Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <328c2dff-7d6d-0dc0-46cf-ae715845903e@fb.com>
References: <20171103065833.8076-1-sandipan@linux.vnet.ibm.com>
 <94a4761f-1b51-8b70-fb7f-3cea91c69717@fb.com> <1509815348.8zu63uatdo.naveen@linux.ibm.com>
 <4acdc081-341d-ee91-a591-b1d331a8c8d5@fb.com> <1509982000.092la4257a.naveen@linux.ibm.com>
 <df92faa5-ca2b-a623-e074-e14b7345f8e5@fb.com> <1510042830.62prnydrlw.naveen@linux.ibm.com>
 <CAH3MdRX-XehVX5A1O7+vxR+UuG46z7mRSMOer4hsxFYdKjWKUg@mail.gmail.com>
 <fc6e7617-ee4e-b799-7565-bdfc4f7e0159@fb.com> <CAH3MdRVm+6FroZ5MAENtqiMCDY5kR_74JY-fY9-Nr5Sw96pCHg@mail.gmail.com>
 <328c2dff-7d6d-0dc0-46cf-ae715845903e@fb.com>
From:   Y Song <ys114321@gmail.com>
Date:   Tue, 7 Nov 2017 14:42:46 -0800
Message-ID: <CAH3MdRXxVX_PX64+2_e_yvy8BMCzKY-vZzdzTCaQWr_XathmNA@mail.gmail.com>
Subject: Re: [RFC PATCH] bpf: Add helpers to read useful task_struct members
To:     Alexei Starovoitov <ast@fb.com>
Cc:     "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        netdev <netdev@vger.kernel.org>,
        Sandipan Das <sandipan@linux.vnet.ibm.com>,
        Brendan Gregg <brendan.d.gregg@gmail.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Kees Cook <keescook@chromium.org>, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Nov 7, 2017 at 2:04 PM, Alexei Starovoitov <ast@fb.com> wrote:
> On 11/8/17 6:47 AM, Y Song wrote:
>>
>> On Tue, Nov 7, 2017 at 1:39 PM, Alexei Starovoitov <ast@fb.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@linux.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 :-)

From 1583464915287567078@xxx Wed Nov 08 02:57:00 +0000 2017
X-GM-THRID: 1583027203607239623
X-Gmail-Labels: Inbox,Category Forums,HistoricalUnread