Re: scripts/gdb: issues when loading modules after lx-symbols

[Jan Kiszka]

On 05.10.20 11:29, Stefano Garzarella wrote:
> On Mon, Oct 05, 2020 at 10:33:30AM +0200, Jan Kiszka wrote:
>> On 05.10.20 10:14, Stefano Garzarella wrote:
>>> On Sun, Oct 04, 2020 at 08:52:37PM +0200, Jan Kiszka wrote:
>>>> On 01.10.20 16:31, Stefano Garzarella wrote:
>>>>> Hi,
>>>>> I had some issues with gdb scripts and kernel modules in Linux 5.9-rc7.
>>>>>
>>>>> If the modules are already loaded, and I do 'lx-symbols', all work fine.
>>>>> But, if I load a kernel module after 'lx-symbols', I had this issue:
>>>>>
>>>>> [ 5093.393940] invalid opcode: 0000 [#1] SMP PTI
>>>>> [ 5093.395134] CPU: 0 PID: 576 Comm: modprobe Not tainted 5.9.0-rc7-ste-00010-gf0b671d9608d-dirty #2
>>>>> [ 5093.397566] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
>>>>> [ 5093.400761] RIP: 0010:do_init_module+0x1/0x270
>>>>> [ 5093.402553] Code: ff ff e9 cf fe ff ff 0f 0b 49 c7 c4 f2 ff ff ff e9 c1 fe ff ff e8 5f b2 65 00 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 cc <1f> 44 00 00 55 ba 10 00 00 00 be c0 0c 00 00 48 89 e5 41 56 41 55
>>>>> [ 5093.409505] RSP: 0018:ffffc90000563d18 EFLAGS: 00010246
>>>>> [ 5093.412056] RAX: 0000000000000000 RBX: ffffffffc010a0c0 RCX: 0000000000004ee3
>>>>> [ 5093.414472] RDX: 0000000000004ee2 RSI: ffffea0001efe188 RDI: ffffffffc010a0c0
>>>>> [ 5093.416349] RBP: ffffc90000563e50 R08: 0000000000000000 R09: 0000000000000002
>>>>> [ 5093.418044] R10: 0000000000000096 R11: 00000000000008a4 R12: ffff88807a0d1280
>>>>> [ 5093.424721] R13: ffffffffc010a110 R14: ffff88807a0d1300 R15: ffffc90000563e70
>>>>> [ 5093.427138] FS:  00007f018f632740(0000) GS:ffff88807dc00000(0000) knlGS:0000000000000000
>>>>> [ 5093.430037] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>> [ 5093.432279] CR2: 000055fbe282b239 CR3: 000000007922a006 CR4: 0000000000170ef0
>>>>> [ 5093.435096] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>> [ 5093.436765] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>> [ 5093.439689] Call Trace:
>>>>> [ 5093.440954]  ? load_module+0x24b6/0x27d0
>>>>> [ 5093.443212]  ? __kernel_read+0xd6/0x150
>>>>> [ 5093.445140]  __do_sys_finit_module+0xd3/0xf0
>>>>> [ 5093.446877]  __x64_sys_finit_module+0x1a/0x20
>>>>> [ 5093.449098]  do_syscall_64+0x38/0x50
>>>>> [ 5093.450877]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>>>> [ 5093.456153] RIP: 0033:0x7f018f75c43d
>>>>> [ 5093.457728] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 2b 6a 0c 00 f7 d8 64 89 01 48
>>>>> [ 5093.466349] RSP: 002b:00007ffd7f080368 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
>>>>> [ 5093.470613] RAX: ffffffffffffffda RBX: 0000557e5c96f9c0 RCX: 00007f018f75c43d
>>>>> [ 5093.474747] RDX: 0000000000000000 RSI: 0000557e5c964288 RDI: 0000000000000003
>>>>> [ 5093.478049] RBP: 0000000000040000 R08: 0000000000000000 R09: 0000000000000000
>>>>> [ 5093.481298] R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000000
>>>>> [ 5093.483725] R13: 0000557e5c964288 R14: 0000557e5c96f950 R15: 0000557e5c9775c0
>>>>> [ 5093.485778] Modules linked in: virtio_vdpa(+) vdpa sunrpc kvm_intel kvm irqbypass virtio_blk virtio_rng rng_core [last unloaded: virtio_vdpa]
>>>>> [ 5093.488695] ---[ end trace 23712ecebc11f53c ]---
>>>>>
>>>>> Guest kernel: Linux 5.9-rc7
>>>>> gdb: GNU gdb (GDB) Fedora 9.1-6.fc32
>>>>> I tried with QEMU 4.2.1 and the latest master branch: same issue.
>>>>>
>>>>>
>>>>> I did some digging, and skipping the gdb 'add-symbol-file' command in symbol.py
>>>>> avoid the issue, but of course I don't have the symbols loaded:
>>>>>
>>>>>     diff --git a/scripts/gdb/linux/symbols.py b/scripts/gdb/linux/symbols.py
>>>>>     index 1be9763cf8bb..eadfaa4d4907 100644
>>>>>     --- a/scripts/gdb/linux/symbols.py
>>>>>     +++ b/scripts/gdb/linux/symbols.py
>>>>>     @@ -129,7 +129,7 @@ lx-symbols command."""
>>>>>                      filename=module_file,
>>>>>                      addr=module_addr,
>>>>>                      sections=self._section_arguments(module))
>>>>>     -            gdb.execute(cmdline, to_string=True)
>>>>>     +            #gdb.execute(cmdline, to_string=True)
>>>>>                  if module_name not in self.loaded_modules:
>>>>>                      self.loaded_modules.append(module_name)
>>>>>              else:
>>>>>
>>>>> I tried several modules and this happens every time after '(gdb) lx-symbols'.
>>>>>
>>>>> Do you have any hints?
>>>>>
>>>> I assume you are debugging a kernel inside QEMU/KVM, right?
>>>
>>> Right!
>>>
>>>>                                                             Does it work
>>>> without -enable-kvm?
>>>
>>> Yes, disabling kvm it works.
>>>
>>
>> OK, there it is, still...
>> What may also "work" is going down to single core.
> 
> No, I tried with single core and kvm enabled and I have the same issue.
> 
>>
>>>>
>>>> Debugging guests in KVM mode at least was unstable for a long time. I
>>>> avoided setting soft-BPs - which is what the script does for the sake of
>>>> tracking modules loading -, falling back to hw-BPs, as I had no time to
>>>> debug that further. /Maybe/ that's the issue here.
>>>
>>> Thanks for the suggestion, I'll try to have a look.
>>>
>>
>> Would be great if this issue could finally be resolved. And then covered
>> by the kvm-unit tests. Those still succeed, I think.
> 
> Yeah, I'm a bit busy, but I'll try to find a fix.
> 
> Just an update, I tried to follow your suggestion using hw-BPs, but
> unfortunately the gdb python module doesn't provide an easy way to set
> them, so I hacked a bit gdb forcing hw-BPs and with this patch applied
> to gdb I don't see the issue anymore:
> 
> diff --git a/gdb/python/py-breakpoint.c b/gdb/python/py-breakpoint.c
> index 7369c91ad9..df8ec92049 100644
> --- a/gdb/python/py-breakpoint.c
> +++ b/gdb/python/py-breakpoint.c
> @@ -57,7 +57,7 @@ struct pybp_code
>  static struct pybp_code pybp_codes[] =
>  {
>    { "BP_NONE", bp_none},
> -  { "BP_BREAKPOINT", bp_breakpoint},
> +  { "BP_BREAKPOINT", bp_hardware_breakpoint},
>    { "BP_WATCHPOINT", bp_watchpoint},
>    { "BP_HARDWARE_WATCHPOINT", bp_hardware_watchpoint},
>    { "BP_READ_WATCHPOINT", bp_read_watchpoint},
> @@ -383,7 +383,7 @@ bppy_get_location (PyObject *self, void *closure)
> 
>    BPPY_REQUIRE_VALID (obj);
> 
> -  if (obj->bp->type != bp_breakpoint)
> +  if (obj->bp->type != bp_hardware_breakpoint)
>      Py_RETURN_NONE;
> 
>    const char *str = event_location_to_string (obj->bp->location.get ());
> @@ -730,7 +730,7 @@ bppy_init (PyObject *self, PyObject *args, PyObject *kwargs)
>                                     "temporary","source", "function",
>                                     "label", "line", "qualified", NULL };
>    const char *spec = NULL;
> -  enum bptype type = bp_breakpoint;
> +  enum bptype type = bp_hardware_breakpoint;
>    int access_type = hw_write;
>    PyObject *internal = NULL;
>    PyObject *temporary = NULL;
> @@ -792,7 +792,7 @@ bppy_init (PyObject *self, PyObject *args, PyObject *kwargs)
>      {
>        switch (type)
>         {
> -       case bp_breakpoint:
> +       case bp_hardware_breakpoint:
>           {
>             event_location_up location;
>             symbol_name_match_type func_name_match_type
> @@ -834,7 +834,7 @@ bppy_init (PyObject *self, PyObject *args, PyObject *kwargs)
>             create_breakpoint (python_gdbarch,
>                                location.get (), NULL, -1, NULL,
>                                0,
> -                              temporary_bp, bp_breakpoint,
> +                              temporary_bp, bp_hardware_breakpoint,
>                                0,
>                                AUTO_BOOLEAN_TRUE,
>                                ops,
> @@ -1007,7 +1007,7 @@ gdbpy_breakpoint_created (struct breakpoint *bp)
>    if (!user_breakpoint_p (bp) && bppy_pending_object == NULL)
>      return;
> 
> -  if (bp->type != bp_breakpoint
> +  if (bp->type != bp_hardware_breakpoint
>        && bp->type != bp_watchpoint
>        && bp->type != bp_hardware_watchpoint
>        && bp->type != bp_read_watchpoint
> 
> Of course it is an hack, but it's a starting point :-)
> 

There are two key differences with soft vs. hard BPs:

 - guest code modification to inject and remove INT3 (looking at your
   panic, this might be the first thing to check)
 - different exception vectors and their reflection to or filtering from
   the guest

Both are similar in that the need to step over the intercepted
instruction in order to resume - except that soft BP needs a
remove-step-restore-INT3 cycle.

You should try debugging that without the lx-symbols script, just by
adding soft BPs and watching what happens to the guest, what becomes
incorrectly visible to it. Report, and maybe KVM folks can jump in
(adding the list).

Jan

-- 
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Corporate Competence Center Embedded Linux