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SubjectRe: [PATCH] livepatch: Convert error about unsupported reliable stacktrace into a warning
On Thu, 18 Apr 2019, Josh Poimboeuf wrote:

> On Thu, Apr 18, 2019 at 01:29:36PM +0200, Petr Mladek wrote:
> > The commit d0807da78e11d46f ("livepatch: Remove immediate feature") caused
> > that any livepatch was refused when reliable stacktraces were not supported
> > on the given architecture.
> >
> > The limitation is too strong. User space processes are safely migrated
> > even when entering or leaving the kernel. Kthreads transition would
> > need to get forced. But it is safe when:
> >
> > + The livepatch does not change the semantic of the code.
> > + Callbacks do not depend on a safely finished transition.
> >
> > Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > Signed-off-by: Petr Mladek <pmladek@suse.com>
> > ---
> > kernel/livepatch/core.c | 5 ++---
> > 1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index eb0ee10a1981..14f33ab6c583 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -1003,11 +1003,10 @@ int klp_enable_patch(struct klp_patch *patch)
> > return -ENODEV;
> >
> > if (!klp_have_reliable_stack()) {
> > - pr_err("This architecture doesn't have support for the livepatch consistency model.\n");
> > - return -EOPNOTSUPP;
> > + pr_warn("This architecture doesn't have support for the livepatch consistency model.\n");
> > + pr_warn("The livepatch transition may never complete.\n");
> > }
> >
> > -
> > mutex_lock(&klp_mutex);
> >
> > ret = klp_init_patch_early(patch);
>
> Thanks Petr. I would also suggest that we remove the WARN_ONCE() from
> the weak version of save_stack_trace_tsk_reliable(), since we expect it
> to be used in this case.

Moreover, we have WARN_ON_ONCE(ret == -ENOSYS) in klp_check_stack() where
save_stack_trace_tsk_reliable() is called.

Miroslav

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