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Date:   Thu, 28 Jul 2022 19:52:10 +0700
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To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Yury Norov <yury.norov@gmail.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Linux fsdevel Mailing List <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <YtR8tPTkL/L1kFkY@yury-laptop> <Yt7jfiSlNOeeookP@yury-laptop>
 <CAHk-=wjroqb0Hr9-1BCATjSuBdfdkWS6qqFaLXrwFCsHvgGH_g@mail.gmail.com>
From:   Ammar Faizi <ammarfaizi2@gmail.com>
Subject: Re: x86_64/kvm: kernel hangs on poweroff
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On 7/26/22 2:02 AM, Linus Torvalds wrote:
> On Mon, Jul 25, 2022 at 11:40 AM Yury Norov <yury.norov@gmail.com> wrote:
>>> [   22.162259] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000000
>>> [   22.163539] CPU: 0 PID: 1 Comm: systemd-shutdow Not tainted 5.19.0-rc6 #198
>>> [   22.164327] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014
>>> [   22.164327] Call Trace:
>>> [   22.164327]  <TASK>
>>> [   22.164327]  dump_stack_lvl+0x34/0x44
>>> [   22.164327]  panic+0x107/0x28a
>>> [   22.164327]  do_exit.cold+0x15/0x15
>>> [   22.164327]  ? sys_off_notify+0x3e/0x60
>>> [   22.164327]  __do_sys_reboot+0x1df/0x220
>>> [   22.164327]  ? vfs_writev+0xc7/0x190
>>> [   22.164327]  ? virtio_crypto_ctrl_vq_request+0xd0/0xd0
>>> [   22.164327]  ? fpregs_assert_state_consistent+0x1e/0x40
>>> [   22.164327]  do_syscall_64+0x3b/0x90
>>> [   22.164327]  entry_SYSCALL_64_after_hwframe+0x46/0xb0
>>> [   22.164327] RIP: 0033:0x7f42b4118443
>>> [   22.164327] Code: 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 89 fa be 69 19 12 28 bf ad de e1 fe b8 a9 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 05 c3 0f 1f 40 00 48 8b 15 f9 c9 0d 00 f7 d8
> 
> That's literally the "exit()" system call.

I took a look and it turned out it's not the "exit()" system call.

The disassembly is:

    0:    64 89 01                 mov    %eax,%fs:(%rcx)
    3:    48 83 c8 ff              or     $0xffffffffffffffff,%rax
    7:    c3                       ret
    8:    66 2e 0f 1f 84 00 00     cs nopw 0x0(%rax,%rax,1)
    f:    00 00 00
   12:    0f 1f 44 00 00           nopl   0x0(%rax,%rax,1)
   17:    89 fa                    mov    %edi,%edx
   19:    be 69 19 12 28           mov    $0x28121969,%esi
   1e:    bf ad de e1 fe           mov    $0xfee1dead,%edi
   23:    b8 a9 00 00 00           mov    $0xa9,%eax
   28:    0f 05                    syscall
   2a:*   48 3d 00 f0 ff ff        cmp    $0xfffffffffffff000,%rax        <-- trapping instruction

The syscall is with orig %rax being 0xa9 (169 in dec). That is
the reboot system call. The problem here is that the reboot
system call calls do_exit(0) with @current being the init.

So I think the way to investigate it is take a look at kernel/reboot.c,
understand why did you end up in do_exit(0) path. Hope that helps.

-- 
Ammar Faizi