string.h: Introduce memset_after() for wiping trailing members/padding

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.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Francis Laniel <laniel_francis@privacyrequired.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Daniel Axtens <dja@axtens.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

2 files changed, 30 insertions, 0 deletions

diff --git a/include/linux/string.h b/include/linux/string.h
index ac1c769a5a80..da490c2154a9 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -272,6 +272,23 @@ static inline void memcpy_and_pad(void *dest, size_t 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
  * @prefix: The string to see if @str starts with
diff --git a/lib/memcpy_kunit.c b/lib/memcpy_kunit.c
index 8b2109bb62df..5c5b4f3221d9 100644
--- a/lib/memcpy_kunit.c
+++ b/lib/memcpy_kunit.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
 }