linux-kernel - Re: [RFC PATCH] bpf: Add helpers to read useful task

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Date:   Tue, 7 Nov 2017 18:29:49 -0600
From:   Atish Patra <atish.patra@...cle.com>
To:     Y Song <ys114321@...il.com>, Alexei Starovoitov <ast@...com>
Cc:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        netdev <netdev@...r.kernel.org>,
        Sandipan Das <sandipan@...ux.vnet.ibm.com>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] bpf: Add helpers to read useful task_struct members

On 11/07/2017 04:42 PM, Y Song wrote:
> On Tue, Nov 7, 2017 at 2:04 PM, Alexei Starovoitov <ast@...com> wrote:
>> On 11/8/17 6:47 AM, Y Song wrote:
>>>
>>> On Tue, Nov 7, 2017 at 1:39 PM, Alexei Starovoitov <ast@...com> wrote:
>>>>
>>>> On 11/8/17 6:14 AM, Y Song wrote:
>>>>>
>>>>>
>>>>> On Tue, Nov 7, 2017 at 12:37 AM, Naveen N. Rao
>>>>> <naveen.n.rao@...ux.vnet.ibm.com> wrote:
>>>>>>
>>>>>>
>>>>>> 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.
>>>>>
>>>>>
>>>>>
>>>>> Just tested this with bcc. bcc actually complains. the rewriter
>>>>> is not able to rewrite prev->pid where prev is "struct task_struct
>>>>> *prev".
>>>>> I will change bcc rewriter to see whether the field value is correct or
>>>>> not.
>>>>>
>>>>> Not sure my understanding is correct or not, but I am afraid that
>>>>> the above approach for clang compiler change may not work.
>>>>> If clang calculates the field offset based on header file, the offset
>>>>> may not be the same as kernel one....
>>>>
>>>>
>>>>
>>>> why is that?
>>>> When randomization is off both gcc and clang must generate the same
>>>> offsets, since it's C standard.
>>>
>>>
>>> The patch changed compiler-clang.h, so gcc still do randomization.
>>
>>
>> gcc_plugins are off by default and randomization will not be
>> turned on for any sane distro or datacenter that cares about
>> performance and stability.
>> So imo above compiler-clang.h patch together with bcc fix would
>> be enough.
>
> Agree that short time the suggested fix should be enough.
> Long time, disto could become "insane" someday :-)
>

Yup it works with clang compiler & bcc hack. Thanks :).
I verified it with task_switch.py & runqlat.py.

Are you going to push the bcc hacks to github repo ?

Regards,
Atish