Messages in this thread | | | From | Marco Elver <> | Date | Fri, 20 Sep 2019 19:51:04 +0200 | Subject | Re: Kernel Concurrency Sanitizer (KCSAN) |
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On Fri, 20 Sep 2019 at 18:47, Dmitry Vyukov <dvyukov@google.com> wrote: > > On Fri, Sep 20, 2019 at 6:31 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > On Fri, Sep 20, 2019 at 04:18:57PM +0200, Marco Elver wrote: > > > We would like to share a new data-race detector for the Linux kernel: > > > Kernel Concurrency Sanitizer (KCSAN) -- > > > https://github.com/google/ktsan/wiki/KCSAN (Details: > > > https://github.com/google/ktsan/blob/kcsan/Documentation/dev-tools/kcsan.rst) > > > > Nice! > > > > BTW kcsan_atomic_next() is missing a stub definition in <linux/kcsan.h> > > when !CONFIG_KCSAN: > > > > https://github.com/google/ktsan/commit/a22a093a0f0d0b582c82cdbac4f133a3f61d207c#diff-19d7c475b4b92aab8ba440415ab786ec > > > > ... and I think the kcsan_{begin,end}_atomic() stubs need to be static > > inline too.
Thanks for catching, fixed and pushed. Feel free to rebase your arm64 branch.
> > It looks like this is easy enough to enable on arm64, with the only real > > special case being secondary_start_kernel() which we might want to > > refactor to allow some portions to be instrumented. > > > > I pushed the trivial patches I needed to get arm64 booting to my arm64/kcsan > > branch: > > > > git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git arm64/kcsan
Cool, thanks for testing!
> > We have some interesting splats at boot time in stop_machine, which > > don't seem to have been hit/fixed on x86 yet in the kcsan-with-fixes > > branch, e.g. > > > > [ 0.237939] ================================================================== > > [ 0.239431] BUG: KCSAN: data-race in multi_cpu_stop+0xa8/0x198 and set_state+0x80/0xb0 > > [ 0.241189] > > [ 0.241606] write to 0xffff00001003bd00 of 4 bytes by task 24 on cpu 3: > > [ 0.243435] set_state+0x80/0xb0 > > [ 0.244328] multi_cpu_stop+0x16c/0x198 > > [ 0.245406] cpu_stopper_thread+0x170/0x298 > > [ 0.246565] smpboot_thread_fn+0x40c/0x560 > > [ 0.247696] kthread+0x1a8/0x1b0 > > [ 0.248586] ret_from_fork+0x10/0x18 > > [ 0.249589] > > [ 0.250006] read to 0xffff00001003bd00 of 4 bytes by task 14 on cpu 1: > > [ 0.251804] multi_cpu_stop+0xa8/0x198 > > [ 0.252851] cpu_stopper_thread+0x170/0x298 > > [ 0.254008] smpboot_thread_fn+0x40c/0x560 > > [ 0.255135] kthread+0x1a8/0x1b0 > > [ 0.256027] ret_from_fork+0x10/0x18 > > [ 0.257036] > > [ 0.257449] Reported by Kernel Concurrency Sanitizer on: > > [ 0.258918] CPU: 1 PID: 14 Comm: migration/1 Not tainted 5.3.0-00007-g67ab35a199f4-dirty #3 > > [ 0.261241] Hardware name: linux,dummy-virt (DT) > > [ 0.262517] ==================================================================>
Thanks, the fixes in -with-fixes were ones I only encountered with Syzkaller, where I disable KCSAN during boot. I've just added a fix for this race and pushed to kcsan-with-fixes.
> > > To those of you who we mentioned at LPC that we're working on a > > > watchpoint-based KTSAN inspired by DataCollider [1], this is it (we > > > renamed it to KCSAN to avoid confusion with KTSAN). > > > [1] http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf > > > > > > In the coming weeks we're planning to: > > > * Set up a syzkaller instance. > > > * Share the dashboard so that you can see the races that are found. > > > * Attempt to send fixes for some races upstream (if you find that the > > > kcsan-with-fixes branch contains an important fix, please feel free to > > > point it out and we'll prioritize that). > > > > > > There are a few open questions: > > > * The big one: most of the reported races are due to unmarked > > > accesses; prioritization or pruning of races to focus initial efforts > > > to fix races might be required. Comments on how best to proceed are > > > welcome. We're aware that these are issues that have recently received > > > attention in the context of the LKMM > > > (https://lwn.net/Articles/793253/). > > > > I think the big risk here is drive-by "fixes" masking the warnings > > rather than fixing the actual issue. It's easy for people to suppress a > > warning with {READ,WRITE}_ONCE(), so they're liable to do that even the > > resulting race isn't benign. > > > > I don't have a clue how to prevent that, though. > > I think this is mostly orthogonal problem. E.g. for some syzbot bugs I > see fixes that also try to simply "shut up" the immediate > manifestation with whatever means, e.g. sprinkling some slinlocks. So > (1) it's not unique to atomics, (2) presence of READ/WRITE_ONCE will > make the reader aware of the fact that this runs concurrently with > something else, and then they may ask themselves why this runs > concurrently with something when the object is supposed to be private > to the thread, and then maybe they re-fix it properly. Whereas if it's > completely unmarked, nobody will even notice that this code accesses > the object concurrently with other code. So even if READ/WRITE_ONCE > was a wrong fix, it's still better to have it rather than not.
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