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

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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: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230106200600.never.735-kees@kernel.org
[jc: minor wording tweaks]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Kees Cook 2023-01-06 12:06:04 -08:00 committed by Jonathan Corbet
parent 5a37deab42
commit 8763a30bc1
1 changed files with 26 additions and 0 deletions
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Documentation/process/deprecated.rst
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@ -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);
};
};