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SubjectStrange hang with gcc 8 of kprobe multiple_kprobes test
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.

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>
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 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.

I included my config (for my virt machine, which I was also able to
trigger it with).

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.

I notice that when I get into the state, journald and the dbus_daemon
are constantly running. Perhaps the userspace time keeping went bad?

-- Steve
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