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

Paul Moore
2021-05-26T18:44:44Z

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
times now, for audit to work we need both the _entry and _exit call.

--
paul moore