linux-kernel - Re: [PATCH] compiler, clang: handle randomizable anonymous structs

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Message-ID: <20171202021806.7ilhguxt4llil6cs@ast-mbp>
Date:   Fri, 1 Dec 2017 18:18:08 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Sandipan Das <sandipan@...ux.vnet.ibm.com>
Cc:     akpm@...ux-foundation.org, gregkh@...uxfoundation.org,
        kstewart@...uxfoundation.org, linux-kernel@...r.kernel.org,
        yhs@...com, atish.patra@...cle.com,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH] compiler, clang: handle randomizable anonymous structs

On Thu, Nov 09, 2017 at 12:16:45PM +0530, Sandipan Das wrote:
> The GCC randomize layout plugin can randomize the member
> offsets of sensitive kernel data structures. To use this
> feature, certain annotations and members are added to the
> structures which affect the member offsets even if this
> plugin is not used.
> 
> All of these structures are completely randomized, except
> for task_struct which leaves out some of its members. All
> the other members are wrapped within an anonymous struct
> with the __randomize_layout attribute. This is done using
> the randomized_struct_fields_start and
> randomized_struct_fields_end defines. When the plugin is
> disabled, the behaviour of this attribute can vary based
> on the GCC version. For GCC 5.1+, this attribute maps to
> __designated_init otherwise it is just an empty define
> but the anonymous structure is still present. For other
> compilers, both randomized_struct_fields_start and
> randomized_struct_fields_end default to empty defines
> meaning the anonymous structure is not introduced at all.
> So, if a module compiled with Clang, such as a BPF program,
> needs to access task_struct fields such as pid and comm,
> the offsets of these members as recognized by Clang are
> different from those recognized by modules compiled with
> GCC. If GCC 4.6+ is used to build the kernel, this can be
> solved by introducing appropriate defines for Clang so
> that the anonymous structure is seen when determining the
> offsets for the members.
> 
> Signed-off-by: Sandipan Das <sandipan@...ux.vnet.ibm.com>
> ---
>  include/linux/compiler-clang.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 54dfef70a072..780b1242bf24 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -16,3 +16,6 @@
>   * with any version that can compile the kernel
>   */
>  #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
> +
> +#define randomized_struct_fields_start	struct {
> +#define randomized_struct_fields_end	};
> -- 
> 2.13.6

Andrew or Greg,
could you please pull this patch into 4.13 and 4.14 releases as well ?
Sorry we missed the stable tag when submitting it originally.
It is commit 4ca59b14e588 ("include/linux/compiler-clang.h: handle randomizable anonymous structs")
Thanks