From 413187f79062634575098653c40f95d439d3b157 Mon Sep 17 00:00:00 2001 From: "Gustavo A. R. Silva" Date: Wed, 17 Sep 2025 15:26:49 +0200 Subject: stddef: Remove token-pasting in TRAILING_OVERLAP() Currently, TRAILING_OVERLAP() token-pastes the FAM parameter into the name of internal pdding member `__offset_to_##FAM`. This forces FAM to be a single identifier, which prevents callers from using a FAM when it's a nested member. For instance, see the following scenario: | struct flex { | size_t count; | int data[]; | }; | struct foo { | int hdr_foo; | struct flex f; | }; | struct composite { | struct foo hdr; | int data[100]; | }; In this case, it'd be useful if TRAILING_OVERLAP() could be used in the following way: | struct composite { | TRAILING_OVERLAP(struct foo, hdr, f.data, | int data[100]; | ); | }; However, this is not current possible due to the token concatenation in `__offset_to_##FAM`, which fails when FAM contains a dot. So, remove token-pasting and use the fixed internal name `__offset_to_FAM` and, with this, expand the capabilities of TRAILING_OVERLAP(). :) Signed-off-by: Gustavo A. R. Silva Link: https://lore.kernel.org/r/13b3e0a69aad837b4e32ca8269b9d91bf1fbe9ef.1758115257.git.gustavoars@kernel.org Signed-off-by: Kees Cook --- include/linux/stddef.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/stddef.h') diff --git a/include/linux/stddef.h b/include/linux/stddef.h index dab49e2ec8c0..701099c67c24 100644 --- a/include/linux/stddef.h +++ b/include/linux/stddef.h @@ -108,7 +108,7 @@ enum { union { \ TYPE NAME; \ struct { \ - unsigned char __offset_to_##FAM[offsetof(TYPE, FAM)]; \ + unsigned char __offset_to_FAM[offsetof(TYPE, FAM)]; \ MEMBERS \ }; \ } -- cgit v1.2.3 From 2bbdcf02c3f3df6c85abd0a2fee0d7f0f4113e96 Mon Sep 17 00:00:00 2001 From: "Gustavo A. R. Silva" Date: Wed, 17 Sep 2025 15:28:07 +0200 Subject: stddef: Introduce __TRAILING_OVERLAP() Introduce underlying __TRAILING_OVERLAP() macro to let callers apply atributes to trailing overlapping members. For instance, the code below: | struct flex { | size_t count; | int data[]; | }; | struct { | struct flex f; | struct foo a; | struct boo b; | } __packed instance; can now be changed to the following, and preserve the __packed attribute: | __TRAILING_OVERLAP(struct flex, f, data, __packed, | struct foo a; | struct boo b; | ) instance; Signed-off-by: Gustavo A. R. Silva Link: https://lore.kernel.org/r/f80c529b239ce11f0a51f714fe00ddf839e05f5e.1758115257.git.gustavoars@kernel.org Signed-off-by: Kees Cook --- include/linux/stddef.h | 22 +++++++++++++++++++--- 1 file changed, 19 insertions(+), 3 deletions(-) (limited to 'include/linux/stddef.h') diff --git a/include/linux/stddef.h b/include/linux/stddef.h index 701099c67c24..80b6bfb944f0 100644 --- a/include/linux/stddef.h +++ b/include/linux/stddef.h @@ -94,7 +94,8 @@ enum { __DECLARE_FLEX_ARRAY(TYPE, NAME) /** - * TRAILING_OVERLAP() - Overlap a flexible-array member with trailing members. + * __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. @@ -102,15 +103,30 @@ enum { * @TYPE: Flexible structure type name, including "struct" keyword. * @NAME: Name for a variable to define. * @FAM: The flexible-array member within @TYPE + * @ATTRS: Any struct attributes (usually empty) * @MEMBERS: Trailing overlapping members. */ -#define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS) \ +#define __TRAILING_OVERLAP(TYPE, NAME, FAM, ATTRS, MEMBERS) \ union { \ TYPE NAME; \ struct { \ unsigned char __offset_to_FAM[offsetof(TYPE, FAM)]; \ MEMBERS \ - }; \ + } ATTRS; \ } +/** + * 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) \ + __TRAILING_OVERLAP(TYPE, NAME, FAM, /* no attrs */, MEMBERS) + #endif -- cgit v1.2.3