On Wed, Jul 15, 2026 at 12:12 PM Kees Cook <[email protected]> wrote: > > On Tue, Jul 14, 2026 at 11:18:18AM -0400, Jesse Taube wrote: > > Add new DECLARE_SIZED_FLEX() helper to set the default size of a > > flexible-array member. The code is identical to the declaration in > > __DEFINE_FLEX() which has also been changed to use DECLARE_SIZED_FLEX(). > > > > Add DECLARE_COUNTED_FLEX_ARRAY() helper which is a variant of > > DECLARE_FLEX_ARRAY() with a counted-by attribute. > > > > Also add default sized variants of > > DECLARE_FLEX_ARRAY(), DECLARE_SIZED_FLEX(), and > > DECLARE_COUNTED_FLEX_ARRAY(), DECLARE_COUNTED_SIZED_FLEX(). > > Where do you want to use these new helpers? > > > Signed-off-by: Jesse Taube <[email protected]> > > --- > > include/linux/overflow.h | 58 +++++++++++++++++++++++++++++++++++++--- > > 1 file changed, 54 insertions(+), 4 deletions(-) > > > > diff --git a/include/linux/overflow.h b/include/linux/overflow.h > > index a8cb6319b4fb..a3384cae49e5 100644 > > --- a/include/linux/overflow.h > > +++ b/include/linux/overflow.h > > @@ -464,6 +464,59 @@ static __always_inline size_t __must_check > > size_sub(size_t minuend, size_t subtr > > */ > > #define struct_offset(p, member) (offsetof(typeof(*(p)), member)) > > > > +/** > > + * __DECLARE_SIZED_FLEX() - helper macro for DECLARE_SIZED_FLEX() family. > > + * Allows for easily declaring a structure with a trailing flexible array > > member > > + * to have a specific size. > > + * > > + * @obj: object to be given a specific size > > + * @name: Name of the array member. > > + * @count: Number of elements in the array; must be compile-time const. > > + */ > > +#define __DECLARE_SIZED_FLEX(obj, name, count) \ > > + _Static_assert(__builtin_constant_p(count), \ > > + "default sized flex array members require compile-time > > const count"); \ > > + union { \ > > + u8 bytes[struct_size_t(obj, name, count)]; \ > > + obj; \ > > + } > > + > > +/** > > + * DECLARE_COUNTED_FLEX_ARRAY() - Declare a counted flexible array > > + * > > + * @type: Type name. > > + * @name: Name of the array member. > > + * @counter: Name of the __counted_by member. > > + */ > > +#define DECLARE_COUNTED_FLEX_ARRAY(type, name, counter)\ > > + struct { \ > > + size_t counter; \ > > + type name[] __counted_by(counter); \ > > + } > > + > > +/** > > + * DECLARE_SIZED_FLEX() - Declare a structure with a trailing flexible > > array > > + * member with a default size. > > + * > > + * @type: Type name. > > + * @name: Name of the array member. > > + * @count: Number of elements in the array; must be compile-time const. > > + */ > > +#define DECLARE_SIZED_FLEX(type, name, count) \ > > + __DECLARE_SIZED_FLEX(type name[], name, count) > > + > > +/** > > + * DECLARE_COUNTED_SIZED_FLEX() - Declare a structure with a trailing > > flexible > > + * array member counted by count, with a default size. > > + * > > + * @type: Type name. > > + * @name: Name of the array member. > > + * @counter: Name of the __counted_by member. > > + * @count: Number of elements in the array; must be compile-time const. > > + */ > > +#define DECLARE_COUNTED_SIZED_FLEX(type, name, counter, count) \ > > + __DECLARE_SIZED_FLEX(DECLARE_COUNTED_FLEX_ARRAY(type, name, counter), > > name, count) > > I ask about where these will be used because one of the things I want to > keep tied together is the "counter" and "count", which usually means the > declaration should be combined with an initializer in some way so that > the counter gets assigned, as DEFINE_FLEX() ultimately does. > > I'm worried that extracting the internal construction of __DEFINE_FLEX > means we may run the risk of increasing the risk of losing that tie.
We can add a comment to make sure to set count, or use DEFINE_FLEX to allocate it. Im not sure if there is a way for this to be a compile time error though. > So, I'd love to understand how you want to use this, as that would help > my understanding and potentially shape the design so we can keep counter > and count strongly associated. Basically, I want to use `DECLARE_COUNTED_SIZED_FLEX` for a version of this patch that uses `__counted_by` https://lore.kernel.org/linux-scsi/[email protected]/ I have checked that it does always set the `counter` variable correctly. As for `DECLARE_COUNTED_FLEX_ARRAY` there is already `DECLARE_FLEX_ARRAY` and the counted variant is already used in a few places. A good example is `struct max77759_maxq_response` which subsiqently gets allocated with `DEFINE_FLEX` https://elixir.bootlin.com/linux/v7.2-rc1/source/include/linux/mfd/max77759.h#L253 I assume you are ok with adding `__DECLARE_SIZED_FLEX` and `DECLARE_SIZED_FLEX` as they are usefull in cases where keeping structure offsets is important, similar to TRAILING_OVERLAP. An example of where `__DECLARE_SIZED_FLEX` could be used is: https://lore.kernel.org/all/[email protected]/ Thanks, Jesse Taube > > > + > > /** > > * __DEFINE_FLEX() - helper macro for DEFINE_FLEX() family. > > * Enables caller macro to pass arbitrary trailing expressions > > @@ -477,10 +530,7 @@ static __always_inline size_t __must_check > > size_sub(size_t minuend, size_t subtr > > #define __DEFINE_FLEX(type, name, member, count, trailer...) > > \ > > _Static_assert(__builtin_constant_p(count), > > \ > > "onstack flex array members require compile-time const > > count"); \ > > - union { > > \ > > - u8 bytes[struct_size_t(type, member, count)]; > > \ > > - type obj; > > \ > > - } name##_u trailer; > > \ > > + __DECLARE_SIZED_FLEX(type obj, member, count) name##_u trailer; > > \ > > type *name = (type *)&name##_u > > -Kees > > -- > Kees Cook >
