Hi Steve, On Mon, 3 Dec 2018 21:18:07 -0500 Steven Rostedt <rost...@goodmis.org> wrote:
> Hi Masami, > > I started testing some of my new code and the system got into a > strange state. Debugging further, I found the cause came from the > kprobe tests. It became stranger to me that I could reproduce it with > older kernels. I went back as far as 4.16 and it triggered. I thought > this very strange because I ran this test on all those kernels in the > past. > > After a bit of hair pulling, I figured out what changed. I upgraded to > gcc 8.1 (and I reproduce it with 8.2 as well). I convert back to gcc 7 > and the tests pass without issue. OK, let me see. > The issue that I notice when the system gets into this strange state is > that I can't log into the box. Nor can I reboot. Basically it's > anything to do with systemd just doesn't work (insert your jokes here > now, and then let's move on). > > I was able to narrow down what the exact function was that caused the > issues and it is: update_vsyscall() > > gcc 7 looks like this: > > ffffffff81004bf0 <update_vsyscall>: > ffffffff81004bf0: e8 0b cc 9f 00 callq ffffffff81a01800 > <__fentry__> > ffffffff81004bf1: R_X86_64_PC32 __fentry__-0x4 > ffffffff81004bf5: 48 8b 07 mov (%rdi),%rax > ffffffff81004bf8: 8b 15 96 5f 34 01 mov 0x1345f96(%rip),%edx > # ffffffff8234ab94 <vclocks_used> > ffffffff81004bfa: R_X86_64_PC32 vclocks_used-0x4 > ffffffff81004bfe: 83 05 7b 84 6f 01 01 addl $0x1,0x16f847b(%rip) > # ffffffff826fd080 <vsyscall_gtod_data> > ffffffff81004c00: R_X86_64_PC32 vsyscall_gtod_data-0x5 > ffffffff81004c05: 8b 48 24 mov 0x24(%rax),%ecx > ffffffff81004c08: b8 01 00 00 00 mov $0x1,%eax > ffffffff81004c0d: d3 e0 shl %cl,%eax > > And gcc 8 looks like this: > > ffffffff81004c90 <update_vsyscall>: > ffffffff81004c90: e8 6b cb 9f 00 callq ffffffff81a01800 > <__fentry__> > ffffffff81004c91: R_X86_64_PC32 __fentry__-0x4 > ffffffff81004c95: 48 8b 07 mov (%rdi),%rax > ffffffff81004c98: 83 05 e1 93 6f 01 01 addl $0x1,0x16f93e1(%rip) > # ffffffff826fe080 <vsyscall_gtod_data> Hm this is a RIP relative instruction, it should be modified by kprobes. > ffffffff81004c9a: R_X86_64_PC32 vsyscall_gtod_data-0x5 > ffffffff81004c9f: 8b 50 24 mov 0x24(%rax),%edx > ffffffff81004ca2: 8b 05 ec 5e 34 01 mov 0x1345eec(%rip),%eax > # ffffffff8234ab94 <vclocks_used> > ffffffff81004ca4: R_X86_64_PC32 vclocks_used-0x4 > > The test adds a kprobe (optimized) at udpate_vsyscall+5. And will > insert a jump on the two instructions after fentry. The difference > between v7 and v8 is that v7 is touching vclocks_used and v8 is > touching vsyscall_gtod_data. > > Is there some black magic going on with the vsyscall area with > vsyscall_gtod_data that is causing havoc when a kprobe is added there? I think it might miss something when preprocessing RIP relative instruction. Could you disable jump optimization as below and test what happen on update_vsyscall+5 AND update_vsyscall+8? (RIP relative preprocess must happen even if the jump optimization is disabled) # echo 0 > /proc/sys/debug/kprobes-optimization > I can dig a little more into this, but I'm currently at my HQ office > with a lot of other objectives that I must get done, and I can't work > on this much more this week. OK, let me try to reproduce it in my environment. > > I included my config (for my virt machine, which I was also able to > trigger it with). Thanks, but I think it should not depend on the kconfig. > > The test that triggers this bug is: > > tools/testing/selftests/ftrace/test.d/kprobe/multiple_kprobes.tc > > It runs the test fine, but other things just start to act up after I > run it. Yeah, thank you for digging it down. It is now much easier to me. > > I notice that when I get into the state, journald and the dbus_daemon > are constantly running. Perhaps the userspace time keeping went bad? Yeah, I think so. Maybe addl instruction becomes broken. Thank you, -- Masami Hiramatsu <mhira...@kernel.org>
