[PATCH for-next 22/25] string.h: Introduce memset_after() for wiping trailing members/padding

[Kees Cook]

A common idiom in kernel code is to wipe the contents of a structure
after a given member. This is especially useful in places where there is
trailing padding. These open-coded cases are usually difficult to read
and very sensitive to struct layout changes. Introduce a new helper,
memset_after() that takes the target struct instance, the byte to write,
and the member name after which the zeroing should start.

Additionally adds memset_startat() for wiping trailing members _starting_
at a specific member instead of after a member, which is more readable
in certain circumstances, but doesn't include any preceding padding.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/linux/string.h | 17 +++++++++++++++++
 lib/test_memcpy.c      | 13 +++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index 9473f81b9db2..d593de2635ba 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -271,6 +271,23 @@ static inline void memcpy_and_pad(void *dest, size_t dest_len,
 		memcpy(dest, src, dest_len);
 }
 
+/**
+ * memset_after - Set a value after a struct member to the end of a struct
+ *
+ * @obj: Address of target struct instance
+ * @v: Byte value to repeatedly write
+ * @member: after which struct member to start writing bytes
+ *
+ * This is good for clearing padding following the given member.
+ */
+#define memset_after(obj, v, member)					\
+({									\
+	u8 *__ptr = (u8 *)(obj);					\
+	typeof(v) __val = (v);						\
+	memset(__ptr + offsetofend(typeof(*(obj)), member), __val,	\
+	       sizeof(*(obj)) - offsetofend(typeof(*(obj)), member));	\
+})
+
 /**
  * str_has_prefix - Test if a string has a given prefix
  * @str: The string to test
diff --git a/lib/test_memcpy.c b/lib/test_memcpy.c
index ec546cec883e..3b485de8c885 100644
--- a/lib/test_memcpy.c
+++ b/lib/test_memcpy.c
@@ -215,6 +215,13 @@ static void memset_test(struct kunit *test)
 			  0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30,
 			},
 	};
+	struct some_bytes after = {
+		.data = { 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x72,
+			  0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72,
+			  0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72,
+			  0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72, 0x72,
+			},
+	};
 	struct some_bytes dest = { };
 	int count, value;
 	u8 *ptr;
@@ -245,6 +252,12 @@ static void memset_test(struct kunit *test)
 	ptr += 8;
 	memset(ptr++, value++, count++);
 	compare("argument side-effects", dest, three);
+
+	/* Verify memset_after() */
+	dest = control;
+	memset_after(&dest, 0x72, three);
+	compare("memset_after()", dest, after);
+
 #undef TEST_OP
 }
 
-- 
2.30.2