Re: [RFC PATCH 2/9] audit,io_uring,io-wq: add some basic audit support to io_uring

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/26/21 7:44 PM, Paul Moore wrote:
> On Wed, May 26, 2021 at 2:01 PM Jens Axboe <axboe@kernel.dk> wrote:
>> On 5/26/21 11:54 AM, Jens Axboe wrote:
>>> On 5/26/21 11:31 AM, Jens Axboe wrote:
>>>> On 5/26/21 11:15 AM, Jens Axboe wrote:
>>>>> On 5/25/21 8:04 PM, Paul Moore wrote:
>>>>>> On Tue, May 25, 2021 at 9:11 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>> On 5/24/21 1:59 PM, Paul Moore wrote:
>>>>>>>> That said, audit is not for everyone, and we have build time and
>>>>>>>> runtime options to help make life easier.  Beyond simply disabling
>>>>>>>> audit at compile time a number of Linux distributions effectively
>>>>>>>> shortcut audit at runtime by adding a "never" rule to the audit
>>>>>>>> filter, for example:
>>>>>>>>
>>>>>>>>  % auditctl -a task,never
>>>>>>>
>>>>>>> As has been brought up, the issue we're facing is that distros have
>>>>>>> CONFIG_AUDIT=y and hence the above is the best real world case outside
>>>>>>> of people doing custom kernels. My question would then be how much
>>>>>>> overhead the above will add, considering it's an entry/exit call per op.
>>>>>>> If auditctl is turned off, what is the expectation in turns of overhead?
>>>>>>
>>>>>> I commented on that case in my last email to Pavel, but I'll try to go
>>>>>> over it again in a little more detail.
>>>>>>
>>>>>> As we discussed earlier in this thread, we can skip the req->opcode
>>>>>> check before both the _entry and _exit calls, so we are left with just
>>>>>> the bare audit calls in the io_uring code.  As the _entry and _exit
>>>>>> functions are small, I've copied them and their supporting functions
>>>>>> below and I'll try to explain what would happen in CONFIG_AUDIT=y,
>>>>>> "task,never" case.
>>>>>>
>>>>>> +  static inline struct audit_context *audit_context(void)
>>>>>> +  {
>>>>>> +    return current->audit_context;
>>>>>> +  }
>>>>>>
>>>>>> +  static inline bool audit_dummy_context(void)
>>>>>> +  {
>>>>>> +    void *p = audit_context();
>>>>>> +    return !p || *(int *)p;
>>>>>> +  }
>>>>>>
>>>>>> +  static inline void audit_uring_entry(u8 op)
>>>>>> +  {
>>>>>> +    if (unlikely(audit_enabled && audit_context()))
>>>>>> +      __audit_uring_entry(op);
>>>>>> +  }
>>>>>>
>>>>>> We have one if statement where the conditional checks on two
>>>>>> individual conditions.  The first (audit_enabled) is simply a check to
>>>>>> see if anyone has "turned on" auditing at runtime; historically this
>>>>>> worked rather well, and still does in a number of places, but ever
>>>>>> since systemd has taken to forcing audit on regardless of the admin's
>>>>>> audit configuration it is less useful.  The second (audit_context())
>>>>>> is a check to see if an audit_context has been allocated for the
>>>>>> current task.  In the case of "task,never" current->audit_context will
>>>>>> be NULL (see audit_alloc()) and the __audit_uring_entry() slowpath
>>>>>> will never be called.
>>>>>>
>>>>>> Worst case here is checking the value of audit_enabled and
>>>>>> current->audit_context.  Depending on which you think is more likely
>>>>>> we can change the order of the check so that the
>>>>>> current->audit_context check is first if you feel that is more likely
>>>>>> to be NULL than audit_enabled is to be false (it may be that way now).
>>>>>>
>>>>>> +  static inline void audit_uring_exit(int success, long code)
>>>>>> +  {
>>>>>> +    if (unlikely(!audit_dummy_context()))
>>>>>> +      __audit_uring_exit(success, code);
>>>>>> +  }
>>>>>>
>>>>>> The exit call is very similar to the entry call, but in the
>>>>>> "task,never" case it is very simple as the first check to be performed
>>>>>> is the current->audit_context check which we know to be NULL.  The
>>>>>> __audit_uring_exit() slowpath will never be called.
>>>>>
>>>>> I actually ran some numbers this morning. The test base is 5.13+, and
>>>>> CONFIG_AUDIT=y and CONFIG_AUDITSYSCALL=y is set for both the baseline
>>>>> test and the test with this series applied. I used your git branch as of
>>>>> this morning.
>>>>>
>>>>> The test case is my usual peak perf test, which is random reads at
>>>>> QD=128 and using polled IO. It's a single core test, not threaded. I ran
>>>>> two different tests - one was having a thread just do the IO, the other
>>>>> is using SQPOLL to do the IO for us. The device is capable than more
>>>>> IOPS than a single core can deliver, so we're CPU limited in this test.
>>>>> Hence it's a good test case as it does actual work, and shows software
>>>>> overhead quite nicely. Runs are very stable (less than 0.5% difference
>>>>> between runs on the same base), yet I did average 4 runs.
>>>>>
>>>>> Kernel              SQPOLL          IOPS            Perf diff
>>>>> ---------------------------------------------------------
>>>>> 5.13                0               3029872         0.0%
>>>>> 5.13                1               3031056         0.0%
>>>>> 5.13 + audit        0               2894160         -4.5%
>>>>> 5.13 + audit        1               2886168         -4.8%
>>>>>
>>>>> That's an immediate drop in perf of almost 5%. Looking at a quick
>>>>> profile of it (nothing fancy, just checking for 'audit' in the profile)
>>>>> shows this:
>>>>>
>>>>> +    2.17%  io_uring  [kernel.vmlinux]  [k] __audit_uring_entry
>>>>> +    0.71%  io_uring  [kernel.vmlinux]  [k] __audit_uring_exit
>>>>>      0.07%  io_uring  [kernel.vmlinux]  [k] __audit_syscall_entry
>>>>>      0.02%  io_uring  [kernel.vmlinux]  [k] __audit_syscall_exit
>>>>>
>>>>> Note that this is with _no_ rules!
>>>>
>>>> io_uring also supports a NOP command, which basically just measures
>>>> reqs/sec through the interface. Ran that as well:
>>>>
>>>> Kernel               SQPOLL          IOPS            Perf diff
>>>> ---------------------------------------------------------
>>>> 5.13         0               31.05M          0.0%
>>>> 5.13 + audit 0               25.31M          -18.5%
>>>>
>>>> and profile for the latter includes:
>>>>
>>>> +    5.19%  io_uring  [kernel.vmlinux]  [k] __audit_uring_entry
>>>> +    4.31%  io_uring  [kernel.vmlinux]  [k] __audit_uring_exit
>>>>      0.26%  io_uring  [kernel.vmlinux]  [k] __audit_syscall_entry
>>>>      0.08%  io_uring  [kernel.vmlinux]  [k] __audit_syscall_exit
>>>
>>> As Pavel correctly pointed it, looks like auditing is enabled. And
>>> indeed it was! Hence the above numbers is without having turned off
>>> auditing. Running the NOPs after having turned off audit, we get 30.6M
>>> IOPS, which is down about 1.5% from the baseline. The results for the
>>> polled random read test above did _not_ change from this, they are still
>>> down the same amount.
>>>
>>> Note, and I should have included this in the first email, this is not
>>> any kind of argument for or against audit logging. It's purely meant to
>>> be a set of numbers that show how the current series impacts
>>> performance.
>>
>> And finally, just checking if we make it optional per opcode if we see
>> any real impact, and the answer is no. Using the below patch which
>> effectively bypasses audit calls unless the opcode has flagged the need
>> to do so, I cannot measure any difference in perf (as expected).
>>
>> To turn this into something useful, my suggestion as a viable path
>> forward would be:
>>
>> 1) Use something like the below patch and flag request types that we
>>    want to do audit logging for.
>>
>> 2) As Pavel suggested, eliminate the need for having both and entry/exit
>>    hook, turning it into just one. That effectively cuts the number of
>>    checks and calls in half.
> 
> I suspect the updated working-io_uring branch with HEAD at
> 1f25193a3f54 (updated a short time ago, see my last email in this
> thread) will improve performance.  Also, as has been mention several

See the email you replied to, ~1.5% was basically an overhead of
two `if (io_op_defs[req->opcode].audit)` in case of nops, where at
least once req->opcode is cached. But to be completely fair, misses
unlikely

-- 
Pavel Begunkov