Date:   Fri, 21 Jun 2019 09:43:00 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Nicolai Stange <nstange@...e.de>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/5] livepatch: Basic API to track system state changes

On Tue, Jun 11, 2019 at 03:56:24PM +0200, Petr Mladek wrote:
> This is another step how to help maintaining more livepatches.
> 
> One big help was the atomic replace and cumulative livepatches. These
> livepatches replaces the already installed ones. Therefore it should

nit: s/replaces/replaces

> be enough when each cumulative livepatch is consistent.
> 
> The problems might come with shadow variables and callbacks. They might
> change the system behavior or state so that it is not longer safe to

nit: s/not longer safe/no longer safe

> go back and use an older livepatch or the original kernel code. Also
> any new livepatch must be able to detect what changes have already been
> done by the already installed livepatches.
> 
> This is where the livepatch system state tracking gets useful. It
> allows to:
> 
>   - find whether a system state has already been modified by
>     previous livepatches
> 
>   - store data needed to manipulate and restore the system state
> 
> The information about the manipulated system states is stored in an
> array of struct klp_state. There are two functions that allow
> to find this structure for a given struct klp_patch or for
> already installed (replaced) livepatches.
> 

suggestion: "Two new functions, klp_get_state() and
klp_get_prev_state(), can find this structure ..." or perhaps drop this
part altogether and let the future reader do a 'git show' or 'git log
-p' to see the code changes and the exact function names.

-- Joe

> The dependencies are going to be solved by a version field added later.
> The only important information is that it will be allowed to modify
> the same state by more non-cumulative livepatches. It is the same logic
> as that it is allowed to modify the same function several times.
> The livepatch author is responsible for preventing incompatible
> changes.
> 
> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  include/linux/livepatch.h | 15 +++++++++
>  kernel/livepatch/Makefile |  2 +-
>  kernel/livepatch/state.c  | 83 +++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 99 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/livepatch/state.c
> 
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index eeba421cc671..591abdee30d7 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -132,10 +132,21 @@ struct klp_object {
>  	bool patched;
>  };
>  
> +/**
> + * struct klp_state - state of the system modified by the livepatch
> + * @id:		system state identifier (non zero)
> + * @data:	custom data
> + */
> +struct klp_state {
> +	int id;
> +	void *data;
> +};
> +
>  /**
>   * struct klp_patch - patch structure for live patching
>   * @mod:	reference to the live patch module
>   * @objs:	object entries for kernel objects to be patched
> + * @states:	system states that can get modified
>   * @replace:	replace all actively used patches
>   * @list:	list node for global list of actively used patches
>   * @kobj:	kobject for sysfs resources
> @@ -150,6 +161,7 @@ struct klp_patch {
>  	/* external */
>  	struct module *mod;
>  	struct klp_object *objs;
> +	struct klp_state *states;
>  	bool replace;
>  
>  	/* internal */
> @@ -220,6 +232,9 @@ void *klp_shadow_get_or_alloc(void *obj, unsigned long id,
>  void klp_shadow_free(void *obj, unsigned long id, klp_shadow_dtor_t dtor);
>  void klp_shadow_free_all(unsigned long id, klp_shadow_dtor_t dtor);
>  
> +struct klp_state *klp_get_state(struct klp_patch *patch, int id);
> +struct klp_state *klp_get_prev_state(int id);
> +
>  #else /* !CONFIG_LIVEPATCH */
>  
>  static inline int klp_module_coming(struct module *mod) { return 0; }
> diff --git a/kernel/livepatch/Makefile b/kernel/livepatch/Makefile
> index cf9b5bcdb952..cf03d4bdfc66 100644
> --- a/kernel/livepatch/Makefile
> +++ b/kernel/livepatch/Makefile
> @@ -1,4 +1,4 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  obj-$(CONFIG_LIVEPATCH) += livepatch.o
>  
> -livepatch-objs := core.o patch.o shadow.o transition.o
> +livepatch-objs := core.o patch.o shadow.o state.o transition.o
> diff --git a/kernel/livepatch/state.c b/kernel/livepatch/state.c
> new file mode 100644
> index 000000000000..f8822b71f96e
> --- /dev/null
> +++ b/kernel/livepatch/state.c
> @@ -0,0 +1,83 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * system_state.c - State of the system modified by livepatches
> + *
> + * Copyright (C) 2019 SUSE
> + */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/livepatch.h>
> +#include "core.h"
> +#include "transition.h"
> +
> +#define klp_for_each_state(patch, state)		\
> +	for (state = patch->states; state && state->id; state++)
> +
> +/**
> + * klp_get_state() - get information about system state modified by
> + *	the given patch
> + * @patch:	livepatch that modifies the given system state
> + * @id:		custom identifier of the modified system state
> + *
> + * Checks whether the given patch modifies to given system state.
> + *
> + * The function can be called either from pre/post (un)patch
> + * callbacks or from the kernel code added by the livepatch.
> + *
> + * Return: pointer to struct klp_state when found, otherwise NULL.
> + */
> +struct klp_state *klp_get_state(struct klp_patch *patch, int id)
> +{
> +	struct klp_state *state;
> +
> +	klp_for_each_state(patch, state) {
> +		if (state->id == id)
> +			return state;
> +	}
> +
> +	return NULL;
> +}
> +EXPORT_SYMBOL_GPL(klp_get_state);
> +
> +/**
> + * klp_get_prev_state() - get information about system state modified by
> + *	the already installed livepatches
> + * @id:		custom identifier of the modified system state
> + *
> + * Checks whether already installed livepatches modify the given
> + * system state.
> + *
> + * The same system state can be modified by more non-cumulative
> + * livepatches. It is expected that the latest livepatch has
> + * the most up-to-date information.
> + *
> + * The function can be called only during transition when a new
> + * livepatch is being enabled or when such a transition is reverted.
> + * It is typically called only from from pre/post (un)patch
> + * callbacks.
> + *
> + * Return: pointer to the latest struct klp_state from already
> + *	installed livepatches, NULL when not found.
> + */
> +struct klp_state *klp_get_prev_state(int id)
> +{
> +	struct klp_patch *patch;
> +	struct klp_state *state, *last_state = NULL;
> +
> +	if (WARN_ON_ONCE(!klp_transition_patch))
> +		return NULL;
> +
> +	klp_for_each_patch(patch) {
> +		if (patch == klp_transition_patch)
> +			goto out;
> +
> +		state = klp_get_state(patch, id);
> +		if (state)
> +			last_state = state;
> +	}
> +
> +out:
> +	return last_state;
> +}
> +EXPORT_SYMBOL_GPL(klp_get_prev_state);
> -- 
> 2.16.4
> 

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