Re: [PATCH 2/3] livepatch: Allow user to specify functions to search for on a stack

Joe Lawrence
2021-11-22T15:53:27Z

On 11/22/21 2:57 AM, Miroslav Benes wrote:
> On Fri, 19 Nov 2021, Josh Poimboeuf wrote:
>
>> Thanks for doing this! And at peterz-esque speed no less :-)
>>
>> On Fri, Nov 19, 2021 at 10:03:26AM +0100, Miroslav Benes wrote:
>>> livepatch's consistency model requires that no live patched function
>>> must be found on any task's stack during a transition process after a
>>> live patch is applied. It is achieved by walking through stacks of all
>>> blocked tasks.
>>>
>>> The user might also want to define more functions to search for without
>>> them being patched at all. It may either help with preparing a live
>>> patch, which would otherwise require additional touches to achieve the
>>> consistency
>>
>> Do we have any examples of this situation we can add to the commit log?
>
> I do not have anything at hand. Joe, do you remember the case you
> mentioned previously about adding a nop to a function?
>

I went looking in my inbox to see... Unfortunately the closest thing I
found was a kpatchset in which we added nops to coax kpatch-build into
reverting previous patch version changes. Not applicable here, but I
was certain we entertained the same idea to increase the task stack
check for some other problem.

Maybe adding a hypothetical scenario to the commit log would suffice?

>>> or it can be used to overcome deficiencies the stack
>>> checking inherently has. For example, GCC may optimize a function so
>>> that a part of it is moved to a different section and the function would
>>> jump to it. This child function would not be found on a stack in this
>>> case, but it may be important to search for it so that, again, the
>>> consistency is achieved.
>>>
>>> Allow the user to specify such functions on klp_object level.
>>>
>>> Signed-off-by: Miroslav Benes <mbenes@suse.cz>
>>> ---
>>> include/linux/livepatch.h | 11 +++++++++++
>>> kernel/livepatch/core.c | 16 ++++++++++++++++
>>> kernel/livepatch/transition.c | 21 ++++++++++++++++-----
>>> 3 files changed, 43 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
>>> index 2614247a9781..89df578af8c3 100644
>>> --- a/include/linux/livepatch.h
>>> +++ b/include/linux/livepatch.h
>>> @@ -106,9 +106,11 @@ struct klp_callbacks {
>>> * struct klp_object - kernel object structure for live patching
>>> * @name: module name (or NULL for vmlinux)
>>> * @funcs: function entries for functions to be patched in the object
>>> + * @funcs_stack: function entries for functions to be stack checked
>>
>> So there are two arrays/lists of 'klp_func', and two implied meanings of
>> what a 'klp_func' is and how it's initialized.
>>
>> Might it be simpler and more explicit to just add a new external field
>> to 'klp_func' and continue to have a single 'funcs' array? Similar to
>> what we already do with the special-casing of 'nop', except it would be
>> an external field, e.g. 'no_patch' or 'stack_only'.
>>
>> Then instead of all the extra klp_for_each_func_stack_static()
>> incantations, and the special cases in higher-level callers like
>> klp_init_object() and klp_init_patch_early(), the lower-level functions
>> like klp_init_func() and klp_init_func_early() can check the field to
>> determine which initializations need to be made. Which is kind of nice
>> IMO as it pushes that detail down more where it belongs. And makes the
>> different types of 'klp_func' more explicit.
>
> I thought about doing this for a moment but then I was worried there would
> be many places which would require special-casing, so I tried to keep it
> separate. But yes, it would be cleaner, so definitely worth trying for v2.
>

I'll add that the first thing that came to mind when you raised this
feature idea in the other thread was to support existing klp_funcs array
with NULL new_func's. I didn't go look to see how invasive it would be,
but it will be interesting to see if a single list approach turns out
any simpler for v2.

--
Joe