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

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Message-ID: <YZ9gfPuCTmDmOj9h@alley>
Date:   Thu, 25 Nov 2021 11:07:56 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>, jikos@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, shuah@...nel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/3] livepatch: Allow user to specify functions to search
 for on a stack

On Mon 2021-11-22 10:53:21, Joe Lawrence wrote:
> 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?
> 
> Maybe adding a hypothetical scenario to the commit log would suffice?

I wonder if we could describe a scenario based on the thread about
.cold code variants, see
https://lore.kernel.org/all/20211112015003.pefl656m3zmir6ov@treble/

This feature would allow to safely livepatch already released
kernels where the unwinder is not able to reliably detect
a newly discovered problems.

> >>> 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@...e.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'.
> 
> 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.

Please, solve this with the extra flag, e.g. .stack_only, as
already suggested. It will help to distinguish mistakes and
intentions. Also it will allow to find these symbols by grep.

> 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.

I am not sure either. But I expect that it will be easier than
the extra array.

Best Regards,
Petr