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

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
...@@ -346,3 +346,29 @@ struct_size() and flex_array_size() helpers:: ...@@ -346,3 +346,29 @@ struct_size() and flex_array_size() helpers::
instance->count = count; instance->count = count;
memcpy(instance->items, source, flex_array_size(instance, items, instance->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);
};
};
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