Re: [RFC 0/5] livepatch: new API to track system state changes

Re: [RFC 0/5] livepatch: new API to track system state changes

[Joe Lawrence]

On Tue, Jun 11, 2019 at 03:56:22PM +0200, Petr Mladek wrote:
> Hi,
> 
> this is another piece in the puzzle that helps to maintain more
> livepatches.
> 
> Especially pre/post (un)patch callbacks might change a system state.
> Any newly installed livepatch has to somehow deal with system state
> modifications done be already installed livepatches.
> 
> This patchset provides, hopefully, a simple and generic API that
> helps to keep and pass information between the livepatches.
> It is also usable to prevent loading incompatible livepatches.
>

Thanks for posting, Petr and aplogies for not getting to this RFC
earlier.  I think this strikes a reasonable balance between the (too) 
"simplified" versioning scheme that I posted a few weeks back, and what
I was afraid might have been too complicated callback-state-version
concept.

This RFC reads fairly straightforward and especially easy to review
given the included documentation and self-tests.  I'll add a few
comments per patch, but again, I like how this came out.
 
> There was also a related idea to add a sticky flag. It should be
> easy to add it later. It would perfectly fit into the new struct
> klp_state.

I think so, too.  It would indicate that the patch is introducing a
state which cannot be safely unloaded.  But we can talk about that at a
later time if/when we want to add that wrinkle to klp_state.
 
-- Joe

Re: [RFC 2/5] livepatch: Basic API to track system state changes

[Joe Lawrence]

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@suse.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
> 

Re: [RFC 2/5] livepatch: Basic API to track system state changes

[Nicolai Stange]

Petr Mladek <pmladek@suse.com> writes:

> ---
>  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;

Can we make this an unsigned long please? It would be consistent with
shadow variable ids and would give more room for encoding bugzilla or
CVE numbers or so.

Nicolai

> +	void *data;
> +};
> +

Re: [RFC 4/5] livepatch: Documentation of the new API for tracking system state changes

[Joe Lawrence]

On Tue, Jun 11, 2019 at 03:56:26PM +0200, Petr Mladek wrote:
> Documentation explaining the motivation, capabilities, and usage
> of the new API for tracking system state changes.
> 
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
>  Documentation/livepatch/index.rst        |  1 +
>  Documentation/livepatch/system-state.rst | 80 ++++++++++++++++++++++++++++++++
>  2 files changed, 81 insertions(+)
>  create mode 100644 Documentation/livepatch/system-state.rst
> 
> diff --git a/Documentation/livepatch/index.rst b/Documentation/livepatch/index.rst
> index edd291d51847..94bbbc2c8993 100644
> --- a/Documentation/livepatch/index.rst
> +++ b/Documentation/livepatch/index.rst
> @@ -9,6 +9,7 @@ Kernel Livepatching
>  
>      livepatch
>      callbacks
> +    system-state
>      cumulative-patches
>      module-elf-format
>      shadow-vars
> diff --git a/Documentation/livepatch/system-state.rst b/Documentation/livepatch/system-state.rst
> new file mode 100644
> index 000000000000..3a35073a0b80
> --- /dev/null
> +++ b/Documentation/livepatch/system-state.rst
> @@ -0,0 +1,80 @@
> [ ... snip ... ]
> +1. Livepatch compatibility

nit: 2. Livepatch compatibility

-- Joe

Re: [RFC 5/5] livepatch: Selftests of the API for tracking system state changes

[Miroslav Benes]

> diff --git a/lib/livepatch/test_klp_state.c b/lib/livepatch/test_klp_state.c
> new file mode 100644
> index 000000000000..c43dc2f2e01d
> --- /dev/null
> +++ b/lib/livepatch/test_klp_state.c
> @@ -0,0 +1,161 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright (C) 2019 SUSE
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/module.h>
> +#include <linux/kernel.h>
> +#include <linux/printk.h>
> +#include <linux/livepatch.h>
> +
> +#define CONSOLE_LOGLEVEL_STATE 1
> +/* Version 1 does not support migration. */
> +#define CONSOLE_LOGLEVEL_STATE_VERSION 1

[...]

> +static struct klp_state states[] = {
> +	{
> +		.id = CONSOLE_LOGLEVEL_STATE,
> +		.version = 1,

s/1/CONSOLE_LOGLEVEL_STATE_VERSION/

Miroslav

Re: [RFC 5/5] livepatch: Selftests of the API for tracking system state changes

[Joe Lawrence]

On Tue, Jun 11, 2019 at 03:56:27PM +0200, Petr Mladek wrote:
> 
> [ ... snip ... ]
> 
> diff --git a/lib/livepatch/test_klp_state.c b/lib/livepatch/test_klp_state.c
> new file mode 100644
> index 000000000000..c43dc2f2e01d
> --- /dev/null
> +++ b/lib/livepatch/test_klp_state.c
> @@ -0,0 +1,161 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright (C) 2019 SUSE
> 
> [ ... snip ... ]
> 
> +MODULE_AUTHOR("Joe Lawrence <joe.lawrence@redhat.com>");

Feel free to update the module author for these.

-- Joe

Re: [RFC 0/5] livepatch: new API to track system state changes

[Nicolai Stange]

Hi Petr,

> this is another piece in the puzzle that helps to maintain more
> livepatches.
>
> Especially pre/post (un)patch callbacks might change a system state.
> Any newly installed livepatch has to somehow deal with system state
> modifications done be already installed livepatches.
>
> This patchset provides, hopefully, a simple and generic API that
> helps to keep and pass information between the livepatches.
> It is also usable to prevent loading incompatible livepatches.

I like it a lot, many thanks for doing this!

Minor remarks/questions will follow inline.

Nicolai

Re: [RFC 3/5] livepatch: Allow to distinguish different version of system state changes

[Miroslav Benes]

> +/* Check if the patch is able to deal with the given system state. */
> +static bool klp_is_state_compatible(struct klp_patch *patch,
> +                                 struct klp_state *state)
> +{
> +     struct klp_state *new_state;
> +
> +     new_state = klp_get_state(patch, state->id);
> +
> +     if (new_state)
> +             return new_state->version < state->version ? false : true;

return new_state->version >= state->version;

?

> +     /* Cumulative livepatch must handle all already modified states. 
*/
> +     return patch->replace ? false : true;

return !patch->replace;

?

> + * Check if the new livepatch will not break the existing system states.
> + * Cumulative patches must handle all already modified states.
> + * Non-cumulative patches can touch already modified states.
> + */
> +bool klp_is_patch_compatible(struct klp_patch *patch)

make C=1 kernel/livepatch/state.o

kernel/livepatch/state.c:107:6: warning: symbol 'klp_is_patch_compatible' was not declared. Should it be static?

#include "state.h" in state.c would solve it.

Miroslav

Re: [RFC 3/5] livepatch: Allow to distinguish different version of system state changes

[Joe Lawrence]

On Tue, Jun 11, 2019 at 03:56:25PM +0200, Petr Mladek wrote:
> The atomic replace runs pre/post (un)install callbacks only from the new
> livepatch. There are several reasons for this:
> 
>   + Simplicity: clear ordering of operations, no interactions between
> 	old and new callbacks.
> 
>   + Reliability: only new livepatch knows what changes can already be made
> 	by older livepatches and how to take over the state.
> 
>   + Testing: the atomic replace can be properly tested only when a newer
> 	livepatch is available. It might be too late to fix unwanted effect
> 	of callbacks from older	livepatches.
> 
> It might happen that an older change is not enough and the same system
> state has to be modified another way. Different changes need to get
> distinguished by a version number added to struct klp_state.
> 
> The version can also be used to prevent loading incompatible livepatches.
> The check is done when the livepatch is enabled. The rules are:
> 
>   + Any completely new system state modification is allowed.
> 
>   + System state modifications with the same or higher version are allowed
>     for already modified system states.
> 

More word play: would it be any clearer to drop the use of
"modification" when talking about klp_states?  Sometimes I read
modification to mean a change to a klp_state itself rather than the
system at large.

In my mind, "modification" is implied, but I already know where this
patchset is going, so perhaps I'm just trying to be lazy and not type
the whole thing out :)  I wish I could come up with a nice, succinct
alternative, but "state" or "klp_state" would work for me.  /two cents

>   + Cumulative livepatches must handle all system state modifications from
>     already installed livepatches.
> 
>   + Non-cumulative livepatches are allowed to touch already modified
>     system states.
> 
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
>  include/linux/livepatch.h |  2 ++
>  kernel/livepatch/core.c   |  8 ++++++++
>  kernel/livepatch/state.c  | 40 +++++++++++++++++++++++++++++++++++++++-
>  kernel/livepatch/state.h  |  9 +++++++++
>  4 files changed, 58 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/livepatch/state.h
> 
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index 591abdee30d7..8bc4c6cc3f3f 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -135,10 +135,12 @@ struct klp_object {
>  /**
>   * struct klp_state - state of the system modified by the livepatch
>   * @id:		system state identifier (non zero)
> + * @version:	version of the change (non-zero)
>   * @data:	custom data
>   */
>  struct klp_state {
>  	int id;
> +	int version;
>  	void *data;
>  };
>  
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 24c4a13bd26c..614642719825 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -21,6 +21,7 @@
>  #include <asm/cacheflush.h>
>  #include "core.h"
>  #include "patch.h"
> +#include "state.h"
>  #include "transition.h"
>  
>  /*
> @@ -1003,6 +1004,13 @@ int klp_enable_patch(struct klp_patch *patch)
>  
>  	mutex_lock(&klp_mutex);
>  
> +	if(!klp_is_patch_compatible(patch)) {
> +		pr_err("Livepatch patch (%s) is not compatible with the already installed livepatches.\n",
> +			patch->mod->name);
> +		mutex_unlock(&klp_mutex);
> +		return -EINVAL;
> +	}
> +
>  	ret = klp_init_patch_early(patch);
>  	if (ret) {
>  		mutex_unlock(&klp_mutex);
> diff --git a/kernel/livepatch/state.c b/kernel/livepatch/state.c
> index f8822b71f96e..b54a69b9e4b4 100644
> --- a/kernel/livepatch/state.c
> +++ b/kernel/livepatch/state.c
> @@ -12,7 +12,9 @@
>  #include "transition.h"
>  
>  #define klp_for_each_state(patch, state)		\
> -	for (state = patch->states; state && state->id; state++)
> +	for (state = patch->states;			\
> +	     state && state->id && state->version;	\
> +	     state++)

Minor git bookkeeping here, but this could be moved to the patch that
introduced the macro.

>  
>  /**
>   * klp_get_state() - get information about system state modified by
> @@ -81,3 +83,39 @@ struct klp_state *klp_get_prev_state(int id)
>  	return last_state;
>  }
>  EXPORT_SYMBOL_GPL(klp_get_prev_state);
> +
> +/* Check if the patch is able to deal with the given system state. */
> +static bool klp_is_state_compatible(struct klp_patch *patch,
> +				    struct klp_state *state)
> +{
> +	struct klp_state *new_state;
> +
> +	new_state = klp_get_state(patch, state->id);
> +
> +	if (new_state)
> +		return new_state->version < state->version ? false : true;
> +
> +	/* Cumulative livepatch must handle all already modified states. */
> +	return patch->replace ? false : true;
> +}
> +
> +/*
> + * Check if the new livepatch will not break the existing system states.

suggestion: "Check that the new livepatch will not break" or
            "Check if the new livepatch will break"

-- Joe

Re: [RFC 3/5] livepatch: Allow to distinguish different version of system state changes

[Joe Lawrence]

On Fri, Jun 21, 2019 at 10:09:11AM -0400, Joe Lawrence wrote:
> More word play: would it be any clearer to drop the use of
> "modification" when talking about klp_states?  Sometimes I read
> modification to mean a change to a klp_state itself rather than the
> system at large.

After reading through the rest of the series, maybe I was premature
about this.  "System state modification" is used consistently throughout
the series, so without having any better suggestion, ignore my comment.

-- Joe

Re: [RFC 3/5] livepatch: Allow to distinguish different version of system state changes

[Nicolai Stange]

Petr Mladek <pmladek@suse.com> writes:

> ---
>  include/linux/livepatch.h |  2 ++
>  kernel/livepatch/core.c   |  8 ++++++++
>  kernel/livepatch/state.c  | 40 +++++++++++++++++++++++++++++++++++++++-
>  kernel/livepatch/state.h  |  9 +++++++++
>  4 files changed, 58 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/livepatch/state.h
>
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index 591abdee30d7..8bc4c6cc3f3f 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -135,10 +135,12 @@ struct klp_object {
>  /**
>   * struct klp_state - state of the system modified by the livepatch
>   * @id:		system state identifier (non zero)
> + * @version:	version of the change (non-zero)
>   * @data:	custom data
>   */
>  struct klp_state {
>  	int id;
> +	int version;
>  	void *data;
>  };
>  
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 24c4a13bd26c..614642719825 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -21,6 +21,7 @@
>  #include <asm/cacheflush.h>
>  #include "core.h"
>  #include "patch.h"
> +#include "state.h"
>  #include "transition.h"
>  
>  /*
> @@ -1003,6 +1004,13 @@ int klp_enable_patch(struct klp_patch *patch)
>  
>  	mutex_lock(&klp_mutex);
>  
> +	if(!klp_is_patch_compatible(patch)) {
> +		pr_err("Livepatch patch (%s) is not compatible with the already installed livepatches.\n",
> +			patch->mod->name);
> +		mutex_unlock(&klp_mutex);
> +		return -EINVAL;
> +	}
> +
>  	ret = klp_init_patch_early(patch);
>  	if (ret) {
>  		mutex_unlock(&klp_mutex);


Just as a remark: klp_reverse_transition() could still transition back
to a !klp_is_patch_compatible() patch.

I don't think it's much of a problem, because for live patches
introducing completely new states to the system, it is reasonable
to assume that they'll start applying incompatible changes only from
their ->post_patch(), I guess.

For state "upgrades" to higher versions, it's not so clear though and
some care will be needed. But I think these could still be handled
safely at the cost of some complexity in the new live patch's
->post_patch().

Another detail is that ->post_unpatch() will be called for the new live
patch which has been unpatched due to transition reversal and one would
have to be careful not to free shared state from under the older, still
active live patch. How would ->post_unpatch() distinguish between
transition reversal and "normal" live patch disabling?  By
klp_get_prev_state() != NULL?

Perhaps transition reversal should be mentioned in the documentation?

Thanks,

Nicolai


-- 
SUSE Linux GmbH, GF: Felix Imendörffer, Mary Higgins, Sri Rasiah, HRB
21284 (AG Nürnberg)

Re: [RFC 3/5] livepatch: Allow to distinguish different version of system state changes

[Petr Mladek]

Hi,

first, I am sorry that I answer this non-trivial mail so late.
I know that it might be hard to remember the context.


On Mon 2019-06-24 12:26:07, Nicolai Stange wrote:
> Petr Mladek <pmladek@suse.com> writes:
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index 24c4a13bd26c..614642719825 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -1003,6 +1004,13 @@ int klp_enable_patch(struct klp_patch *patch)
> >  
> >  	mutex_lock(&klp_mutex);
> >  
> > +	if(!klp_is_patch_compatible(patch)) {
> > +		pr_err("Livepatch patch (%s) is not compatible with the already installed livepatches.\n",
> > +			patch->mod->name);
> > +		mutex_unlock(&klp_mutex);
> > +		return -EINVAL;
> > +	}
> > +
> >  	ret = klp_init_patch_early(patch);
> >  	if (ret) {
> >  		mutex_unlock(&klp_mutex);
> 
> 
> Just as a remark: klp_reverse_transition() could still transition back
> to a !klp_is_patch_compatible() patch.

I am slightly confused. The new livepatch is enabled only when the new
states have the same or higher version. And only callbacks from
the new livepatch are used, including post_unpatch() when
the transition gets reverted.

The "compatible" livepatch should be able to handle all situations:

    + Modify the system state when it was not modified before.

    + Take over the system state when it has already been modified
      by the previous livepatch.

    + Restore the previous state when the transition is reverted.


> I don't think it's much of a problem, because for live patches
> introducing completely new states to the system, it is reasonable
> to assume that they'll start applying incompatible changes only from
> their ->post_patch(), I guess.
>
> For state "upgrades" to higher versions, it's not so clear though and
> some care will be needed. But I think these could still be handled
> safely at the cost of some complexity in the new live patch's
> ->post_patch().

Just to be sure. The post_unpatch() from the new livepatch
will get called when the transitions is reverted. It should
be able to revert any changes made by its own pre_patch().

You are right that it will need some care. Especially because
the transition revert is not easy to test.

I think that this is the main reason why Joe would like
to introduce the sticky flag. It might be used to block
the transition revert and livepatch disabling when it would
be to complicated, error-prone, or even impossible.


> Another detail is that ->post_unpatch() will be called for the new live
> patch which has been unpatched due to transition reversal and one would
> have to be careful not to free shared state from under the older, still
> active live patch. How would ->post_unpatch() distinguish between
> transition reversal and "normal" live patch disabling?  By
> klp_get_prev_state() != NULL?

Exactly. klp_get_prev_state() != NULL can be used in the
post_unpatch() to restore the original state when
the transition gets reverted.

See restore_console_loglevel() in lib/livepatch/test_klp_state2.c

> Perhaps transition reversal should be mentioned in the documentation?

Good point. I'll mention it in the documentation.

Best Regards,
Petr

Re: [RFC 3/5] livepatch: Allow to distinguish different version of system state changes

[Petr Mladek]

On Tue 2019-06-11 15:56:25, Petr Mladek wrote:
> It might happen that an older change is not enough and the same system
> state has to be modified another way. Different changes need to get
> distinguished by a version number added to struct klp_state.
> 
> The version can also be used to prevent loading incompatible livepatches.
> The check is done when the livepatch is enabled. The rules are:
> 
>   + Any completely new system state modification is allowed.
> 
>   + System state modifications with the same or higher version are allowed
>     for already modified system states.
> 
>   + Cumulative livepatches must handle all system state modifications from
>     already installed livepatches.
> 
>   + Non-cumulative livepatches are allowed to touch already modified
>     system states.
> 
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
>  include/linux/livepatch.h |  2 ++
>  kernel/livepatch/core.c   |  8 ++++++++
>  kernel/livepatch/state.c  | 40 +++++++++++++++++++++++++++++++++++++++-
>  kernel/livepatch/state.h  |  9 +++++++++
>  4 files changed, 58 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/livepatch/state.h
> 
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index 591abdee30d7..8bc4c6cc3f3f 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -135,10 +135,12 @@ struct klp_object {
>  /**
>   * struct klp_state - state of the system modified by the livepatch
>   * @id:		system state identifier (non zero)
> + * @version:	version of the change (non-zero)
>   * @data:	custom data
>   */
>  struct klp_state {
>  	int id;

As suggested by Nicolay, there will be in v2:

	unsigned long id;

> +	int version;

It would make sense to make "version" unsigned as well.
I am just unsure about the size:

  + "unsigned long" looks like an overhead to me
  + "u8" might be enough

But I would stay on the safe side and use:

	unsigned int version;

Is anyone against?

Best Regards,
Petr