* [PATCH] docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage
@ 2023-01-06 20:06 Kees Cook
2023-01-11 23:11 ` Jonathan Corbet
0 siblings, 1 reply; 3+ messages in thread
From: Kees Cook @ 2023-01-06 20:06 UTC (permalink / raw)
To: Jonathan Corbet
Cc: Kees Cook, Vincent Mailhol, Gustavo A. R. Silva, linux-doc,
linux-kernel, linux-hardening
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
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage
2023-01-06 20:06 [PATCH] docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage Kees Cook
@ 2023-01-11 23:11 ` Jonathan Corbet
2023-01-12 19:38 ` Kees Cook
0 siblings, 1 reply; 3+ messages in thread
From: Jonathan Corbet @ 2023-01-11 23:11 UTC (permalink / raw)
To: Kees Cook
Cc: Kees Cook, Vincent Mailhol, Gustavo A. R. Silva, linux-doc,
linux-kernel, linux-hardening
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
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage
2023-01-11 23:11 ` Jonathan Corbet
@ 2023-01-12 19:38 ` Kees Cook
0 siblings, 0 replies; 3+ messages in thread
From: Kees Cook @ 2023-01-12 19:38 UTC (permalink / raw)
To: Jonathan Corbet
Cc: Vincent Mailhol, Gustavo A. R. Silva, linux-doc, linux-kernel,
linux-hardening
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
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