Messages in this thread | | | Date | Fri, 20 Sep 2019 17:31:25 +0100 | From | Mark Rutland <> | Subject | Re: Kernel Concurrency Sanitizer (KCSAN) |
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On Fri, Sep 20, 2019 at 04:18:57PM +0200, Marco Elver wrote: > Hi all,
Hi,
> 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.
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
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] ==================================================================
> 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.
> * How/when to upstream KCSAN?
I would love to see this soon!
Thanks, Mark.
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