Add new TRAILING_OVERLAP() helper macro to create a union between a flexible-array member (FAM) and a set of members that would otherwise follow it. This overlays the trailing members onto the FAM while preserving the original memory layout.
Co-developed-by: Kees Cook <[email protected]> Signed-off-by: Kees Cook <[email protected]> Signed-off-by: Gustavo A. R. Silva <[email protected]> --- Changes in v2: - Use `__offset_of_##FAM` instead of `__offset_of_fam` for the offset variable. - Update changelog text. v1: - Link: https://lore.kernel.org/linux-hardening/aFDIBBeYgRSx1ekO@kspp/ include/linux/stddef.h | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/include/linux/stddef.h b/include/linux/stddef.h index 929d67710cc5..dab49e2ec8c0 100644 --- a/include/linux/stddef.h +++ b/include/linux/stddef.h @@ -93,4 +93,24 @@ enum { #define DECLARE_FLEX_ARRAY(TYPE, NAME) \ __DECLARE_FLEX_ARRAY(TYPE, NAME) +/** + * TRAILING_OVERLAP() - Overlap a flexible-array member with trailing members. + * + * Creates a union between a flexible-array member (FAM) in a struct and a set + * of additional members that would otherwise follow it. + * + * @TYPE: Flexible structure type name, including "struct" keyword. + * @NAME: Name for a variable to define. + * @FAM: The flexible-array member within @TYPE + * @MEMBERS: Trailing overlapping members. + */ +#define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS) \ + union { \ + TYPE NAME; \ + struct { \ + unsigned char __offset_to_##FAM[offsetof(TYPE, FAM)]; \ + MEMBERS \ + }; \ + } + #endif -- 2.43.0
