Re: [PATCH] scripts/gdb: rework lx-symbols gdb script

[Jan Kiszka]

On 11.08.21 22:10, Maxim Levitsky wrote:
> On Wed, 2021-08-11 at 21:01 +0200, Jan Kiszka wrote:
>> On 11.08.21 15:31, Maxim Levitsky wrote:
>>> Fix several issues that are present in lx-symbols script:
>>>
>>> * Track module unloads by placing another software breakpoint at
>>>   'free_module'
>>>   (force uninline this symbol just in case), and use remove-symbol-file
>>>   gdb command to unload the symobls of the module that is unloading.
>>>
>>>   That gives the gdb a chance to mark all software breakpoints from
>>>   this module as pending again.
>>>   Also remove the module from the 'known' module list once it is unloaded.
>>>
>>> * Since we now track module unload, we don't need to reload all
>>>   symbols anymore when 'known' module loaded again
>>>   (that can't happen anymore).
>>>   This allows reloading a module in the debugged kernel to finish
>>>   much faster, while lx-symbols tracks module loads and unloads.
>>>
>>> * Disable/enable all gdb breakpoints on both module load and unload
>>>   breakpoint hits, and not only in 'load_all_symbols' as was done before.
>>>   (load_all_symbols is no longer called on breakpoint hit)
>>>   That allows gdb to avoid getting confused about the state of the
>>>   (now two) internal breakpoints we place.
>>>   Otherwise it will leave them in the kernel code segment, when
>>>   continuing which triggers a guest kernel panic as soon as it skips
>>>   over the 'int3' instruction and executes the garbage tail of the optcode
>>>   on which the breakpoint was placed.
>>>
>>> * Block SIGINT while the script is running as this seems to crash gdb
>>>
>>> * Add a basic check that kernel is already loaded into the guest memory
>>>   to avoid confusing errors.
>>>
>>> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
>>> ---
>>>  kernel/module.c              |   8 +-
>>>  scripts/gdb/linux/symbols.py | 203 +++++++++++++++++++++++------------
>>>  2 files changed, 143 insertions(+), 68 deletions(-)
>>>
>>> diff --git a/kernel/module.c b/kernel/module.c
>>> index ed13917ea5f3..242bd4bb0b55 100644
>>> --- a/kernel/module.c
>>> +++ b/kernel/module.c
>>> @@ -906,8 +906,12 @@ int module_refcount(struct module *mod)
>>>  }
>>>  EXPORT_SYMBOL(module_refcount);
>>>  
>>> -/* This exists whether we can unload or not */
>>> -static void free_module(struct module *mod);
>>> +/* This exists whether we can unload or not
>>> + * Keep it uninlined to provide a reliable breakpoint target,
>>> + * e.g. for the gdb helper command 'lx-symbols'.
>>> + */
>>> +
>>> +static noinline void free_module(struct module *mod);
>>>  
>>>  SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
>>>  		unsigned int, flags)
>>
>> You likely want and need to push that as separate patch, analogously to
>> be02a1862304.
> 
> I will do.
> 
>>
>> And as you are factoring the patch, maybe think about whether you can
>> split the changes to symbols.py into logical steps as well. The commit
>> messages suggests that, thought that might be misleading.
> 
> I can try doing that.
> 
>>
>>> diff --git a/scripts/gdb/linux/symbols.py b/scripts/gdb/linux/symbols.py
>>> index 08d264ac328b..78e278fb4bad 100644
>>> --- a/scripts/gdb/linux/symbols.py
>>> +++ b/scripts/gdb/linux/symbols.py
>>> @@ -14,45 +14,23 @@
>>>  import gdb
>>>  import os
>>>  import re
>>> +import signal
>>>  
>>>  from linux import modules, utils
>>>  
>>>  
>>>  if hasattr(gdb, 'Breakpoint'):
>>> -    class LoadModuleBreakpoint(gdb.Breakpoint):
>>> -        def __init__(self, spec, gdb_command):
>>> -            super(LoadModuleBreakpoint, self).__init__(spec, internal=True)
>>> +
>>> +    class BreakpointWrapper(gdb.Breakpoint):
>>> +        def __init__(self, callback, **kwargs):
>>> +            super(BreakpointWrapper, self).__init__(internal=True, **kwargs)
>>>              self.silent = True
>>> -            self.gdb_command = gdb_command
>>> +            self.callback = callback
>>>  
>>>          def stop(self):
>>> -            module = gdb.parse_and_eval("mod")
>>> -            module_name = module['name'].string()
>>> -            cmd = self.gdb_command
>>> -
>>> -            # enforce update if object file is not found
>>> -            cmd.module_files_updated = False
>>> -
>>> -            # Disable pagination while reporting symbol (re-)loading.
>>> -            # The console input is blocked in this context so that we would
>>> -            # get stuck waiting for the user to acknowledge paged output.
>>> -            show_pagination = gdb.execute("show pagination", to_string=True)
>>> -            pagination = show_pagination.endswith("on.\n")
>>> -            gdb.execute("set pagination off")
>>> -
>>> -            if module_name in cmd.loaded_modules:
>>> -                gdb.write("refreshing all symbols to reload module "
>>> -                          "'{0}'\n".format(module_name))
>>> -                cmd.load_all_symbols()
>>> -            else:
>>> -                cmd.load_module_symbols(module)
>>> -
>>> -            # restore pagination state
>>> -            gdb.execute("set pagination %s" % ("on" if pagination else "off"))
>>> -
>>> +            self.callback()
>>>              return False
>>>  
>>> -
>>>  class LxSymbols(gdb.Command):
>>>      """(Re-)load symbols of Linux kernel and currently loaded modules.
>>>  
>>> @@ -61,15 +39,52 @@ are scanned recursively, starting in the same directory. Optionally, the module
>>>  search path can be extended by a space separated list of paths passed to the
>>>  lx-symbols command."""
>>>  
>>> -    module_paths = []
>>> -    module_files = []
>>> -    module_files_updated = False
>>> -    loaded_modules = []
>>> -    breakpoint = None
>>> -
>>>      def __init__(self):
>>>          super(LxSymbols, self).__init__("lx-symbols", gdb.COMMAND_FILES,
>>>                                          gdb.COMPLETE_FILENAME)
>>> +        self.module_paths = []
>>> +        self.module_files = []
>>> +        self.module_files_updated = False
>>> +        self.loaded_modules = {}
>>> +        self.internal_breakpoints = []
>>> +
>>> +    # prepare GDB for loading/unloading a module
>>> +    def _prepare_for_module_load_unload(self):
>>> +
>>> +        self.blocked_sigint = False
>>> +
>>> +        # block SIGINT during execution to avoid gdb crash
>>> +        sigmask = signal.pthread_sigmask(signal.SIG_BLOCK, [])
>>> +        if not signal.SIGINT in sigmask:
>>> +            self.blocked_sigint = True
>>> +            signal.pthread_sigmask(signal.SIG_BLOCK, {signal.SIGINT})
>>> +
>>> +        # disable all breakpoints to workaround a GDB bug where it would
>>> +        # not correctly resume from an internal breakpoint we placed
>>> +        # in do_module_init/free_module (it leaves the int3
>>
>> Seems the comment ends prematurely.
> 
> Yep, GDB leaves the int3 instruction in the guest memory, and the guest dies after
> it encounters the truncated instruction that follows it.
> 
>>
>> Any reference to a gdb bug tracker entry? Or affected versions? The
>> description is a bit too fuzzy.
> 
> Well stricly speaking this isn't a GDB bug.
> GDB documentation explictly prohibits what we are doing in this script.
> 
> https://sourceware.org/gdb/current/onlinedocs/gdb/Breakpoints-In-Python.html
> 
> "You should not alter the execution state of the inferior (i.e., step, next, etc.), alter the current frame context 
> (i.e., change the current active frame), or alter, add or delete any breakpoint. 
> As a general rule, you should not alter any data within GDB or the inferior at this time."
> 
> However we are reloading the whole symbol table as a response to a breakpoint.
> 
> However there is pretty much no other way to do what this script does so the next best thing
> is to workaround this as was already partially done, and I just made it more robust.
> 
> Same for blocking SIGINT which I added, which otherwise crashes GDB
> while the symbols are reloading.
> It probably will also be blamed on this.
> 
> Do you think I might have some luck taking with GDB maintainers and asking them to support
> this use case of updating symbol table when a breakpoint hits?
> 

We should at least clarify if it's a GDB bug or our use case is outside
of the envisioned ones, thus need to account for that. Then we should
not call it a bug.

[...]

>>> +            if not module_list:
>>> +                gdb.write("no modules found\n")
>>> +            else:
>>> +                [self._do_load_module_symbols(module) for module in module_list]
>>
>> Is that common python style? Elsewhere, you do
>>
>> for var in list:
>>     function(var)
> 
> It is a code I moved verbatim from the above. 
> I can change it to use the more common syntax.

Oh, missed that. And it seems I once wrote it this way - no idea anymore
why...

Jan

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