Re: [PATCH v15 07/11] livepatch: Add atomic replace

[Miroslav Benes]

On Wed, 9 Jan 2019, Petr Mladek wrote:

> From: Jason Baron <jbaron@xxxxxxxxxx>
> 
> Sometimes we would like to revert a particular fix. Currently, this
> is not easy because we want to keep all other fixes active and we
> could revert only the last applied patch.
> 
> One solution would be to apply new patch that implemented all
> the reverted functions like in the original code. It would work
> as expected but there will be unnecessary redirections. In addition,
> it would also require knowing which functions need to be reverted at
> build time.
> 
> Another problem is when there are many patches that touch the same
> functions. There might be dependencies between patches that are
> not enforced on the kernel side. Also it might be pretty hard to
> actually prepare the patch and ensure compatibility with the other
> patches.
> 
> Atomic replace && cumulative patches:
> 
> A better solution would be to create cumulative patch and say that
> it replaces all older ones.
> 
> This patch adds a new "replace" flag to struct klp_patch. When it is
> enabled, a set of 'nop' klp_func will be dynamically created for all
> functions that are already being patched but that will no longer be
> modified by the new patch. They are used as a new target during
> the patch transition.
> 
> The idea is to handle Nops' structures like the static ones. When
> the dynamic structures are allocated, we initialize all values that
> are normally statically defined.
> 
> The only exception is "new_func" in struct klp_func. It has to point
> to the original function and the address is known only when the object
> (module) is loaded. Note that we really need to set it. The address is
> used, for example, in klp_check_stack_func().
> 
> Nevertheless we still need to distinguish the dynamically allocated
> structures in some operations. For this, we add "nop" flag into
> struct klp_func and "dynamic" flag into struct klp_object. They
> need special handling in the following situations:
> 
>   + The structures are added into the lists of objects and functions
>     immediately. In fact, the lists were created for this purpose.
> 
>   + The address of the original function is known only when the patched
>     object (module) is loaded. Therefore it is copied later in
>     klp_init_object_loaded().
> 
>   + The ftrace handler must not set PC to func->new_func. It would cause
>     infinite loop because the address points back to the beginning of
>     the original function.
> 
>   + The various free() functions must free the structure itself.
> 
> Note that other ways to detect the dynamic structures are not considered
> safe. For example, even the statically defined struct klp_object might
> include empty funcs array. It might be there just to run some callbacks.
> 
> Also note that the safe iterator must be used in the free() functions.
> Otherwise already freed structures might get accessed.
> 
> Special callbacks handling:
> 
> The callbacks from the replaced patches are _not_ called by intention.
> It would be pretty hard to define a reasonable semantic and implement it.
> 
> It might even be counter-productive. The new patch is cumulative. It is
> supposed to include most of the changes from older patches. In most cases,
> it will not want to call pre_unpatch() post_unpatch() callbacks from
> the replaced patches. It would disable/break things for no good reasons.
> Also it should be easier to handle various scenarios in a single script
> in the new patch than think about interactions caused by running many
> scripts from older patches. Not to say that the old scripts even would
> not expect to be called in this situation.
> 
> Removing replaced patches:
> 
> One nice effect of the cumulative patches is that the code from the
> older patches is no longer used. Therefore the replaced patches can
> be removed. It has several advantages:
> 
>   + Nops' structs will no longer be necessary and might be removed.
>     This would save memory, restore performance (no ftrace handler),
>     allow clear view on what is really patched.
> 
>   + Disabling the patch will cause using the original code everywhere.
>     Therefore the livepatch callbacks could handle only one scenario.
>     Note that the complication is already complex enough when the patch
>     gets enabled. It is currently solved by calling callbacks only from
>     the new cumulative patch.
> 
>   + The state is clean in both the sysfs interface and lsmod. The modules
>     with the replaced livepatches might even get removed from the system.
> 
> Some people actually expected this behavior from the beginning. After all
> a cumulative patch is supposed to "completely" replace an existing one.
> It is like when a new version of an application replaces an older one.
> 
> This patch does the first step. It removes the replaced patches from
> the list of patches. It is safe. The consistency model ensures that
> they are no longer used. By other words, each process works only with
> the structures from klp_transition_patch.
> 
> The removal is done by a special function. It combines actions done by
> __disable_patch() and klp_complete_transition(). But it is a fast
> track without all the transaction-related stuff.
> 
> Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
> [pmladek@xxxxxxxx: Split, reuse existing code, simplified]
> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> Cc: Jiri Kosina <jikos@xxxxxxxxxx>
> Cc: Miroslav Benes <mbenes@xxxxxxx>

Acked-by: Miroslav Benes <mbenes@xxxxxxx>

Miroslav