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: Jonathan Corbet <corbet@lwn.net>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 Documentation/process/deprecated.rst | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index c8fd53a11a20..fcac0a929efa 100644
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -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.) 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);
+		};
+	};
-- 
2.34.1

Kees Cook <keescook@chromium.org> writes:

> 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: Jonathan Corbet <corbet@lwn.net>
> Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> Cc: linux-doc@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  Documentation/process/deprecated.rst | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)

I've applied this, but couldn't resist making one tweak...

> diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
> index c8fd53a11a20..fcac0a929efa 100644
> --- a/Documentation/process/deprecated.rst
> +++ b/Documentation/process/deprecated.rst
> @@ -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.) When the flexible array is either alone in a struct,
> +or is part of a union. 

The pedant in me couldn't stand leaving that sentence fragment, so I
made a complete sentence out of it.

Thanks,

jon

On Wed, Jan 11, 2023 at 04:11:41PM -0700, Jonathan Corbet wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > 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: Jonathan Corbet <corbet@lwn.net>
> > Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> > Cc: linux-doc@vger.kernel.org
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  Documentation/process/deprecated.rst | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> 
> I've applied this, but couldn't resist making one tweak...
> 
> > diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
> > index c8fd53a11a20..fcac0a929efa 100644
> > --- a/Documentation/process/deprecated.rst
> > +++ b/Documentation/process/deprecated.rst
> > @@ -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.) When the flexible array is either alone in a struct,
> > +or is part of a union. 
> 
> The pedant in me couldn't stand leaving that sentence fragment, so I
> made a complete sentence out of it.

Thank you; I appreciate the help! :)

-- 
Kees Cook