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

[Jens Axboe]

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 at 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

-- 
Jens Axboe