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

[Stefano Garzarella]

On Mon, Oct 05, 2020 at 01:48:35PM +0200, Jan Kiszka wrote:
> On 05.10.20 13:05, Stefano Garzarella wrote:
> > On Mon, Oct 05, 2020 at 11:45:41AM +0200, Jan Kiszka wrote:
> >> 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.
> > 
> > Thanks for the explanation!
> > 
> >>
> >> 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).
> > 
> > It works well. Also using lx-symbols, without loading new modules in the
> > guest after it, I can debug the guest kernel with soft-BP.
> 
> Even if putting the BP manually at the same location as lx-symbols does?

Yes. I put the BP manually and I did 'continue' after the break without
panic.

> BTW, that location is sane?

Yes, it seems sane.

> 
> > 
> > The issue with soft-BP seems related to 'add-symbol-file' commands;
> > if I skip it in the python script, I don't have the panic.
> 
> So, it's the pattern of stopping at a soft-BP, reloading symbols,
> resuming after the BP?

Yes, but only if the module X was not loaded.

I just tried the following pattern and I don't have the panic:

guest$ modprobe X
                                    (gdb) lx-symbols
                                    loading vmlinux
                                    scanning for modules in /linux/build
                                    loading @0xffffffffc0147000: X.ko
                                    (gdb) c
                                    Continuing.
guest$ rmmod X
guest$ modprobe X
                                    refreshing all symbols to reload module 'X'
                                    loading vmlinux
                                    loading @0xffffffffc0147000: X.ko


Stefano