Re: [RFC PATCH 0/9] livepatch: consistency model

Masami Hiramatsu
2015-02-10T11:17:10Z

(2015/02/10 2:31), Josh Poimboeuf wrote:
> This patch set implements a livepatch consistency model, targeted for 3.21.
> Now that we have a solid livepatch code base, this is the biggest remaining
> missing piece.
>
> This code stems from the design proposal made by Vojtech [1] in November. It
> makes live patching safer in general. Specifically, it allows you to apply
> patches which change function prototypes. It also lays the groundwork for
> future code changes which will enable data and data semantic changes.

Interesting, How would you do that?

> It's basically a hybrid of kpatch and kGraft, combining kpatch's backtrace
> checking with kGraft's per-task consistency. When patching, tasks are
> carefully transitioned from the old universe to the new universe. A task can
> only be switched to the new universe if it's not using a function that is to be
> patched or unpatched. After all tasks have moved to the new universe, the
> patching process is complete.
>
> How it transitions various tasks to the new universe:
>
> - The stacks of all sleeping tasks are checked. Each task that is not sleeping
> on a to-be-patched function is switched.
>
> - Other user tasks are handled by do_notify_resume() (see patch 9/9). If a
> task is I/O bound, it switches universes when returning from a system call.
> If it's CPU bound, it switches when returning from an interrupt. If it's
> sleeping on a patched function, the user can send SIGSTOP and SIGCONT to
> force it to switch upon return from the signal handler.

Ah, OK. So you can handle those without hooking switch_to :)

>
> - Idle "swapper" tasks which are sleeping on a to-be-patched function can be
> switched from within the outer idle loop.
>
> - An interrupt handler will inherit the universe of the task it interrupts.
>
> - kthreads which are sleeping on to-be-patched functions are not yet handled
> (more on this below).
>
>
> I think this approach provides the best benefits of both kpatch and kGraft:
>
> advantages vs kpatch:
> - no stop machine latency

Good! :)

> - higher patch success rate (can patch in-use functions)
> - patching failures are more predictable (primary failure mode is attempting to
> patch a kthread which is sleeping forever on a patched function, more on this
> below)
>
> advantages vs kGraft:
> - less code complexity (don't have to hack up the code of all the different
> kthreads)
> - less impact to processes (don't have to signal all sleeping tasks)
>
> disadvantages vs kpatch:
> - no system-wide switch point (not really a functional limitation, just forces
> the patch author to be more careful. but that's probably a good thing anyway)

OK, we must check carefully that the old function and new function can be co-exist.

> My biggest concerns and questions related to this patch set are:
>
> 1) To safely examine the task stacks, the transition code locks each task's rq
> struct, which requires using the scheduler's internal rq locking functions.
> It seems to work well, but I'm not sure if there's a cleaner way to safely
> do stack checking without stop_machine().

We'd better ask scheduler people.

>
> 2) As mentioned above, kthreads which are always sleeping on a patched function
> will never transition to the new universe. This is really a minor issue
> (less than 1% of patches). It's not necessarily something that needs to be
> resolved with this patch set, but it would be good to have some discussion
> about it regardless.
>
> To overcome this issue, I have 1/2 an idea: we could add some stack checking
> code to the ftrace handler itself to transition the kthread to the new
> universe after it re-enters the function it was originally sleeping on, if
> the stack doesn't already have have any other to-be-patched functions.
> Combined with the klp_transition_work_fn()'s periodic stack checking of
> sleeping tasks, that would handle most of the cases (except when trying to
> patch the high-level thread_fn itself).

It makes sense to me. (I just did similar thing)

>
> But then how do you make the kthread wake up? As far as I can tell,
> wake_up_process() doesn't seem to work on a kthread (unless I messed up my
> testing somehow). What does kGraft do in this case?

Hmm, at a glance, the code itself can work on kthread too...
Maybe you can also send you testing patch too.

Thank you,

>
>
> [1] https://lkml.org/lkml/2014/11/7/354
>
>
> Josh Poimboeuf (9):
> livepatch: simplify disable error path
> livepatch: separate enabled and patched states
> livepatch: move patching functions into patch.c
> livepatch: get function sizes
> sched: move task rq locking functions to sched.h
> livepatch: create per-task consistency model
> proc: add /proc/<pid>/universe to show livepatch status
> livepatch: allow patch modules to be removed
> livepatch: update task universe when exiting kernel
>
> arch/x86/include/asm/thread_info.h | 4 +-
> arch/x86/kernel/signal.c | 4 +
> fs/proc/base.c | 11 ++
> include/linux/livepatch.h | 38 ++--
> include/linux/sched.h | 3 +
> kernel/fork.c | 2 +
> kernel/livepatch/Makefile | 2 +-
> kernel/livepatch/core.c | 360 ++++++++++---------------------------
> kernel/livepatch/patch.c | 206 +++++++++++++++++++++
> kernel/livepatch/patch.h | 26 +++
> kernel/livepatch/transition.c | 318 ++++++++++++++++++++++++++++++++
> kernel/livepatch/transition.h | 16 ++
> kernel/sched/core.c | 34 +---
> kernel/sched/idle.c | 4 +
> kernel/sched/sched.h | 33 ++++
> 15 files changed, 747 insertions(+), 314 deletions(-)
> create mode 100644 kernel/livepatch/patch.c
> create mode 100644 kernel/livepatch/patch.h
> create mode 100644 kernel/livepatch/transition.c
> create mode 100644 kernel/livepatch/transition.h
>

--
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com