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Date:   Tue, 07 Nov 2017 14:07:34 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [RFC PATCH] bpf: Add helpers to read useful task_struct members
To:     Alexei Starovoitov <ast@fb.com>, netdev@vger.kernel.org,
        Sandipan Das <sandipan@linux.vnet.ibm.com>
Cc:     Brendan Gregg <brendan.d.gregg@gmail.com>, daniel@iogearbox.net,
        Martin KaFai Lau <kafai@fb.com>,
        Kees Cook <keescook@chromium.org>, linux-kernel@vger.kernel.org
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        <1509815348.8zu63uatdo.naveen@linux.ibm.com>
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Alexei Starovoitov wrote:
> On 11/7/17 12:55 AM, Naveen N. Rao wrote:
>>> I thought such struct shouldn't change layout.
>>> If it is we need to fix include/linux/compiler-clang.h to do that
>>> anon struct as well.
>>
>> We considered that, but it looked to be very dependent on the version of
>> gcc used to build the kernel. But, this may be a simpler approach for
>> the shorter term.
>>
> 
> why it would depend on version of gcc?

>From what I can see, randomized_struct_fields_start is defined only for 
gcc >= 4.6. For older versions, it does not get mapped to an anonymous 
structure. We may not care for older gcc versions, but..

The other issue was that __randomize_layout maps to __designated_init 
when randstruct plugin is not enabled, which is in turn an attribute on 
gcc >= v5.1, but not otherwise.

> We just need this, no?
> 
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index de179993e039..4e29ab6187cb 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -15,3 +15,6 @@
>    * 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   };
> 
> since offsets are mandated by C standard.

Yes, this is what we're testing with and is probably sufficient for our 
purposes.

- Naveen


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