On Sat, 18 Sept 2021 at 10:07, Liu Shixin <liushixin2@xxxxxxxxxx> wrote: > > On 2021/9/16 16:49, Marco Elver wrote: > > On Thu, 16 Sept 2021 at 03:20, Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote: > >> Hi Marco, > >> > >> We found kfence_test will fails on ARM64 with this patch with/without > >> CONFIG_DETECT_HUNG_TASK, > >> > >> Any thought ? > > Please share log and instructions to reproduce if possible. Also, if > > possible, please share bisection log that led you to this patch. > > > > I currently do not see how this patch would cause that, it only > > increases the timeout duration. > > > > I know that under QEMU TCG mode, there are occasionally timeouts in > > the test simply due to QEMU being extremely slow or other weirdness. > > > > > Hi Marco, > > There are some of the results of the current test: > 1. Using qemu-kvm on arm64 machine, all testcase can pass. > 2. Using qemu-system-aarch64 on x86_64 machine, randomly some testcases fail. > 3. Using qemu-system-aarch64 on x86_64, but removing the judgment of kfence_allocation_key in kfence_alloc(), all testcase can pass. > > I add some printing to the kernel and get very strange results. > I add a new variable kfence_allocation_key_gate to track the > state of kfence_allocation_key. As shown in the following code, theoretically, > if kfence_allocation_key_gate is zero, then kfence_allocation_key must be > enabled, so the value of variable error in kfence_alloc() should always be > zero. In fact, all the passed testcases fit this point. But as shown in the > following failed log, although kfence_allocation_key has been enabled, it's > still check failed here. > > So I think static_key might be problematic in my qemu environment. > The change of timeout is not a problem but caused us to observe this problem. > I tried changing the wait_event to a loop. I set timeout to HZ and re-enable/disabled > in each loop, then the failed testcase disappears. Nice analysis, thanks! What I gather is that static_keys/jump_labels are somehow broken in QEMU. This does remind me that I found a bug in QEMU that might be relevant: https://bugs.launchpad.net/qemu/+bug/1920934 Looks like it was never fixed. :-/ The failures I encountered caused the kernel to crash, but never saw the kfence test to fail due to that (never managed to get that far). Though the bug I saw was on x86 TCG mode, and I never tried arm64. If you can, try to build a QEMU with ASan and see if you also get the same use-after-free bug. Unless we observe the problem on a real machine, I think for now we can conclude with fairly high confidence that QEMU TCG still has issues and cannot be fully trusted here (see bug above). Thanks, -- Marco
