Re: [PATCH 1/3] scripts/gdb: rework lx-symbols gdb script

[Jan Kiszka <jan.kiszka@siemens.com>]

On 15.03.21 23:10, 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.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  kernel/module.c              |   8 ++-
>  scripts/gdb/linux/symbols.py | 106 +++++++++++++++++++++++++----------
>  2 files changed, 83 insertions(+), 31 deletions(-)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 30479355ab850..ea81fc06ea1f5 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -901,8 +901,12 @@ 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)
> diff --git a/scripts/gdb/linux/symbols.py b/scripts/gdb/linux/symbols.py
> index 1be9763cf8bb2..4ce879548a1ae 100644
> --- a/scripts/gdb/linux/symbols.py
> +++ b/scripts/gdb/linux/symbols.py
> @@ -17,6 +17,24 @@ import re
>  
>  from linux import modules, utils
>  
> +def save_state():

Naming is a bit too generic. And it's not only saving the state, it's
also disabling things.

> +        breakpoints = []
> +        if hasattr(gdb, 'breakpoints') and not gdb.breakpoints() is None:
> +            for bp in gdb.breakpoints():
> +                breakpoints.append({'breakpoint': bp, 'enabled': bp.enabled})
> +                bp.enabled = False
> +
> +        show_pagination = gdb.execute("show pagination", to_string=True)
> +        pagination = show_pagination.endswith("on.\n")
> +        gdb.execute("set pagination off")
> +
> +        return {"breakpoints":breakpoints, "show_pagination": show_pagination}
> +
> +def load_state(state):

Maybe rather something with "restore", to make naming balanced. Or is
there a use case where "state" is not coming from the function above?

> +    for breakpoint in state["breakpoints"]:
> +        breakpoint['breakpoint'].enabled = breakpoint['enabled']
> +    gdb.execute("set pagination %s" % ("on" if state["show_pagination"] else "off"))
> +
>  
>  if hasattr(gdb, 'Breakpoint'):
>      class LoadModuleBreakpoint(gdb.Breakpoint):
> @@ -30,26 +48,38 @@ if hasattr(gdb, 'Breakpoint'):
>              module_name = module['name'].string()
>              cmd = self.gdb_command
>  
> +            # module already loaded, false alarm
> +            if module_name in cmd.loaded_modules:
> +                return False

Possibly at all, now that we track unloading? Can our state tracking
become out-of-sync?

> +
>              # 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")
> +            state = save_state()
> +            cmd.load_module_symbols(module)
> +            load_state(state)
> +            return False
>  
> -            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)
> +    class UnLoadModuleBreakpoint(gdb.Breakpoint):
> +        def __init__(self, spec, gdb_command):
> +            super(UnLoadModuleBreakpoint, self).__init__(spec, internal=True)
> +            self.silent = True
> +            self.gdb_command = gdb_command
> +
> +        def stop(self):
> +            module = gdb.parse_and_eval("mod")
> +            module_name = module['name'].string()
> +            cmd = self.gdb_command
>  
> -            # restore pagination state
> -            gdb.execute("set pagination %s" % ("on" if pagination else "off"))
> +            if not module_name in cmd.loaded_modules:
> +                return False
>  

Same question as above. For robustness, checking is not bad. But maybe
it's worth reporting as well.

> +            state = save_state()
> +            cmd.unload_module_symbols(module)
> +            load_state(state)
>              return False
>  
>  
> @@ -64,8 +94,9 @@ lx-symbols command."""
>      module_paths = []
>      module_files = []
>      module_files_updated = False
> -    loaded_modules = []
> -    breakpoint = None
> +    loaded_modules = {}
> +    module_load_breakpoint = None
> +    module_unload_breakpoint = None
>  
>      def __init__(self):
>          super(LxSymbols, self).__init__("lx-symbols", gdb.COMMAND_FILES,
> @@ -129,21 +160,32 @@ lx-symbols command."""
>                  filename=module_file,
>                  addr=module_addr,
>                  sections=self._section_arguments(module))
> +
>              gdb.execute(cmdline, to_string=True)
> -            if module_name not in self.loaded_modules:
> -                self.loaded_modules.append(module_name)
> +            self.loaded_modules[module_name] = {"module_file": module_file,
> +                                                "module_addr": module_addr}
>          else:
>              gdb.write("no module object found for '{0}'\n".format(module_name))
>  
> +    def unload_module_symbols(self, module):
> +        module_name = module['name'].string()
> +
> +        module_file = self.loaded_modules[module_name]["module_file"]
> +        module_addr = self.loaded_modules[module_name]["module_addr"]
> +
> +        gdb.write("unloading @{addr}: {filename}\n".format(
> +            addr=module_addr, filename=module_file))
> +        cmdline = "remove-symbol-file {filename}".format(
> +            filename=module_file)
> +
> +        gdb.execute(cmdline, to_string=True)
> +        del self.loaded_modules[module_name]
> +
> +
>      def load_all_symbols(self):
>          gdb.write("loading vmlinux\n")
>  
> -        # Dropping symbols will disable all breakpoints. So save their states
> -        # and restore them afterward.
> -        saved_states = []
> -        if hasattr(gdb, 'breakpoints') and not gdb.breakpoints() is None:
> -            for bp in gdb.breakpoints():
> -                saved_states.append({'breakpoint': bp, 'enabled': bp.enabled})
> +        state = save_state()
>  
>          # drop all current symbols and reload vmlinux
>          orig_vmlinux = 'vmlinux'
> @@ -153,15 +195,14 @@ lx-symbols command."""
>          gdb.execute("symbol-file", to_string=True)
>          gdb.execute("symbol-file {0}".format(orig_vmlinux))
>  
> -        self.loaded_modules = []
> +        self.loaded_modules = {}
>          module_list = modules.module_list()
>          if not module_list:
>              gdb.write("no modules found\n")
>          else:
>              [self.load_module_symbols(module) for module in module_list]
>  
> -        for saved_state in saved_states:
> -            saved_state['breakpoint'].enabled = saved_state['enabled']
> +        load_state(state)
>  
>      def invoke(self, arg, from_tty):
>          self.module_paths = [os.path.expanduser(p) for p in arg.split()]
> @@ -174,11 +215,18 @@ lx-symbols command."""
>          self.load_all_symbols()
>  
>          if hasattr(gdb, 'Breakpoint'):
> -            if self.breakpoint is not None:
> -                self.breakpoint.delete()
> -                self.breakpoint = None
> -            self.breakpoint = LoadModuleBreakpoint(
> -                "kernel/module.c:do_init_module", self)
> +            if self.module_load_breakpoint is not None:
> +                self.module_load_breakpoint.delete()
> +                self.module_load_breakpoint = None
> +            self.module_load_breakpoint = \
> +                LoadModuleBreakpoint("kernel/module.c:do_init_module", self)
> +
> +            if self.module_unload_breakpoint is not None:
> +                self.module_unload_breakpoint.delete()
> +                self.module_unload_breakpoint = None
> +            self.module_unload_breakpoint = \
> +                UnLoadModuleBreakpoint("kernel/module.c:free_module", self)
> +
>          else:
>              gdb.write("Note: symbol update on module loading not supported "
>                        "with this gdb version\n")
> 

Good improvement!

Jan

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