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

Miroslav Benes
2021-11-22T07:57:56Z

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?

> > 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.

Thanks

Miroslav