Messages in this thread | | | From | Dmitry Vyukov <> | Date | Wed, 27 Mar 2019 18:20:00 +0100 | Subject | syzbot bisection analysis |
| |
Hello,
As most of you probably already noticed, syzbot started bisecting cause commits for crashes about 2 weeks ago and sending emails like this: https://groups.google.com/d/msg/syzkaller-bugs/2XhfN2Kfbqs/0U3YnKsGBQAJ The bisection results are also available on the dashboard, e.g.: https://syzkaller.appspot.com/bug?id=02bde0600a225e8efa31bdce2e7f1b822542fef1
Bisection was probably the most popular feature request for syzbot. Cause commits allow to add the right people to CC and also should help to pin-point the harder bugs. If you are interested in details of the bisection process, some are described here: https://github.com/google/syzkaller/blob/master/docs/syzbot.md#bisection The next step step will be fix commit bisection to help identify and close bugs that are already fixed but syzbot is not aware yet.
As expected automatic bisection of kernel bugs is not completely trivial and we've got lots of incorrect results. To better understand what happens, why and how we are doing, I've analyzed the 118 bisections that we have so far for the following metrics: - if the bisection was correct or not - the crash has multiple manifestations (on the same commit or on different commits) - if the fact that bug hard to reproduce contributed to incorrect bisection - if unrelated bugs contributed to incorrect bisection - if skipped commits contributed to incorrect bisection - if disabled configs contributed to incorrect bisection There are also some auto-extracted metrics like the start release of bisection, start/end crash, etc. I won't claim that the analysis is 100% correct, which would require spending a day on each case. But it should be 95% correct or so. The results are here (there is a second tab with raw data): https://docs.google.com/spreadsheets/d/1WdBAN54-csaZpD3LgmTcIMR7NDFuQoOZZqPZ-CUqQgA
Total success rate is slightly above 50%. But there is strong correlation with how far back in history we have to go: for recently introduced bugs the rate is 70+%. And for bugs introduced since v5.0 it's 95%. So hopefully this is a good forecast for future.
The 2 major contributors to incorrect results look quite fundamental: - unrelated bugs contributed to 66% of incorrect results - hard to reproduce bugs contributed to 46% of incorrect results
I've started collecting feedback/ideas re improving bisection quality here: https://github.com/google/syzkaller/issues/1051 But so far no magic bullet come up. So please continue treating the results with understanding. The incorrect results were usually easy to identify: commit to a completely unrelated subsystem, or even non-current arch. There is always a detailed bisection log attached as well.
If you are still here, there were some curious cases too, e.g.: A bug bisected to a comment-only commit: https://groups.google.com/d/msg/syzkaller-bugs/1BSkmb_fawo/vz7GhBd0CQAJ A bug bisected to a release tag: https://groups.google.com/d/msg/syzkaller-bugs/38HP_pUXJ3s/ehD37HSxDAAJ And a fault-injection-provoked bug bisected to addition of the fault injection facility by me (which is, well, kinda expected): https://groups.google.com/d/msg/syzkaller-bugs/GYiA5CKTPXw/MA4mO01wDAAJ
Thanks
| |