On 7/28/26 05:49, Bence Csokas wrote:
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.
This actually affects users of flexible structs that have tail padding[1].
-Gustavo
[1] https://godbolt.org/z/KPq59EYej
Cc: Mark Rutland <[email protected]>
Signed-off-by: Bence Csokas <[email protected]>
---
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
