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

On Wed, May 26, 2021 at 2:01 PM Jens Axboe <axboe(a)kernel.dk&gt; 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(a)kernel.dk&gt; 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.