Watch Performance

Steve Grubb
2006-04-08T16:21:57Z

Hello,

Over the last day or two, I re-worked the user space audit code to be able to
control the new file system audit subsystem. As I was doing the work, I
became concerned about the performance impact since it appears to be using
the syscall exit filter.

The syscall exit filter (and entry filter) is expensive to use except in cases
where you need to use it. This is because each rule in it must be examined
during each syscall to see if the current syscall is of interest. The current
lspp configuration has 10 syscall audit rules.

I became curious what the measured impact would be with the current file
system audit implementation. I decide to run the same performance test that I
tested the audit system with a couple weeks ago when inode and IPC problems
were noticed. I used the lspp.16 kernel with profile=2 boot param. The
following table shows the results:

rules seconds
0 49
10 56
25 75
50 115
75 143
90 185

0 rules had this for function usage:

1284 __d_lookup 4.7380
1170 __link_path_walk 0.3098
1065 avc_has_perm_noaudit 1.2144
706 _atomic_dec_and_lock 8.4048
612 do_path_lookup 0.8204
561 dput 1.2986
509 _raw_spin_lock 2.1477

10 rules had this:

1295 __d_lookup 4.7786
1089 audit_filter_syscall 6.3684
1081 __link_path_walk 0.2862
889 avc_has_perm_noaudit 1.0137
676 audit_getname 2.6000
658 do_path_lookup 0.8820
596 _atomic_dec_and_lock 7.0952

25 rules had this:

3193 audit_filter_rules 3.0009
2178 audit_filter_syscall 12.7368
1280 __d_lookup 4.7232
1131 __link_path_walk 0.2994
956 avc_has_perm_noaudit 1.0901
652 _atomic_dec_and_lock 7.7619
530 dput 1.2269

50 rules had this:

11213 audit_filter_rules 10.5385
4654 audit_filter_syscall 27.2164
4100 selinux_task_ctxid 141.3793
1212 __d_lookup 4.4723
1103 __link_path_walk 0.2920
1012 avc_has_perm_noaudit 1.1539
788 _atomic_dec_and_lock 9.3810

75 had this:

15351 audit_filter_rules 14.4276
6032 audit_filter_syscall 35.2749
2066 selinux_task_ctxid 71.2414
1237 __d_lookup 4.5646
1184 __link_path_walk 0.3135
1014 avc_has_perm_noaudit 1.1562
592 _atomic_dec_and_lock 7.0476

and 90 rules had this:

18287 audit_filter_rules 17.1870
9173 audit_filter_syscall 53.6433
4346 selinux_task_ctxid 149.8621
1314 __link_path_walk 0.3479
1218 __d_lookup 4.4945
1070 avc_has_perm_noaudit 1.2201
682 _atomic_dec_and_lock 8.1190

As you can see, the audit_filter_rules and audit_filter_syscall overwhelmed
the profile quickly. It would not be unreasonable for a system to have 40
watches. The lspp rules have 56 of them. With 10 syscall rules added, the
performance of a correctly configured lspp machine will be similar to the 75
rules test. This represents a 186% performance hit compared to no audit
rules.

I do not believe optimizing the audit_filter_rules function will solve the
problem. I think the file system audit algorithm needs to be re-thought. It
simply cannot penalize every syscall.

There are several ways to solve the problem. Maybe what we need to do is use
the watch list to store watches on and add a new field to the context. If a
watch is triggered it sets the flag in the context. When syscall exit is
done, it checks the flag and if set, does both the watch list and the exit
list. Otherwise, it skips the watch list. I don't know if this is feasible,
or a preferred solution, but we need to start looking at how to decouple the
exit list and watches.

-Steve