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Marco Elver edited this page Oct 21, 2019 · 15 revisions

Kernel Concurrency Sanitizer (KCSAN)

Kernel Concurrency Sanitizer (KCSAN) is a sampling watchpoint-based data-race detector. More details can be found in Documentation.

There is one main branch related to KCSAN:

  • kcsan: Contains KCSAN only, rebased on top of stable upstream kernel (as long as KCSAN is not yet upstream).

Some tags of interest:

  • kcsan_v5.3-with-fixes: Contains initial KCSAN release with various bug-fixes for races detected -- not exhaustive, only for bugs reported very frequently in order to suppress them in initial testing; the commit messages for those bugfixes include the KCSAN report as-is.

Testing & Fuzzing

Initially (Sep 2019) we have been running KCSAN via Syzkaller for several weeks, and have found numerous bugs (running for just 2 days found over 300 unique data-races).

We now have a public Syzkaller instance. Bugs will slowly appear on the dashboard, currently after moderation only, to keep the volume of reported bugs manageable.

Note on Data-Races

KCSAN detects data-races according to the LKMM. Although some data-races are due to logic bugs, or missed synchronization elsewhere, we expect a large number of data-races to be due to concurrent plain accesses.

We want to point out that racing accesses that are not considered logic bugs ("benign"), are still data-races while not marked atomic (READ_ONCE, WRITE_ONCE, atomic_t, etc.) and behaviour may still be undefined -- this LWN article provides good background why such data-races are problems, with some more info on our wiki page.

Rebase Strategy

To rebase KCSAN against latest upstream tree:

  • git rebase <upstream> kcsan
  • Then force-push kcsan.

Rebasing KCSAN with fixes on top can be done as follows:

  • git rebase -i --onto kcsan $(git log --format=oneline <my-branch-with-fixes> | grep -v 'BUG: KCSAN' | head -n 1 | cut -d " " -f 1) <my-branch-with-fixes>

Upstream Fixes of Data-Races found by KCSAN

This is a list of known upstream fixes for data-races found by KCSAN. Last updated: Oct 21, 2019.


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