Commit 8763a30b authored by Kees Cook's avatar Kees Cook Committed by Jonathan Corbet
Browse files

docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage 



There wasn't any mention of when/where DECLARE_FLEX_ARRAY() should be
used, so add the rationale and an example to the deprecation docs.

Suggested-by: default avatarVincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230106200600.never.735-kees@kernel.org


[jc: minor wording tweaks]
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 5a37deab

Documentation/process/deprecated.rst

+26 −0
Original line number Diff line number Diff line
@@ -346,3 +346,29 @@ struct_size() and flex_array_size() helpers:: 
        instance->count = count;



        memcpy(instance->items, source, flex_array_size(instance, items, instance->count));



There are two special cases of replacement where the DECLARE_FLEX_ARRAY()

helper needs to be used. (Note that it is named __DECLARE_FLEX_ARRAY() for

use in UAPI headers.) Those cases are when the flexible array is either

alone in a struct or is part of a union. These are disallowed by the C99

specification, but for no technical reason (as can be seen by both the

existing use of such arrays in those places and the work-around that

DECLARE_FLEX_ARRAY() uses). For example, to convert this::



	struct something {

		...

		union {

			struct type1 one[0];

			struct type2 two[0];

		};

	};



The helper must be used::



	struct something {

		...

		union {

			DECLARE_FLEX_ARRAY(struct type1, one);

			DECLARE_FLEX_ARRAY(struct type2, two);

		};

	};