (q_len >= size) ? size - 1 : q_len;
+	if (__builtin_constant_p(size) && __builtin_constant_p(q_len) && size) {
+		/* Write size is always larger than destination. */
+		if (len >= p_size)
 			__write_overflow();
+	}
+	if (size) {
 		if (len >= p_size)
 			fortify_panic(__func__);
 		__underlying_memcpy(p, q, len);
 		p[len] = '\0';
 	}
-	return ret;
+	return q_len;
 }
 
 /* defined after fortified strnlen to reuse it */