2024-02-05 16:02:22

by Heiko Carstens

Subject: [PATCH 1/2] Compiler Attributes: Add __uninitialized macro

With INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO enabled the kernel will
be compiled with -ftrivial-auto-var-init=<...> which causes initialization
of stack variables at function entry time.

In order to avoid the performance impact that comes with this users can use
the "uninitialized" attribute to prevent such initialization.

Therefore provide the __uninitialized macro which can be used for cases
where INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO is enabled, but only
selected variables should not be initialized.

Signed-off-by: Heiko Carstens <[email protected]>
---
include/linux/compiler_attributes.h | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 28566624f008..f5859b8c68b4 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -333,6 +333,18 @@
*/
#define __section(section) __attribute__((__section__(section)))

+/*
+ * Optional: only supported since gcc >= 12
+ *
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-uninitialized-variable-attribute
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#uninitialized
+ */
+#if __has_attribute(__uninitialized__)
+# define __uninitialized __attribute__((__uninitialized__))
+#else
+# define __uninitialized
+#endif
+
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-unused-function-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-unused-type-attribute
--
2.40.1

2024-02-05 16:22:11

by Kees Cook

Subject: Re: [PATCH 1/2] Compiler Attributes: Add __uninitialized macro

On Mon, Feb 05, 2024 at 04:48:43PM +0100, Heiko Carstens wrote:
> With INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO enabled the kernel will
> be compiled with -ftrivial-auto-var-init=<...> which causes initialization
> of stack variables at function entry time.
>
> In order to avoid the performance impact that comes with this users can use
> the "uninitialized" attribute to prevent such initialization.
>
> Therefore provide the __uninitialized macro which can be used for cases
> where INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO is enabled, but only
> selected variables should not be initialized.
>
> Signed-off-by: Heiko Carstens <[email protected]>

Thanks! We had something similar a while back with syscall entry:
efa90c11f62e ("stack: Constrain and fix stack offset randomization with Clang builds")

Acked-by: Kees Cook <[email protected]>

--
Kees Cook

2024-02-06 01:28:54

by Nathan Chancellor

Subject: Re: [PATCH 1/2] Compiler Attributes: Add __uninitialized macro

On Mon, Feb 05, 2024 at 04:48:43PM +0100, Heiko Carstens wrote:
> With INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO enabled the kernel will
> be compiled with -ftrivial-auto-var-init=<...> which causes initialization
> of stack variables at function entry time.
>
> In order to avoid the performance impact that comes with this users can use
> the "uninitialized" attribute to prevent such initialization.
>
> Therefore provide the __uninitialized macro which can be used for cases
> where INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO is enabled, but only
> selected variables should not be initialized.
>
> Signed-off-by: Heiko Carstens <[email protected]>

Reviewed-by: Nathan Chancellor <[email protected]>

> ---
> include/linux/compiler_attributes.h | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 28566624f008..f5859b8c68b4 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -333,6 +333,18 @@
> */
> #define __section(section) __attribute__((__section__(section)))
>
> +/*
> + * Optional: only supported since gcc >= 12
> + *
> + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-uninitialized-variable-attribute
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#uninitialized
> + */
> +#if __has_attribute(__uninitialized__)
> +# define __uninitialized __attribute__((__uninitialized__))
> +#else
> +# define __uninitialized
> +#endif
> +
> /*
> * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-unused-function-attribute
> * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-unused-type-attribute
> --
> 2.40.1
>