[PATCH] overflow: Support sub-union members in struct_size()

[Bence Csokas <bence.csokas@arm.com>]

Original definition of struct_size() assumed that the total size of an
object with a flex-array member can be calculated simply by adding the
flex-array size to what sizeof() returns. This is however not the case
if the struct has a union inside it, and `member` refers to a field
inside that sub-union:

  struct my_inner {
    int a;
    char b[];
  };

  struct my_outer {
    char c;
    union {
      struct my_inner inner;
      long l;
    };
  };

  /* This will give the wrong result */
  struct_size_t(struct my_outer, inner.b, n_elems);

Replacing sizeof() by the offsetof() of the member fixes this.

Existing users specifying non-union members should not be affected.

Cc: Mark Rutland <mark.rutland@arm.com>

Signed-off-by: Bence Csokas <bence.csokas@arm.com>
---
 include/linux/overflow.h | 4 ++--
 include/linux/stddef.h   | 8 ++++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index a8cb6319b4fb..a11c1135f617 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -433,8 +433,8 @@ static __always_inline size_t __must_check size_sub(size_t minuend, size_t subtr
  */
 #define struct_size(p, member, count)					\
 	__builtin_choose_expr(__is_constexpr(count),			\
-		sizeof(*(p)) + flex_array_size(p, member, count),	\
-		size_add(sizeof(*(p)), flex_array_size(p, member, count)))
+		offsetofp((p), member) + flex_array_size(p, member, count),	\
+		size_add(offsetofp((p), member), flex_array_size(p, member, count)))
 
 /**
  * struct_size_t() - Calculate size of structure with trailing flexible array
diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index e1851c50c89b..b8f0dbd2de55 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -32,6 +32,14 @@ enum {
 #define offsetofend(TYPE, MEMBER) \
 	(offsetof(TYPE, MEMBER)	+ sizeof_field(TYPE, MEMBER))
 
+/**
+ * offsetofp() - Get the offset of a struct field from a pointer to the struct
+ *
+ * @P: Pointer to the structure
+ * @MEMBER: The member within the structure to get the end offset of
+ */
+#define offsetofp(P, MEMBER) offsetof(typeof(*(P)), MEMBER)
+
 /**
  * struct_group() - Wrap a set of declarations in a mirrored struct
  *

base-commit: 62cc90241548d5570ee68e01aaba6506964e9811
-- 
2.54.0