linux-kernel - Re: [PATCH] livepatch: Convert error about unsupported reliable stacktrace into a warning

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Date:   Thu, 18 Apr 2019 08:58:58 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jiri Kosina <jikos@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] livepatch: Convert error about unsupported reliable
 stacktrace into a warning

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@...hat.com>
> Signed-off-by: Petr Mladek <pmladek@...e.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.

-- 
Josh